FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIAETHIOPIAN ROADS AUTHORITY GEOTECHNICAL DESIGN MANUAL 2013 Geotechnical Design Manual - 2013 Foreword FOREWORD The road network in Ethiopia provides the dominant mode of freight and passenger transport and thus plays a vital role in the economy of the country. The network comprises a huge national asset that requires adherence to appropriate standards for design, construction and maintenance in order to provide a high level of service. As the length of the road network is increasing, appropriate choice of methods to preserve this investment becomes increasingly important. In 2002 the Ethiopian Roads Authority (ERA) published road design manuals to provide a standardized approach for the design, construction and maintenance of roads in the country. Due to technological development and change, these manuals require periodic updating and expanding. The new series of manuals has particular reference to the prevailing conditions in Ethiopia and reflects the experience gained through activities within the road sector during the last 10 years. The updating of existing manuals and preparation of new manuals was undertaken in close consultation with the federal and regional roads authorities and stakeholders in the road sector. Most importantly, a series of thematic peer review panels was established that comprised local experts from the public and private sector who provided guidance and review for the project team. The Geotechnical Design Manual is a new addition to the ERA series of manuals. The standards set out shall be adhered to unless otherwise directed by ERA. However, I should emphasize that careful consideration to sound engineering practice shall be observed in the use of the manual, and under no circumstances shall the manual waive professional judgment in applied engineering. For simplification in reference this manual may be cited as ERA’s Geotechnical Design Manual - 2013. On behalf of ERA I would like to thank DFID, Crown Agents and the AFCAP management team for their cooperation, contribution and support in the development of the manual. I would also like to extend my gratitude and appreciation to all of the industry stakeholders and participants who contributed their time, knowledge and effort during the development of the documents. Special thanks are extended to the members of the various Peer Review Panels, whose active support and involvement guided the authors of the manual and the process. It is my sincere hope that this manual will provide all users with a standard reference and a ready source of good practice for the geotechnical design of roads, and will assist in a cost effective operation, and environmentally sustainable development of our road network. I look forward to the practices contained in this manual being quickly adopted into our operations, thereby making a sustainable contribution to the improved infrastructure of our country. Comments and suggestions on all aspects from any concerned body, group or individual as feedback during its implementation is expected and will be highly appreciated. Addis Ababa, 2013 Zaid Wolde Gebriel Director General, Ethiopian Roads Authority Ethiopian Roads Authority Page i standard specifications and bidding documents that are written for the practicing engineer in Ethiopia. is as follows: 1.gov. 14. will be issued and inserted into the relevant chapter. 9. 5. The Geotechnical Design Manual -2013 forms part of the Ethiopian Roads Authority series of Road and Bridge Design documents. The series describes current and recommended practice and sets out the national standards for roads and bridges. 4. 3. 8. The complete series of documents. 6. together with the revision date.000. These documents are available to registered users through the ERA website: www. Standard Drawings Best Practice Manual for Thin Bituminous Surfacings Standard Bidding Documents for Road Work Contracts – A series of Bidding Documents covering a full range from large scale projects unlimited in value to minor works with an upper threshold of $300. procedures or any other relevant issues related to new policies or revised laws of the land or that are mandated by the relevant Federal Government Ministry or Agency should be incorporated into the manual from their date of effectiveness.era. 13. The documents are based on national experience and international practice and are approved by the Director General of the Ethiopian Roads Authority. 12.et Manual Updates Significant changes to criteria. Route Selection Manual Site Investigation Manual Geotechnical Design Manual Geometric Design Manual Pavement Design Manual Volume I Flexible Pavements Pavement Design Manual Volume II Rigid Pavements Pavement Rehabilitation and Asphalt Overlay Design Manual Drainage Design Manual Bridge Design Manual Low Volume Roads Design Manual Standard Environmental Procedures Manual Standard Technical Specifications. When changes are made and approved. 7. The higher level documents have both Local Competitive Bidding and International Competitive Bidding versions. 11. 15. Page ii Ethiopian Roads Authority . 10. covering all roads and bridges in Ethiopia. new page(s) incorporating the revision. Other minor changes that will not significantly affect the whole nature of the manual may be accumulated and made periodically. 2.Preface Geotechnical Design Manual – 2013 PREFACE The Ethiopian Roads Authority is the custodian of the series of technical manuals. Proposed changes should be outlined on the Manual Change Form and forwarded with a covering letter of its need and purpose to the Director General of the Ethiopian Roads Authority. Ethiopian Roads Authority Ethiopian Roads Authority Page iii . 4. 3.2013 Preface All suggestions to improve the manual should be made in accordance with the following procedures: 1. Users of the manual must register on the ERA website: www.et 2. 2013 Zaid Wolde Gebriel Director General.era. Addis Ababa. The release date will be notified to all registered users and authorities.gov.Geotechnical Design Manual . Agreed changes will be approved by the Director General of the Ethiopian Roads Authority on recommendation from the Deputy Director General (Engineering Operations). __________________ Suggested Modification Submitted by: Name:____________________________________Designation:_______________________________ Company/Organisation Address ____________________________________________________________________ _______________________________email:___________________________________________Date:___________ Manual Change Action Authority Date Signature Recommended Action Approval Registration Director Quality Assurance Deputy Director General Eng.___________________ SECTION NO. Ops Approval / Provisional Approval / Rejection of Change: Director General ERA:__________________________________ Date: __________________ Page iv Ethiopian Roads Authority .Preface Geotechnical Design Manual – 2013 ETHIOPIAN ROADS AUTHORITY CHANGE CONTROL DESIGN MANUAL This area to be completed by the ERA Director of Quality Assurance MANUAL CHANGE Manual Title:____________________________ _______________________________________ Section Table Figure Page Explanation CHANGE NO. Our own extensive local experience and expertise was supplemented by inputs from an international team of experts and shared through review workshops to discuss and debate the contents of the draft manual. where possible. Ato BEACON Consulting Engineers 8 Fikert Arega. Name Organization 1 Abebe Asefa. Ato OMEGA Consulting Engineers 4 Asrat Sewit. Ato Ethiopian Roads Authority 3 Asnake Haile. the intent was. Ato Spice Consult 11 Tewodros Alene. As with the other manuals of this series. The contribution of the Peer Review Group participants is gratefully acknowledged.Geotechnical Design Manual . Ato Ethiopian Roads Authority 7 Efrem Degefu. From the outset. ERA would like to thank Crown Agents for their overall management of the project. Ato Saba Engineering 5 Colin Gourley. Ato Ethiopian Roads Authority 12 Zerihun Nuru. Ato Ethiopian Roads Authority 10 Shimelis Tesfaye. It will be used by all authorities and organisations responsible for the provision of roads in Ethiopia. other references are used. Peer Review Groups comprising specialists drawn from within the local industry were established to provide advice and comments in their respective areas of expertise. W/ro Ethiopian Roads Authority 9 Muse Belew. Finally. ERA wishes to thank all the individuals who gave their time to attend the workshops and provide valuable inputs to the compilation of the manual. Where no such reference exists for tests and specifications mentioned in this document. and in the interests of uniformity. to use those tests and specifications included in the AASHTO and/or ASTM Materials references. List of Persons Contributing to Peer Group Review No. Ato Ethiopian Roads Authority 2 Alemayehu Ayele. In addition to the workshops. the approach to the development of the manual was to include all sectors and stakeholders in Ethiopia. Dr. Ato Gondwana Engineering Ethiopian Roads Authority Page v .2013 Acknowledgements ACKNOWLEDGEMENTS The Ethiopian Roads Authority (ERA) wishes to thank the UK Government’s Department for International Development (DFID) through the Africa Community Access Programme (AFCAP) for their support in developing this manual. ERA/DRID 6 Daniel Nebro. 1 Name Organization Role Bekele Negussie ERA AFCAP Coordinator for Ethiopia 2 Abdo Mohammed ERA Project Coordinator 3 Daniel Nebro ERA Project Coordinator 4 Frew Bekele ERA Project Coordinator 5 Lulseged Ayalew. Dr Consultant Final Review Addis Ababa. 2013 Zaid Wolde Gebriel Director General. Ethiopian Roads Authority Page vi Ethiopian Roads Authority .Acknowledgements Geotechnical Design Manual – 2013 Project Team No. Dr AFCAP Consultant Lead Author 6 Robert Geddes AFCAP/Crown Agents Technical Manager 7 Les Sampson AFCAP/Crown Agents Project Director 8 Gareth Hearn URS Technical Review 9 Tim Hunt URS Technical Review 10 Alemgena Araya. ..4..........................1................................ 3-2 Design Considerations ..............................................4 Dispersive soils ...........................................................XIX 1 1..................3 Moisture and density .......... 2-6 2............................................................................. 2-13 2..........................................................................3 Removal and replacement ............2 Earth-fill embankments ........................................................................ 3-13 3..... II ACKNOWLEDGEMENTS ..................................................................................................................................................................................................................VII LIST OF ILLUSTRATIONS ...3 Special Considerations ................ 3-21 Fill Slope Stabilization ..........................................................................3 3...........7 Ethiopian Roads Authority Page vii ...................4.. 2-1 2............... 2-5 2.............................................................. 3-2 3..............2....... 3-16 Bridge Approach Embankments.................1 2..............3 Secondary compression ......................................................................................................3 Embankments on soft ground .......1 Primary consolidation .................2 Vertical drains ........................2 Stiffness .2013 Table of Contents TABLE OF CONTENTS FOREWORD ........1....................... 3-1 3............. XV ABBREVIATIONS .....................................................................2 2 INTRODUCTION .................................. 2-1 2...................................................Geotechnical Design Manual ...................................................................................... 2-6 2.......................3..............................................................................2 Time for settlement ......................................................... 2-13 2............ 3-5 3............................................................................... 2-2 2.................................................................................3........ 2-22 3 ROAD EMBANKMENTS ..................1..2............. I PREFACE .2 Compressible soils .................................. 3-17 Stability Assessment ...1 Types of Embankments ... 3-10 3....................................................................................................................... 2-21 2............................................ X LIST OF TABLES .............................................1 Rock fill embankments ................................................. 1-1 Structure .............................3 Soil stabilization .................................................................................................................................................................................... 3-6 3................. 2-20 2............................1 Strength..................... 1-1 Scope .......4........................... 1-2 PAVEMENT SUBGRADE ............................................................................................................................................................................... 2-1 Geotechnical Design Considerations .....................................................................................3 Collapsible soils .....2 3..................................................... 3-24 3..1 Moisture control .......................................................................................................................................................................... V TABLE OF CONTENTS ............................................................1 Expansive soils ........................................................2 Removal and replacement ... 2-17 2.3. 3-15 3.......................................... 2-1 2..........4 3...................1 1........................6 3.......................2 General............................................................................................................................................................................................................................................................................................3........4...........................5 3....................................... XIII GLOSSARY OF TERMS ...... 3-3 Settlement Analysis ......................................4..............1 Preloading and Surcharge.............. 3-13 3.................................................. 3-1 3.............4.................................3................................ 3-11 Settlement Mitigation .........4 Subgrade Treatment ........................................................ 2-19 2...........................................................................................................................................................................................3................................................ 3-1 3...........................................2........................3.......................................................................................................................................... ....... 4-29 4... Landslide stabilisation .......................... 3-28 3.................... 3-29 3...............13 Reinforced Embankments .. 4-59 4................................ 4-10 4.. The use of computer programs ........................6................. 4-53 4.........................7..............4 Lightweight fills ...3............ 4-5 Landslides...... Soil Slope Stability Analyses ................ 4-1 4.................. profiles and benches ............ 3-34 3......... 4-23 4.................... 4-31 4..................................................................................... 5-7 Page viii Ethiopian Roads Authority ... 4-43 4............3................................. 4-59 4...........3........................................... Rock Cut Slopes .......................... 4-45 4................................. Collapsible soils .................................. Cut slope angles............. 4-15 4...........................................................................................6...............................................11 Mortared Masonry Walls ................... Latosols .........................................................................7.....................5......1.7.. 4-18 4...............3............ Residual soils...................................................5.........................2..... The role of discontinuities .............................................................................................................. Safe angles of cut and benches .. Determination of shear strength parameters .1 5............5............................................ 3-29 3................................. Design considerations ...................... 5-2 General Geotechnical Report Outline ............................6................1................................. Soil slope cuts........ Mode of failures ..............3.................................................................................................4 GEOTECHNICAL REPORT AND CHECKLIST .......................... 3-42 4 ROADSIDE SLOPES ........................ 4................ 3-40 3...........1.................5..........5............................................... 4-60 4..6..............................2....................................................... 3-24 3...........................................................7...................... 4-26 4....................... 4-20 4...................................................................... 4-35 4.....................................................1........................ Colluvium and talus slopes ................................................. Stability analysis procedures ..6....................................4....... 4-40 4............4............................. 4-11 4. 4-38 4............................................ 4-54 4................. 4-27 4......................... Causes of landslides ........8 Embankments in Hilly Areas .........3 Ground improvement.................................................1........................................................10 Wall-supported Embankments ...................................................................................... 3-37 3..4........................ Limit equilibrium analyses ...............................2 Base reinforcement ........... 3-29 3.......... 5-5 Checklist ...............3.......................................9 Fill-slope Angles and Benches .............................................6................2................7...7............3 5............... 5-1 Final Level Geotechnical Report......... 4-36 4.... Limit equilibrium methods .......................................7...........................................................7............3.................................................... Kinematic analysis .....................4.......................2 5...........5....12 Gabion Supported Embankments .......5 Removal and replacement .......................................4............................................................................ Roadbed landslides ...................... 4-61 4.................5....4..................... 4-21 4........................ 3-29 3.................................................7.............7........2......................................................................5................. Design considerations ....... 5-1 Preliminary Level Geotechnical Report .........................................................................6 Toe berms and shear keys .4........... 4-62 5 5.........................................................................4... 4-9 4...................................................................................... The role of weathering .................. 3-40 3..... 4-2 Cut Slopes ............. 4-31 4....4.......... Methods of rock excavation..........................................5.....................................1 Staged construction .............. 4-24 4.......... Types of landslides ...........Table of Contents Geotechnical Design Manual – 2013 3.......... 4................2......3..................................................................................3....................... Natural Slopes ............. 4-61 4.............2......................... 4-30 4...........................................................................3................... 3-31 3.......5......6................................ Rock slope stability analyses................4...........4............................................ Roadside landslides ...1.....5........................................................................................................................ The role of groundwater ...................................... .Geotechnical Design Manual ...................... 6-1 APPENDIX A ...................................2013 6 Table of Contents REFERENCES AND BIBLIOGRAPHY .......................................................................SOIL STABILIZATION . 1 Ethiopian Roads Authority Page ix .................................... ................................................. 3-14 Figure 3-8: Use of vertical drains to accelerate settlement...... 3-7 Figure 3-3: Typical consolidation curve for over-consolidated soil............. 3-16 Figure 3-9: Removal and replacement beneath an embankment ......... From FAO (1998) ................................................................................ From US DOT FHWA (2006B) ............ 3-8 Figure 3-4: Typical consolidation curve for under-consolidated soils (a) void ratio versus vertical effective stress and (b) vertical strain versus vertical effective stress.............................................. 2-16 Figure 2-5: Dispersive potential determined based on percentage sodium and total dissolved solids...................... 3-19 Figure 3-13: Modes of side slope failures in embankments....................................... 3-13 Figure 3-7: Effect of surcharge on magnitude and time of settlement .... From Briaud et al (1997) .............................................. (a) void ratio versus vertical effective stress and (b) vertical strain versus vertical effective stress.................. (a) void ratio versus vertical effective stress and (b) vertical strain versus vertical effective stress....... From US DOT FHWA (2006B) .................................... 3-31 Page x Ethiopian Roads Authority ........... 3-12 Figure 3-6: Concept of pre-loading and its effect on magnitude and time of settlement ............................................. 3-28 Figure 3-17: Use of a counterweight berm (a) and a shear key (b)................ From NCHRP (1989) reproduced in US DOT FHWA (2006B) ................................................. 3-23 Figure 3-15: Effect of flooding and rapid-drawdown on embankment stability................................... From Washington State DOT (2013) ................................................................................... Modified from US DOT FHWA 2006B ....... 2-18 Figure 2-6: Sources of moisture in pavements.......... From IOWA State (2013) and US DOT FHWA (2006B) ............. Modified from US DOT FHWA (2006) 2-20 Figure 3-1: Example of stress-strain incompatibility.............................................. Modified from BRAB (1978) .......................... Modified from US DOT FHWA 2006B .... From Abramson et al (2002) .......................................... 3-30 Figure 3-18: Typical construction of embankments in hilly areas................... 3-3 Figure 3-2: Typical consolidation curve for normally consolidated soil................................................................................ 3-17 Figure 3-11: Settlement and down-drag in bridge abutments and piles................ 3-17 Figure 3-10: Elements of a bridge approach embankment.............................................. Modified from Univ Iowa (2013) .................................... 2-14 Figure 2-4: Typical results of a one dimensional oedometer test. 2-5 Figure 2-2: Graphical illustration of the depth of desiccation and moisture fluctuation........................................................ From US DOT FHWA (2006B)........................................................................List of Illustrations Geotechnical Design Manual – 2013 LIST OF ILLUSTRATIONS Figure 2-1: Typical Resilient Modulus correlations to empirical soil properties and Classification categories...... 3-18 Figure 3-12: Suggested details of a bridge approach embankment............................................... From US DOT FHWA (2006B) ............................... 3-9 Figure 3-5: Log t method of determining the coefficient of consolidation.............................. From NCHRP (2001) . 3-28 Figure 3-16: Concept of calculating the percent consolidation in staged construction...................................................................................................... 2-9 Figure 2-3: Mechanism of soil collapse in natural soils................................. 3-21 Figure 3-14: Typical circular arc failure mechanism................................................................................ From US DOT FHWA (2006B)................... From US DOI Bureau of the Interior (1991) ......................................................... From US DOT FHWA (2006B)............ .......................................... 3-41 Figure 3-29: Application of reinforced slopes in road construction....................................................... From New York State DOT (2007) . From MPWT (2008) .................................................................................................................................................................. 3-35 Figure 3-22: Fill embankment on a hill-side with outward dipping berms .......... 4-1 Figure 4-2: Natural and cut slopes adjacent to a road ............................................................... 4-4 Figure 4-4: Graphic description of the evolution and extent of slope failure................................................................ 4-18 Figure 4-16: Example of a failure affecting the entire road ................................................... 3-36 Figure 3-24: Benched fill on a benched hill-side slope.............................................................................. 4-9 Figure 4-9: Adversely jointed rock mass that can fail if joints become undercut by excavation ............................ 4-9 Figure 4-8: Schematic profile of cut slope benches .. From JKR (2010) ..........................................................................................................Geotechnical Design Manual ............................................................ 3-37 Figure 3-25: Typical use of retaining structures in road embankments ............. 4-17 Figure 4-14: Landslide affecting both the cut and natural slope above..... Modified from Keller and Sherar (2011) ........................... From Hearn and Hunt (2011) ............................ 4-16 Figure 4-12: Retrogressive landslide developed on a natural slope ................................................ 4-5 Figure 4-5: Types of cross-section design... 3-39 Figure 3-27: Terminology associated with semi-gravity retaining walls ..................................... 4-2 Figure 4-3: Illustration of the terms used to describe stages of slope failure....................................................................................... 4-28 Ethiopian Roads Authority Page xi ................... 3-36 Figure 3-23: Embankment on inward inclined hill-side benches........................................ From US DOT FHWA (2001).... 3-38 Figure 3-26: Gravity and semi-gravity retaining walls .... 3-32 Figure 3-20: Typical side-slopes of a rock fill embankment ............................. From Keller and Sherar (2011) ............................ 4-16 Figure 4-13: Landslide on a cut slope .......................................... 3-46 Figure 4-1: Terms used commonly to define a road and associated slopes ................................................ 4-27 Figure 4-21: Simplified illustration of the formation of residual soils .............................................................................................................................................. 4-15 Figure 4-10: Perched water table and the formation of landslides on road cuts ....................... 3-35 Figure 3-21: Typical side-slopes of an earth fill embankment ....... 4-7 Figure 4-6: Example of a box cut .......................... From US DOT FHWA (2001) .................................................................................... 3-39 Figure 3-28: Typical types of gabion walls .... 3-44 Figure 3-31: Failure modes for reinforced soil embankments.......................................................................................................... 4-17 Figure 4-15: Failure of a fill slope ............... 4-19 Figure 4-17: Stability condition of a clay cut slope over time From Bishop and Bjerrum (1960) reproduced in Abramson et al (2002) .................................................................. 4-15 Figure 4-11: Landslides above a road......... From NY State DOT (2007) ...................... 4-24 Figure 4-19: Types of cut-slope profiles ........................................................................................... 3-43 Figure 3-30: Typical construction of reinforced fills....................................................... From Cruden and Varnes (1996) ............................................................2013 List of Illustrations Figure 3-19: Types of slope instability commonly seen in fills and the underlying hillslope.......... 4-26 Figure 4-20: Road cut on a colluvial slope ...... From Cruden and Varnes (1996) .......... 4-22 Figure 4-18: Typical cut slope ratios in most soil types ............................................................................. 4-8 Figure 4-7: Full cut cross-section .................. 3-45 Figure 3-32: Steps for design of reinforced soil slopes........................................ ... 4-41 Figure 4-31: General guide to select a method of rock excavation.......List of Illustrations Geotechnical Design Manual – 2013 Figure 4-22: Relatively steep 1:1 cut slope in residual clay ...... 4-33 Figure 4-26: Graphical illustration of the rock quality designation (RQD) .............................. 4-46 Figure 4-32: Blast design parameters................... 4-35 Figure 4-27: Effect of discontinuity characteristics on groundwater level ....... 4-47 Figure 4-33: Blasting in limestone ................................................. 4-38 Figure 4-29: Modes of failure in rock slopes........................................................................................ 4-37 Figure 4-28: Rock slide initiated by groundwater seepage out of the slope ............................................................................................ Modified from US DOI Bureau of Reclamation (1998) ..... 4-30 Figure 4-25: Description of different discontinuity parameters ........ 4-29 Figure 4-23: Failure in black cotton soil due to infiltration of water ............................................................................................ 4-55 Page xii Ethiopian Roads Authority ............................................................................................................ 4-29 Figure 4-24: Cut through weathered rock and residual soils ................ 4-49 Figure 4-34: Rock displacement as a result of uncontrolled blasting ..................................... 4-50 Figure 4-35: Equilibrium conditions used to define the factor of safety ........................... From Pettifer and Fookes (1994) .......................................... Modified from GWP Consultants (2008) 4-39 Figure 4-30: Example of toppling failure ......................................... ...................... From Soil and water Management (2008) ..................................................... 2-15 Table 2-5: A summary of remedial measures to reduce the effect of dispersive soils............................ From US DOT FHWA 2006B .................................................................. 3-33 Table 3-8: Preliminary fill slope angles ...........................Geotechnical Design Manual .......................... 4-25 Table 4-9: Types of discontinuities and their characteristics................................. 4-20 Table 4-7: Common landslide remedial measures........ 4-14 Table 4-6: Indications of slope instability at or above a road ................... 2-3 Table 2-3: Swelling characteristics of subgrade soils................................... 4-51 Ethiopian Roads Authority Page xiii ................................... 4-32 Table 4-10: Fracture density............................................ 4-21 Table 4-8 Soil cut slope ratios (H:V) for preliminary design purposes ........... Modified from Hearn and Hunt (2011) ............................................. From US DOT FHWA CFLHD (2011) ....................................................................................................... 4-45 Table 4-14: Controlled blasting techniques................................................... From United States DOT FHWA (2006B) .......... 4-3 Table 4-2: Simple classification of landslide types............ Modified from Nettleton et al (2005) .......................... Modified from Montana DOT (2008) .... 4-34 Table 4-11: Aspects to be considered during rock cut slope design..... Modified from US DOT FHWA (2006) ................... 3-34 Table 4-1: Slope stability problems associated with natural slopes............................................................................ From Washington State DOT (2013) ................................................................................ 2-19 Table 3-1: Engineering properties and field and laboratory tests for embankment design... From US DOT FHWA (2006)........... Modified by US DOT FWHA (2006B) from Holtz & Kovacs (1981) ...................... 3-20 Table 3-5: Slope assessment guidelines for the design of embankments and cuts.................................. 3-25 Table 3-6: Design techniques useful for mitigating embankment failure...................................... From US DOT FHWA (2006B) ...................................................................................................................... 2-7 Table 2-4: Collapsible soils......... 2-2 Table 2-2: Resilient modulus (MR)...................... From Sassa and Canuti (2008) ........................................................................................................ topographical and hydrological causal factors .................................................................. 4-13 Table 4-5: Geological....................................... 4-12 Table 4-4: Natural and artificial causes of landslides... Modified from GWP Consultants (2008)..... 3-4 Table 3-2: Correlations between Cc & soil index parameters....................................... 4-44 Table 4-13: Factors controlling bench dimension on rock slopes........... Modified from US DOI Bureau of Reclamation (1998) .... Modified from Cruden and Varnes (1996) ............. From GWP Consultants (2008) ........................................................................................................................ 3-20 Table 3-4: Suggested gradation for drainage aggregate............................................................................................................................................................................................................................................................... 4-11 Table 4-3: Common landslide causal factors...2013 List of Tabless LIST OF TABLES Table 2-1: California Bearing Ratio (CBR).............................. ........ 4-42 Table 4-12: Indicative maximum cut slope angles (H:V) for rock slopes (without discontinuity control)............................... 3-26 Table 3-7: Slope stabilization techniques for embankments on hill slopes......................................... Modified from MPWT (2008) ............ From US DOT FHWA (2006B) . 3-7 Table 3-3: General considerations for specification of selected structural backfill............... After US DOT FHWA (2006) .............. .................. 4-57 Table 4-17: Advantages and limitations of conventional rock slope analyses....List of Tables Geotechnical Design Manual – 2013 Table 4-15: Parameters used for drilling in presplit.................... modified from Coggan et al (1998) ......................................................... From Eberhardt (2003)........ From US DOT FHWA CFLHD (2011) ... smooth and cushion blasting................. 5-8 Page xiv Ethiopian Roads Authority ................. 4-52 Table 4-16: Commonly used limit equilibrium methods ............................................................ 4-62 Table 5-1: Checklist of important information in geotechnical reports ...................... Collapsible Soils Soils that appear to be strong and stable in their natural (dry) state. Bench A near-horizontal step on a cut slope. or both. such as basalt. or pile group anchors. Black Cotton Soil A soil in which there is a high content of expansive clay known as smectite that forms deep cracks in drier seasons or years.Geotechnical Design Manual . semi-solid. Ethiopian Roads Authority Page xv . particularly in Ethiopia. Collapse Potential The change in void ratio (∆e) upon wetting.2013 Glossary of Terms GLOSSARY OF TERMS Anchored Wall Walls that derive their capacity to resist lateral loads by their structural components being restrained by tension elements connected to anchors (and additionally by partial embedment of their structural components into existing ground). usually to provide additional weight at the toe or to act as a buffer against surface erosion. usually to act as a buffer against surface erosion. or a combination of all three. Also used to facilitate the keying in of a fill slope into an underlying natural slope. or to permit the construction of a bench drain. Compressible Soils Soil deposits which are susceptible to large settlements and deformations because of a relatively rapid decrease in void volume upon loading. they can range from grey or red to the more familiar deep black. California Bearing Ratio (CBR) An indirect measurement of soil strength based on resistance to penetration. In each state the consistency and behaviour of a soil is different and thus so are its engineering properties. Atterberg Limits A basic measure of the nature of a fine-grained soil. The anchors may be ground anchors (tiebacks). in climates that are seasonally humid or subject to erratic droughts and floods. Black cotton soils typically form from highly basic rocks. or to impeded drainage. passive pile anchors. The bearing capacity of soil is the maximum average contact pressure between the foundation and the soil which should not produce shear failure in the soil. or to act as an intermediate rock fall area. but which rapidly consolidate under wetting. Depending on the parent material and the climate. plastic and liquid. Berm A near-horizontal step on an embankment slope. Bearing capacity The capacity of soil to support the loads applied to the ground. Depending on the water content of the soil. generating large and often unexpected settlements. it may appear in four states: solid. passive concrete anchors. the dispersed particles are carried away or eroded. Geogrid Comprises a regular grid of plastic with large openings (called apertures) between the tensile elements. Earth fill Embankment Embankments typically built by compacting earthen materials such as natural soils. Expansive Soils Soils which exhibit significant volume changes in the presence of water. Gravity Wall A wall that derives its capacity to resist lateral loads through its dead weight. polyethylene. Pavement The constructed portion of a road that includes the sub-base. Liquid Limit The moisture content at the point between the liquid and plastic limits of a clay as determined using a liquid limit device. including geotextiles. This results in the colloidal fraction going into suspension and.gravity Wall A wall that relies on structural components of the wall partially embedded in foundation materials to mobilize passive resistance to resist lateral loads. staying in suspension. Geomembrane Used to retard or prevent fluid from penetrating the soil and consists of continuous sheets of low permeability materials. geogrids. or as a result of their structural components being restrained by tension elements connected to anchors. The function of the apertures is to allow the surrounding soil materials to interlock across the plane of the geogrid. most commonly polypropylene. Page xvi Ethiopian Roads Authority . A geotextile is usually classified as either woven or non-woven. Non. have repulsive forces between the clay particles that exceed the attractive forces. Modulus of Elasticity The slope of the stress–strain curve in the elastic deformation region. and geomembranes. Geosynthetic Often used to cover a wide range of different artificially manufactured materials. Geotextile A permeable geo-synthetic comprised solely of textile. or nylon. usually created from polymer.Glossary of Terms Geotechnical Design Manual – 2013 Dispersive Soils Soils that. when placed in water. base. Oedometer Laboratory apparatus for carrying out one-dimensional consolidation tests for the determination of settlement. or a combination of the two. In moving water. Gabion Wall Constructed from rectangular steel wire mesh baskets that are filled on site with stone or rock to form a gravity retaining structure. but also potentially including polyester. in still water. The plasticity index is the size of the range of water contents where the soil exhibits plastic properties. Resilient Modulus The resilient modulus (MR) measures the amount of recoverable deformation at any stress level for a dynamically loaded test specimen and is an indication of the stiffness of the layer immediately under the pavement.Geotechnical Design Manual . geosynthetics. 100 mm or greater in size. It can be made from a variety of rock types. Subgrade The supporting ground beneath a pavement structure Surcharging The process of subjecting the ground to a higher pressure than that during the service life in order to achieve a higher initial rate of Ethiopian Roads Authority Page xvii . and other structures against scour.2013 Glossary of Terms and surface layers. and polymer and wire grids are used as reinforcing elements. Metallic strips. Rip-rap Rock or other material used to armour streambeds. Plasticity Index A measure of the plasticity of a soil. Primary Consolidation Settlement of a soil associated with the readjustment of soil particles due to the migration of water out of the voids Reinforced Embankment This is a form of mechanically stabilized earth that incorporates planar reinforcing elements in the constructed embankment with face inclinations of up to 70 degrees. Semi-gravity Wall Similar to gravity walls. commonly granite or limestone. bridge abutments. Plastic Limit The moisture content at the lower limit of the plastic state of clay. or water erosion. Secondary Compression Occurs when a soil continues to settle after the excess pore water pressures are dissipated to a negligible level. and often used on a waterway or water containment where there is potential for water erosion Rock fill Embankment Embankments usually containing more than 75% by volume of large fragments. Shrinkage Limit The moisture content where further loss of moisture will not result in any more volume reduction. Soil Stabilisation The use of mechanical or chemical modifiers to enhance the strength of soils and reduce the change in moisture. The PI is the difference between the liquid limit and the plastic limit Preloading The process of compressing the subsoil prior to placing the permanent load. The occurrence of secondary compression is independent of the stress state. except that it relies on its structural components to mobilize the dead weight of fill to derive its capacity to resist lateral loads. Page xviii Ethiopian Roads Authority . i. Vertical Drain Sometimes called a ‘wick drain’. and is a prefabricated drain consisting of a plastic core that is wrapped with geo-textile and is installed using a mandrel Void Ratio The ratio of the volume of voids in a soil to the volume of solids. Triaxial Compression Test A compression (or shear) test where the stress applied in the vertical direction (along the axis of the cylindrical sample) can be different from the stresses applied in the horizontal directions perpendicular to the sides of the cylinder. Swell Potential The swell of a laterally-confined specimen when it is surcharged and flooded.Glossary of Terms Geotechnical Design Manual – 2013 settlement and thus reducing long term deformation.e. the confining pressure). Geotechnical Design Manual .2013 Abbreviations ABBREVIATIONS AASHTO American Association of State Highway and Transportation Officials Cα Coefficient of Secondary Compression CBR California Bearing Ratio (as described in AASHTO T 193 or ASTM D 1883) Cc Compression Index CD Consolidated Drained CPT Cone Penetration Test Cv Coefficient of Consolidation CU Consolidated Undrained DCP Dynamic Cone Penetrometer DI Design Index E Modulus of Elasticity eo Initial Voids Ratio ERA Ethiopian Roads Authority ESP Exchangeable Sodium Percentage FS Factor of Safety FWD Falling Weight Deflectometer FWHA Federal Highway Agency Gs Specific Gravity H:V Horizontal:Vertical KPa Kilopascals LL Liquid Limit LSPI Lime Slurry Pressure Injection MR Resilient Modulus Nc Bearing Capacity factor NCHRP National Cooperative Highway Research Program NHI National Highway Institute pH Potential Hydrogen PI Plasticity Index RF Reduction Factor RQD Rock Quality Designation SPT Standard Penetration Test TDS Total Dissolved Solids Tv Time Factor USCS Unified Soils Classification System UU Unconsolidated Undrained Ethiopian Roads Authority Page xix . . and cut slopes. the determination of the geotechnical inputs needed for embankment design.1 Scope The subject of Geotechnical Engineering is very wide and for roads its application ranges from simple geotechnical investigation for route selection to foundation analysis and design of bridges.2013 1 Chapter 1 Introduction INTRODUCTION This manual has been prepared to provide guidance on the standards of practice used for solving geotechnical problems relating to the design and construction of roads in Ethiopia. Experience has shown that an inadequate geotechnical investigation can lead to excessive risk both in terms of schedule and cost. The level of geotechnical field investigation necessary for analysis and design depends on the type of the road project and the geotechnical issue under consideration. US Bureau of Reclamation and others. This interaction is important in the design of reliable and cost-effective structures. and the standard of geotechnical reporting. the evaluation and selection of appropriate slope stabilization techniques. the scope of services for each project should be formulated using this manual and others as references. It must be emphasized that the manual is a guidance document. highway and structural engineers needs to take place at all times to ensure compatibility with the various design criteria. Much of the technical guidelines and design requirements provided in this manual have been taken from international references such as those produced by the United States Federal Highway Agency (FHWA). This manual focuses on geotechnical parameters required for subgrade design and construction. This information is given in the Site Investigation Manual which must be consulted in conjunction with this document. Instead. or road rehabilitation projects. The first step in performing any geotechnical analysis and design is a thorough review of any test data and engineering parameters available for the proposed project and any associated geotechnical investigation. Each project presents unique considerations and requires an approach that involves a thorough knowledge of the ground conditions. embankments. Its purpose is to present procedures and guidelines useful to design stable subgrades. and especially between the geotechnical engineer and other personnel. Close and effective communication between geotechnical. the geotechnical aspects of cut-slope design and slope stability. This manual is not intended to serve as the sole reference of geotechnical services on individual projects. In such circumstances the use of alternative approaches and engineering judgement will be Ethiopian Roads Authority Page 1-1 .Geotechnical Design Manual . and that occasions may arise when the methods and recommendations given are inadequate or unsuitable. It also aims to achieve consistency in the way geotechnical problems and solutions are approached in the practice of road design for new road construction. The design and construction of roads requires good communication and coordination between all team members. These sources are listed in the Bibliography at the end of the manual. upgrading. 1. United States Army Corps of Engineers (US ACE). Users are strongly recommended to use this manual in conjunction with the latest edition of the Site Investigation Manual of the Ethiopian Road Authority (ERA). Chapter 1 Introduction Geotechnical Design Manual – 2013 necessary. In a typical application. Chapter 4 also contains general guidelines and engineering practices needed to satisfy the overall stability requirements during road construction for the type of materials and topographic conditions encountered in Ethiopia. and soil type. Chapter 4 deals with the design of slopes as they relate to road construction. staged construction. Some of the subjects covered are the geological aspects that control the nature of rocks and soils exposed in cut slopes and their stability. the design procedures given in Chapter 3 are brief and users of this manual are advised to consult other references for more detailed outlines. consolidation and settlement. and geotextiles or geo-grids are used as the reinforcing materials. Reinforcement allows embankments to be constructed to greater heights and/or with steeper side slopes. Chapter 3 presents a detailed discussion of these geotechnical issues with recommendations on material requirements. and the soil treatment and stabilization methods available. the subgrade must be able to support loads transmitted from above. Page 1-2 Ethiopian Roads Authority . moisture content. Chapter 2 contains a detailed discussion on factors that affect the strength and stiffness characteristics of pavement subgrades. settlement monitoring as well as details of other mitigation measures used to increase embankment performance. 1. A subgrade is the in-situ material upon which the pavement structure is placed. types of slope failures. As landslides and unstable slopes are common in many parts of Ethiopia. earthworks design and excavation techniques. with Chapter 1 being this Introduction. Chapter 3 addresses the geotechnical aspects important for the design of road embankments. the methodologies used for evaluating and describing different subgrade soils. Chapter 2 discusses the characteristics of pavement subgrades. For the pavement to resist deformation and other types of failures. In addition. Chapter 5 summarises the requirements for geotechnical reporting. and stability analysis. Each situation will require its own investigation and design. the reinforcement is placed at the base of the embankment. The performance of a pavement often depends on the quality of its subgrade. and the contents of this manual are intended to be for indicative purposes only. They must understand how these principles are applied to the design of stable roads through various geological materials and landscapes. Appendix A covers the topic of soil stabilisation. it is vital that engineers engaged in road design activities are aware of the basic principles of slope stability. including rock and earth embankments and lightweight fills. etc. internal stability. Chapter 3 includes a discussion of design procedures for reinforced embankment slopes. As with many topics.2 Structure The manual is structured into four chapters and an appendix. The primary geotechnical issues that impact the performance of all these types of embankments are global stability. This load bearing capacity is often affected by the degree of compaction. 1 Strength The strength and stiffness properties control the ability of the subgrade to support loads transmitted from the pavement layers (load-bearing capacity). The subgrade and base layers must be strong enough to resist shear failure and should have adequate stiffness to minimize vertical deflection.2 Geotechnical Design Considerations The performance of a road pavement surface is significantly affected by the characteristics of the subgrade. This means that a thinner pavement structure could be designed on a subgrade with higher CBR compared to a lower CBR value. moisture content. Desirable properties that the subgrade should possess include high strength and stiffness. methods of improvement must be considered. stiffness. It is usually investigated to such depth as may be important to structural design and pavement life. a one-unit change in CBR from 5 to 4 requires a greater increase in pavement thickness than does a change in CBR from 10 to 9. the strength and stiffness of fine-grained soils are low. therefore. In a fill section. and moisture characteristics of materials forming the subgrade can have a significant influence on pavement performance. and low compressibility and swelling. Generally. good drainage.1 General Chapter 2 Pavement Subgrade The subgrade is the supporting ground beneath a pavement structure. The California Bearing Ratio (CBR) is an indirect measure of the strength of the subgrade (Table 2-1). Sands and gravels with high CBR values are often considered as the best subgrades for formation. 2. and it may consist of materials forming the natural ground surface or exposed in excavations. the change in pavement thickness needed to carry a given traffic load is not directly proportional to the change in CBR value of the subgrade soil. soil type. Stronger and stiffer materials provide a more effective foundation for the riding surface and will be more resistant to stresses from repeated loadings and environmental conditions. The fines reduce the overall strength of the subgrade because they reduce the particle-to-particle contact that provides friction to a soil matrix. The strength.2.Geotechnical Design Manual . The higher the CBR value of a subgrade. A critical component of pavement design is. the subgrade is the upper part of the embankment. it should be noted that although the CBR value is directly correlated with strength. It is also the most widely used method for designing pavement structures. the investigation and testing of the subgrade upon which the pavement structure will be constructed. the more strength it has to support the pavement. especially when they are exposed to water. 2. For example. a subgrade having a CBR of 10 or greater can usually support heavy loads and repetitious loading without excessive deformation. In cases where the subgrade is of inadequate strength. Ethiopian Roads Authority Page 2-1 . This ability is often influenced by the degree of compaction.2013 2 PAVEMENT SUBGRADE 2. However. and history of consolidation. ease of compaction. In general. It is located below the base and sub-base courses. AASHTO recommends the use of a resilient modulus (MR) value obtained from a repeated triaxial test for the design of pavements. 2. For field measurement. CBR values measured from as-compacted samples at optimum moisture and density conditions can be significantly greater than CBR values measured from similar samples after soaking. care should be taken to make certain that the deflection dial is anchored well outside the loaded area. Each series of tests is run for a given relative compaction and moisture content. The geotechnical or materials engineer must specify the conditions (dry. After US DOT FHWA (2006) Description Uses in pavements Laboratory determination Field measurement Comment The California Bearing Ratio or CBR is an indirect measurement of soil strength based on resistance to penetration. • Direct input to some empirical pavement design methods. Page 2-2 Ethiopian Roads Authority . The laboratory test is determined based on AASTHO T 193 or ASTM D 1883. particularly for fine grained soils. etc. In the past. • Correlations with resilient modulus and other engineering properties. soaked conditions are used to simulate anticipated long term conditions in the field.) under which each test should be performed. such as vehicle loads on pavements. The CBR test is run on three identically compacted samples. especially for heavily trafficked pavements. It is based on the resistance to penetration by a standardized piston moving at a standardized rate for a prescribed penetration distance. The purpose of using seasonal modulus values. It is important that the testing conditions be clearly stated. Field measurement is made at the field moisture content while laboratory testing is typically performed for soaked conditions. Field measurements are conducted based on ASTM D 4429. Typically. As shown in Table 2-2. Test procedure is similar to that for laboratory determination. after soaking. For example.2 Stiffness The subgrade soil stiffness is measured by resilient modulus (Table 2-2). at optimum moisture. so soil-specific correlations between field and laboratory CBR values are often required. there are currently five test protocols in use for resilient modulus testing in the laboratory. MR measures the amount of recoverable deformation at any stress level for a dynamically loaded test specimen. 95% relative compaction. While the modulus of elasticity is stress divided by strain for a slowly applied load. Δ? ?? = Δ? Where Δ? is the repeated deviator stress and Δ? is the recoverable resilient axial strain. The resilient modulus (MR) of a material is an estimate of its modulus of elasticity (E). Most CBR testing is laboratory based.Chapter 2 Pavement Subgrade Geotechnical Design Manual – 2013 Table 2-1: California Bearing Ratio (CBR). thus the results will be highly dependent on the representativeness of the samples tested. is to qualify the relative damage of a pavement during each season of the year and include this as part of the overall design. the resilient modulus is stress divided by strain for rapidly applied loads. numerous efforts have been made for obtaining appropriate MR values that are representative of field conditions and seasonal moisture variations.2. It is defined simply as the ratio of the cyclic axial stress to resilient axial strain as given below. Axial deformation is measured on the sample using clamps positioned one quarter and three quarters from the base of the test specimen. and fine-grained subgrades. No definitive studies have been conducted to date to provide guidance on differences between measured MR from the various laboratory test protocols. The harmonized protocol developed in NCHRP Project 1-28A attempts to combine the best features from all of the earlier test methods with a loading sequence that minimizes the potential for premature failure of the test specimen. National Cooperative Highway Research Program NCHRP 2004 1-37A.2013 Chapter 2 Pavement Subgrade Table 2-2: Resilient modulus (MR).25 for rigid pavements to adjust to design MR values.67 for granular bases and sub-bases under flexible pavements. Seasonal variations of soil properties are included directly in the NCHRP 2004 1-37A design methodology.Geotechnical Design Manual . time intervals of two weeks or one month duration are used for this analysis. Methods for determining the MR value for each time interval include: Ethiopian Roads Authority Page 2-3 .8 second rest time for subgrade materials. Different specimen sizes. NCHRP 2004 1-37A recommends adjustment factors of 0. Mechanistic-Empirical). compaction procedures.40 for subgrade soils and 0. For very soft specimens. Field measurement Comment In-situ resilient modulus values can be estimated from back-calculation of falling weight deflectometer (FWD) test results or correlations with Dynamic Cone Penetrometer (DCP) values. • Determination of structural layer coefficients in flexible pavements (AASHTO 1986/1993). Field MR values determined from FWD back-calculation are often significantly higher than design MR values measured from laboratory tests because of differences in stress states. AASHTO T P46-94. The 1993 AASHTO guide includes procedures for incorporating seasonal variations into an effective MR for the subgrade. • Characterization of unbound layer stiffness (NCHRP 2004 1-37A). the state of stress specific to each layer in the pavement structure. AASHTO T 307-99. Typically. and loading conditions are usually recommended for granular base/sub-base materials. the displacement may be measured between the top and bottom plates. The procedure in the 1993 AASHTO guide for incorporating seasonal variations into the effective subgrade (MR) can be briefly summarized as follows: • Determine an MR value for each time interval during a year.2 second loading time followed by a 0. The 1993 AASHTO guide recommends for subgrade soils that field MR values be multiplied by a factor of up to 0. Modified from US DOT FHWA (2006) Description Uses in pavements Laboratory determination The resilient modulus (MR) measures the amount of recoverable deformation at any stress level for a dynamically loaded test specimen and is an indication of the stiffness of the layer immediately under the pavement • Characterization of subgrade stiffness for flexible and rigid pavements (AASHTO 1986/1993.33 for flexible pavements and up to 0. coarse-grained subgrades. and the mechanical behaviour of the material type. AASHTO T 294-92. All of the test procedures employ a closed loop electro-hydraulic testing machine to apply repeated cycles of load-pulse at 0. NCHRP 1997 1-28A. These different procedures reflect the different particle sizes of the materials. There are currently five test protocols in use for resilient modulus testing in the laboratory: AASHTO T 292-91. For fine-grained soils. and moisture content.32 Compute the average relative damage ?�as the sum of the relative damage values for each season divided by the number of seasons. and load and cycle duration to a cylindrical test specimen. surface texture. o Back calculation from falling weight deflectometer (FWD) tests performed during each season.18 ?108 (?? )−2. the MR test applies a repeated axial cyclic stress of fixed magnitude.431 However. and soil consolidation affect the resilient modulus. the MR value of granular soils is significantly dependent on gradation. and correlations should always use local experience. AASHTO (1993) and NCHRP (2004) provide different methodologies to obtain MR values. estimation of the resilient modulus can be made from standard CBR and soil index properties. It should. the practice at present is to use the CBR value only during design. For example. the specific gravity. clay content. This is partly because of the difficulty to determine the MR values as these values depend on many factors. Page 2-4 Ethiopian Roads Authority . While the specimen is subjected to this dynamic cyclic stress. angularity. As mentioned above. plasticity index. The test is essentially a cyclic version of a triaxial compression test with the cyclic load application thought to accurately simulate actual traffic loading. In the laboratory. shape. Figure 2-1 provides a graphical chart suggested by NCHRP (2001) which is useful to correlate the CBR and MR values based on AASHTO and USCS soil groups. When test facilities are unavailable for performing the test.Chapter 2 Pavement Subgrade • • • Geotechnical Design Manual – 2013 o Laboratory measurement at the estimated in-situ water content for the time interval. be noted that the CBR value is a static property that cannot account for the actual response of the pavement under the dynamic loads of moving vehicles. Determine the effective subgrade MR from using the inverse of the above equation: ?? = 3015(?�)−0. Estimate a relative damage X corresponding to each seasonal modulus value using the empirical relationship: ? = 1. it is also confined by a static stress provided by a triaxial pressure chamber. however. the AASHTO guide recommends the resilient modulus test be performed on laboratory samples using relevant stress levels and moisture conditions simulating the primary moisture seasons. Geotechnical Design Manual . either along the pavement or within cuts can cause fluctuations in subgrade moisture conditions. Generally. There are numerous ways through which water can percolate and affect the moisture content of the subgrade. and may also produce large changes in strength and stiffness. In order to determine the possible consequences. In silty soils. From NCHRP (2001) 2.3 Moisture and density Moisture is one of the main factors which affects the strength and stiffness of a subgrade. Ethiopian Roads Authority Page 2-5 . in sandy and gravelly soils. however. It also controls the ease of compaction (density) and the compressibility and swelling/shrinkage characteristics of subgrade soils. the sensitivity of the subgrade strength and stiffness to changes in moisture content should be evaluated during or before design. and there may be large changes in strength and stiffness too. Typically these soils attract and retain water through capillary action.2013 Chapter 2 Pavement Subgrade Figure 2-1: Typical Resilient Modulus correlations to empirical soil properties and Classification categories. small fluctuations in water content produce little change in their strength and stiffness. and do not drain well.2. The change in water content in clays often results in large variations in volume. For example seepage from higher ground. any fluctuation in water content can bring about a certain change in volume. 3 Special Considerations Considering variables such as soil type and clay mineralogy along a length of a roadway. In addition. The purpose of compaction is generally to enhance the strength of a soil by increasing density. minimizes long term settlement. several collapsible or highly compressible. and reduces the swelling potential of expansive soils. climate and topography make each project unique with respect to subgrade materials and conditions. Typically these soils attract and retain water through matrix suction.0 m below the pavement surface. Pavement performance also depends on subgrade density. The evaluation of density reached as a result of the compactive efforts of compaction equipment is the most common quality-control measurement made at construction sites. Table 2-3 summarizes the general characteristics of swelling subgrade soils. and longitudinal cracking. In-situ soils used as subgrades for the construction of road pavements are invariably compacted to improve their density. and to their natural and imposed environments. the depth of influence for wheel loading varies between 1. If the density varies. past and present loading history. 2. One of the factors which determine the depth of influence for wheel loading is the inherent characteristic of the subgrade and its natural density. 2.Chapter 2 Pavement Subgrade Geotechnical Design Manual – 2013 particularly if the moisture content is near or above optimum.5 and 3. Page 2-6 Ethiopian Roads Authority .1 Expansive soils Soils which exhibit significant volume changes in the presence of water are termed as expansive soils. Identification of these problematic subgrade soils is reviewed in the ERA Site Investigation Manual. in addition to suitable foundation materials. Swelling may also be due to chemical processes acting on certain non-clay minerals which result in the formation of new minerals of lesser density. Compaction also increases stiffness. Expansive soils generally owe their expansive character to their constituent clay minerals. Usually. expansive or swelling. and dispersive soils occur in many countries and in various regions of Ethiopia. Typical damage to roads on expansive soils includes longitudinal unevenness and bumpiness. then an adequate subgrade compaction is necessary for obtaining a high-quality travel surface. The nature of volume change beneath pavements in the vertical direction often takes the form of a general upward movement beginning shortly after the start of construction and continuing until an equilibrium subgrade moisture condition is achieved. Local expansion may also occur from poor drainage. Additional information can also be obtained in the ERA Pavement Design Manual. Cyclic expansion and contractions of subgrade soils usually occur at the perimeter of pavements and on shoulders exposed to rainfall and evaporation. cut sections may lead to local heaving due to removal of surcharge and subsequent increase of moisture content. decreases the sensitivity of the subgrade soil to changes in moisture content. For this reason.3. differential movement near culverts. The characteristics of these soils and the design alternatives to achieve adequate subgrades to support pavement structures are given below. the geology (soil genesis). These soils exhibit behaviour opposite to consolidation and compression. surcharge. Field measurements Indirect techniques. Alternatively. and poor drainage. Uses in pavements Swelling subgrade soils can have a seriously detrimental effect on pavement performance. The intrinsic factors are soil composition and thickness. remoulded. stabilized. Swelling soils must be identified so that they can be removed. surcharge load. diagenetic factors. compared with those composed of kaolinites and illites. initial dry density. If the soil structure is not confined such that swelling may occur laterally and vertically. These conditions are promoted by insufficient leaching of the soil by downward moving water. clay mineralogy. stress history (loading sequence).Geotechnical Design Manual . Soil composition and genesis. Montmorillonite is formed from the weathering of volcanic ash or primary silicate minerals such as feldspars. Long term physical and chemical weathering or alterations of clay soils as a result of changes in overburden conditions or groundwater environment are generally termed Ethiopian Roads Authority Page 2-7 . pyroxenes or amphiboles under those conditions which result in the retention of bases and silica. Laboratory determination Swell potential is measured using either the AASHTO T 258 or ASTM D 4546 test protocols. time allowed for swell. The swell test is typically performed in a consolidation apparatus. triaxial tests can be used to determine three dimensional swell characteristics. Clays containing montmorillonite expand highly. the potential for volume change is mainly determined by mineralogical composition of the clay content.2013 Chapter 2 Pavement Subgrade Table 2-3: Swelling characteristics of subgrade soils. variation in Intrinsic properties thickness. Comment The swell test can be performed on undisturbed. The volume changes exhibited by expansive soils are related to the interactions of various intrinsic and external factors. Often. and dry density. after the specimen is inundated. time. or compacted specimens. soil fabric. dry density. swell potential can be estimated using some soil physical and index properties. 2. depth below the ground surface. the presence of permeable layers. Modified from Montana DOT (2008) Description Swelling is a large change in soil volume induced by changes in moisture content. solution characteristics. vegetation. The swell potential is determined by observing the swell of a laterally-confined specimen when it is surcharged and flooded.1 Intrinsic properties Soil composition involves the type and amount of clay mineral within a soil and the size and specific surface area of these minerals. soil thickness. soil fabric and moisture content while the external factors include climate and time. or treated in the subgrade preparation and pavement design. seasonal moisture fluctuation. the height of the specimen is kept constant by adding loads. The vertical stress necessary to maintain zero volume change is the swelling pressure. Laboratory related variables which influence the measurement of volume change are initial moisture content. sample size and shape and temperature.3.1. External factors Climatic factors. The volume change of expansive soils is primarily due to the hydration of clay minerals. This particle interaction. the dry density of a material influences the soil fabric or inter-particle arrangement. however. and soil fabric. In general. the permeability is greater at low moisture contents and dry densities and decreases to some relatively constant value at about the optimum moisture content. just like differential settlement. This holds true for both disturbed and undisturbed materials. differential volume change. dry density. and desiccation cracks. double-layer water interaction. For example. The reason is that higher densities result in closer particle spacing. Pore fluids containing high concentrations of cations tend to reduce the magnitude of volume change of an expansive soil. Variations in thickness of the layer will result in variations of the magnitudes of volume change. therefore causing greater particle interaction. the differences in behaviour of expansive soils between the undisturbed and disturbed states are related to the presence and absence of diagenetic bonds. Another important influence of dry density on volume change is its interrelationships with some of the other intrinsic factors. the properties of the soil profile which influence volume change include the total layer thickness and its variation. The soil fabric refers to the orientation of the constituent particles. fractures. The arrangement of clay minerals influences the amount and to some degree the direction (lateral or vertical) of volume change exhibited by an expansive soil. The degree of hydration is influenced by the amount and type of ions present in pore fluids. or more precisely. The depth of desiccation is important to the magnitude and rate of volume change and can be defined as the depth to which evaporation influences are reflected in the soil profile (Fig Page 2-8 Ethiopian Roads Authority . In the case of expansive soils. provided that a source of moisture is available throughout the layer. Above optimum. permeability can normally be enhanced by fissures. Obviously. the permeability is essentially constant. the fabric consists of the arrangements of the plate-like clay minerals with each other and with the non-clay components. The obvious reason for this minimum permeability near the optimum moisture content and maximum dry density is that the voids available for moisture movement are at a minimum because of the close particle spacing. For compacted soils. The diagenetic factors are generally reflected in such phenomena as inter-particle bonding due to recrystallization of the contacts between clay minerals under high overburden stress conditions or by cementation of particles as a result of precipitation of cementing agents from the groundwater. The permeability of a soil is a function of the initial moisture content. depth below the ground surface. In many cases. The dry density is an important factor in determining the magnitude of volume change. and the presence of lenses and layers of more permeable materials. is a major problem to pavement structures. Permeability also plays an important role in the time rate of volume change. the thicker the layer of expansive soil. or more precisely. the greater is the total potential volume change. Apart from soil composition. Differential expansion. The phenomenon of volume change in expansive soils is also the direct result of the availability and variation in the quantity and chemical property of water in the soil.Chapter 2 Pavement Subgrade Geotechnical Design Manual – 2013 diagenetic factors. results in higher osmotic repulsive forces and a greater volume change. The swell or swelling pressure of an expansive soil increases with increasing dry density for constant moisture content. Ambient temperature conditions also influence the depth of seasonal variations.2 External factors Both natural (climate and time) and man-made (surcharge. Modified from BRAB (1978) 2.Geotechnical Design Manual . Seasonal moisture variations in the central part of Ethiopia and in some northern and western highlands have been reported to occur up to a depth of between 3 and 5m. Generally. long dry season followed by excessive rainfall. For instance. Seasonal moisture variations are relatively constant for given climatic conditions and identical soil profile since the general trend is toward accumulation or loss of total moisture content until the weather changes. surface drainage.e. The influence of time on volume change is another interrelated property which has its major impact on the rate at which expansion occurs.2013 Chapter 2 Pavement Subgrade 2-2). the larger depths of seasonal moisture change occur in areas where the seasonal climatic changes are greatest.3. As would be expected. In lowland and semi-arid areas with reduced sources of water. vegetation. etc) external factors play a role to determine the magnitude of volume change in expansive soils. i. the depth of seasonal moisture variation comprises some thickness of the surface material which is influenced by variations in climatic conditions. the depth of desiccation is high in warm climate with enough moisture in the soil. During rainfall and colder seasons. Figure 2-2: Graphical illustration of the depth of desiccation and moisture fluctuation. Changes in the overburden conditions and the proximity of the groundwater table have an important influence on the depth of desiccation. moisture accumulates closer to the surface and dissipates by evaporation when the climate is warm. The time to the first occurrence of volume change and the rate of expansion are functions of the permeability of the soil and Ethiopian Roads Authority Page 2-9 .1. variation is limited to a depth of 2m. The most widely used indicators for identification of expansive soils are the index properties that are routinely determined by most road agencies. Experience has shown that the volume change behaviour correlates reasonably well with liquid limit. For instance. index properties of soils. Generally. The first is an indirect technique in which one or more of the intrinsic properties are described or measured. there are three ways of identification and testing. The problem may be eliminated or reduced by locating the pavement on an embankment constructed with non-expansive materials. and shrinkage limit. It is a frequent problem associated with highways on expansive soils. Poor surface drainage leads to moisture accumulation or ponding which can provide a source of moisture for expansive subgrades. Vegetation with large root systems located in close proximity to pavements will result in differential moisture conditions and thus differential volume change. then some type of Page 2-10 Ethiopian Roads Authority . The obvious need for qualitative identification is to inform the design engineer of the potential for expansion and to generally classify this potential on the basis of probable severity. In most cases. It may be noted that confinement has its greatest influence on expansive soils in a stress-related sense. If the liquid limit is between 40 and 70. then the material is often highly expansive and may not be used for fills. plasticity index. This means the greater the confinement. and complemented with experience to determine potential volume change. The information may come from soil composition and genesis. Quantitative testing is necessary to obtain measurable properties for predicting or estimating the magnitude of volume change the soil will experience in order to ascertain approximate treatment or design alternatives. and soil classification systems. in road construction. Often. The application of surcharge to an expansive material reduces the amount of volume change that is likely to occur. the moisture that was being used by plants will tend to accumulate beneath the pavement structure and enhance the volume change. 2. and ensuring sufficient lateral and longitudinal gradients so that surface water is removed away from the road with minimum surface infiltration. a combination of observed Atterberg limits and prior experience with materials within a given area could be the main identification methods used for expansive soils. Expansion occurs as soon as moisture is made available and continues until an equilibrium condition is reached with regard to the source of water.Chapter 2 Pavement Subgrade Geotechnical Design Manual – 2013 the availability of water. Vegetation such as trees. the climate of the area. chemical and physical properties of parent material. the load applied by a pavement is often far less than that needed to control deformation.1.3 Identification and testing The identification and testing of expansive soils are discussed in the ERA Site Investigation Manual. In areas where vegetation is removed. the presence of a layer of non-expansive overburden may reduce the effect of the underlying expansive material. locating the side ditches as far away as possible from the edge of the road.3. and some grasses are conducive to moisture movement or depletion by transpiration. if the liquid limit is above 70. the greater the stress and the smaller would be the deformation. However. shrubs. The purpose is to qualitatively and quantitatively describe the volume change behaviour of soils. 2. In addition. stabilize. This map can be used as guide to locate expansive soils in the region. In practice. The second and direct technique involves actual measurement of volume change in a laboratory. The size of polygons is also an indicator. they are used only to assess the magnitude of volume change and to assist in the selection of appropriate treatment techniques. and its lateral or regional distribution. In nature. a measure of the activity (the ratio of the plasticity index to the clay fraction) is a good indication of the presence of smectites (expansive clay minerals). This technique provides swell or swelling pressure values. it is highly unlikely that in-situ subgrade soils would have a sufficient source of water for complete saturation. remove and replace the expansive soil with select borrow materials.1. However. it should also be noted that any clay-rich soil may exhibit polygonal cracking upon drying and it is important to distinguish this from an expansive soil. • • • For relatively thin (up to say 600 mm) layers of expansive clays near the ground surface.4 Treatment options When expansive soils are encountered in areas where significant moisture fluctuations in the subgrade are expected. the plasticity index is also useful in that if it is below 15. If the plasticity index is greater than 35.Geotechnical Design Manual . however.3. The surfaces exhibit polygonal shrinkage cracks which reflect the percentage of clay and possibly the presence of expandable clay minerals. the determination of the extent of volume change from indirect techniques and performing laboratory tests only on selected samples is often the most cost effective and faster way of dealing with expansive soils. in that small polygons are often the result of a high clay content. increase the roadway camber to reduce the possibility of surface infiltration. The problem is that the laboratory tests are conservative because of the method by which water is made available to specimens. Moreover. both the swell and swelling pressure values are rarely taken directly into account in the design of pavements. Examining these and other fissures in the field and considering the regional distribution of expansive soils from other sources could help the pavement design engineer to estimate the magnitude of volume change. The third approach involves data from indirect and direct techniques that are correlated either directly or by statistical means to determine the comparative severity of the degree of expansion. Instead. The amount of heave of expansive soils can also be reduced if compacted to low Ethiopian Roads Authority Page 2-11 . Scarify. In addition to the use of Atterberg limits to determine swell behaviour. The swell defines deformation while the swelling pressure is related to stress generated by the volume change. consideration should be given to the following measures to minimize future volume changes and damage to the pavement structure.2013 Chapter 2 Pavement Subgrade treatment may be necessary to avoid distress. Extend the width of the bottom of the pavement layers on both sides of the road to reduce the change in subgrade moisture at the edges of the pavement. In pavement design. then minimal problems are anticipated. and re-compact the upper portion of the expansive clay subgrade. Since subgrade loading conditions are minimal. the material must be treated to minimize the problem or it should be discarded. its probable severity. For instance. usually it is the swell rather than the swelling pressure which is used by many road agencies. an engineering soil map is sometimes prepared during site investigation where pedological soil groups along route corridors are described on the basis of index properties. Many laboratories use the odometer swell test with a minimal surcharge to determine volume change. it is known that the appearance of highly expansive soils such as Black Cotton soils is distinct after desiccation. Similarly. Pre-wetting the subgrade is one way of reducing the effect of swelling soils. and reduce the potential for swell.g. volume change should be reduced. lime may be detrimental in soils containing sulphates. Page 2-12 Ethiopian Roads Authority .Chapter 2 Pavement Subgrade • • • • • • • Geotechnical Design Manual – 2013 densities at moisture contents wet of optimum. Potassium solutions injected into expansive soils can cause a base exchange. Sealed pavements (e. Constructing the pavement during the time of the year when the moisture content of subgrade soils is close to the anticipated equilibrium value may reduce further moisture movement and hence expansion. In this context. The time needed for ponding and the depth of moisture penetration in the subgrade is determined based on the characteristics of the soils and their swelling potential. if the soils are compacted below optimum. Since the change in moisture content is the main factor influencing the volume change of swelling soils. compared to gravel roads where surface water infiltration is impossible to prevent. The objective of pre-wetting is to allow desiccated swelling soils to reach equilibrium prior to placement of the pavement. However. Although pavement loads are generally insufficient. Loading expansive soils with a stress greater than the swelling pressure is also a way of preventing swelling. completing the excavation to the design elevation. but they may fail to satisfy specified density requirements. waterproof asphaltic membranes are sometimes used in different forms to limit the surface penetration of water. the strength of this material could also significantly reduce. increase the soil permeability. they may exhibit excellent immediate stability. pre-loading or placing 1. it is obvious that if the soil is isolated from water. The placement of geotextile between the subgrade and the pavement layers (subbase) to help distribute differential movements associated with heave more evenly. Fissures should exist in situ to promote the penetration of lime. vertical moisture barriers placed adjacent to pavements or around the perimeter of foundations down to the maximum depth of moisture changes may also be an effective method in maintaining uniform soil moisture. Lime injected or mixed into expansive soil can reduce the potential for heave. With pre-loading. In addition. In areas with deep cuts in over-consolidated expansive clay soils. swelling tends to be more uniform. Lime or cement stabilization are accepted methods for this purpose. The most commonly applied method for accelerating swelling by this technique is ponding. Partial encapsulation along the edge of the pavement or full encapsulation can also be used to reduce change in subgrade moisture. asphalt surfaced) will always be more successful in maintaining a more constant moisture content in the underlying subgrade soils. and allowing the subsurface soils to rebound prior to placing the pavement layers may also reduce further expansion. Chemical stabilization has been used for altering the clay structure to prevent or minimize the swelling of expansive clays. In addition. However. Encapsulation involves maintaining the moisture content at desired constant level by wrapping the subgrade soils in waterproof membranes. Upon saturation. using relatively thick bases and sub-bases (or capping layers) may also be useful to reduce the effect of swelling to a certain extent.0m or more of permanent compacted fill on the existing ground surface prior to pavement construction will reduce the negative (suction) pore-water pressure and thereby decrease the potential for swell. Others. Often. Collapsible soils Collapsible soils are those that appear to be strong and stable in their natural (dry) state. All soils compress when load is applied. This movement is largely unrecoverable. A short description of collapsible soils is given in Table 2-4. After construction. potentially creating further problems. If soils are non-plastic.3 Remove and process soil to attain the approximate optimum moisture content.3. When dry or at low moisture content. Some. When existing subgrade soils do not meet minimum design requirements and are susceptible to large settlements over time. which can occur beneath pavement structures. If compressible soils are not treated. Consolidate deep deposits of very weak saturated soils with large fills (surcharge) prior to construction. connection to storm drainage pipes or roadside ditches may be required. If the soil is extremely wet or saturated and relatively permeable. consider dewatering using well points or deep horizontal drains. which is reflected as settlement or deformation on the pavement surface.3. the following treatment options should be considered: • • • • • 2. generating large and often unexpected settlements.2 Chapter 2 Pavement Subgrade Compressible soils Compressible soils are soil deposits which are susceptible to large settlements and deformations because of a relatively rapid decrease in void volume upon loading. large surface depressions with random cracking can develop. such as those containing large amounts of silt and clay. Consider wick drains to accelerate consolidation. such as gravels and sands.Geotechnical Design Manual . compress very little. compress significantly. Organic and peaty soils are also prone to compression. Ethiopian Roads Authority Page 2-13 . water molecules in silt or clays flow out of the soil mass. The condition for collapse is that the soil mass must be in a partially saturated condition and then wetted up and loaded simultaneously. When a load is applied. collapsible soils give the appearance of a stable deposit. the loose structure of these soils is held together by small amounts of clay minerals or calcium carbonate (Figure 2-3). the overall volume of the soil mass decreases. If horizontal drains cannot be daylighted. The introduction of water dissolves the bonds created by these cementing materials and allows the soil to take a denser packing under any type of compressive loading. Compressible soils usually have very low density.2013 2. When a road is to be built in areas with thick deposits of highly compressible soils. The surface depressions can allow water to pond on the road surface and readily infiltrate the pavement structure. consider dynamic compaction of the soils from the surface to increase the dry density. They are usually found in low lying areas that are prone to flooding. the soil grains move closer. the fills can either be left in-place or removed. Remove and replace subgrade soil with suitable compacted borrow or select embankment materials. and replace and compact. As this happens. specific index properties must be examined to estimate settlement. As the water flows out. depending on the final elevation. but which rapidly consolidate under wetting. loess deposits are common in the southern part of the Omo River valley. accumulating sandy and silty soils. causing them to become difficult to manage and susceptible to flow failure. Loess. residual soils formed as a result of the removal of organics by decomposition or the leaching of certain minerals (calcium carbonate). Other types of collapsible deposits include alluvial soils formed in flood plains and semi-arid lowlands. often found in Ethiopia. Collapsible soils are found in areas where there are loess deposits (windblown silts).Chapter 2 Pavement Subgrade Geotechnical Design Manual – 2013 Figure 2-3: Mechanism of soil collapse in natural soils. can stand on nearly vertical slopes until saturated. These soils can pose significant problems if present in the subgrade or used as fill. In Ethiopia. and are easily crushed during compaction. typically have low density and high porosity. unlike other non-cohesive soils. Page 2-14 Ethiopian Roads Authority . Volcanic ashes (andosols with allophane clay minerals). Collapsible soils may also be suspected in undeveloped areas that have young. a low unit weight. and residual soils formed by weathering of volcanic rocks such as welded tuff and ash. and a high void ratio. It has a low relative density. as their structure is lost through compaction and they can have very high water contents. especially if the difference in levels is high after tamping and levelling. If a pit is excavated. the actual quantification is not that important. The severity of collapse depends on the extent of wetting. compacted. The collapse potential of suspected soils is determined by placing an undisturbed. such rutting and unevenness is more likely to be due to other factors such as poor compaction or the presence of compressible soils. depth of the deposit and loading from the overburden weight and structure. The collapse during wetting occurs due to the destruction of clay binding. where the heave is restricted by the overlying load.Geotechnical Design Manual . or remoulded specimen in an oedometer. Among the common artificial sources of wetting in the surroundings of pavements are: (a) irrigation of agricultural lands. and (c) seepages from ponded areas such as retention basins. Indirect techniques such as excavating and refilling of pits and measuring the depth below the original ground surface. Unlike expansive soils. Laboratory determination Field measurement Comment Collapse potential is measured using the ASTM D 5333 test protocol. collapsible soils can be subject to collapse over their entire thickness (Paige-Green 2008).3. then filling the hole with the same amount of material can indicate whether the soils are collapsible. the potential degree of collapse is best determined using oedometer tests (various tests including the double oedometer. The subsurface conditions are then evaluated by ground investigation. 2. the result of collapse of the subgrade is mostly manifested by the development of a deeply rutted and often uneven road surface and significant deterioration of the riding quality of the road with or without cracking (Page Green 2008).1 Identification The potential for collapse at a specific location is initially evaluated based on the geological and environmental setting.3. which provides the original strength of these soils. (b) leakage from unlined drains. These may be supplemented by boreholes that can extend to depths sufficient to define the thickness of the collapsible soil. On pavements. resulting in a collapse of the soil skeleton and large decreases in soil volume. The primary objective is to recognize the problem and implement measures to disrupt the collapse structure as far as Ethiopian Roads Authority Page 2-15 . However. For subgrades under roads. long term infiltration can lead to soil wetting at depth below the surface which in extreme circumstances can be quite serious and can lead to significant settlements and ground cracking. A load is applied and the soil is saturated to measure the magnitude of the vertical displacement. Remoulding and compacting may also destroy the original structure. In the laboratory. Uses in pavements Collapsible subgrade soils can have a seriously detrimental effect on pavement performance.2013 Chapter 2 Pavement Subgrade Table 2-4: Collapsible soils. From US DOT FHWA (2006) Description Collapsible soils exhibit large decreases in strength at moisture contents approaching saturation. Sustained. single oedometer and collapse potential test are used) in which the collapse potential under specific loads is determined. trial pits and open excavations. Minor artificial and natural wetting is often confined to the top layers beneath the ground surface. for example using power auger borings. Collapsible soils must be identified so that they can be removed or stabilized. Collapse potential in the order of 1% is considered to be mild. Compaction of subgrades using conventional compaction plant has been shown to be only moderately effective in removing the collapse potential to any significant depth. Clearly. however. more than 60% of the mass of collapsible material lies in the 0. while that above 10% is considered to be severe. Figure 2-4: Typical results of a one dimensional oedometer test. so that differential settlement is minimized (Page-Green 2008).Chapter 2 Pavement Subgrade Geotechnical Design Manual – 2013 possible and to produce a subgrade that is relatively uniform. If this work is done in the wet season. the more collapsible the soil is considered to be. with or without the addition of water. special remedial measures may be required to prevent large-scale cracking and differential settlement. Typical collapsible soils have densities of less than 1600 kg/m3 (mostly in the range of 1000 to 1585 kg/m3). Modified from Univ Iowa (2013) Usually. gradation.3. a more economical and effective result is obtained as a result of the improved lubrication offered by water (Page-Green 2008). The most obvious remedial measure is to preclude the presence of water. In addition. An example of a typical stress-strain curve obtained by a one dimensional oedometer test is shown in Fig. have proved most effective in reducing the collapse potential to a significant depth. However. and the extent of loading at the time of wetting.4. the collapse potential of the soil depends on density. impractical. The collapse potential is defined as the change in void ratio (∆e) upon wetting compared to eo + 1. the initial water content. the larger the collapse potential. For a given initial water content. the collapse potential decreases with increase in the energy of compaction. In addition. even after the addition of water. modern high energy compaction techniques using large impact rollers.075 mm (Page-Green 2008). Page 2-16 Ethiopian Roads Authority . Settlement estimates are generally made by taking the collapse potential over the potential depth of wetting. high collapse potentials are recorded for low initial moisture contents.075 to 2 mm range and less than 20% is finer than 0. A good indication of the potential for high collapse potential is a very low density. 2.3. 2. composition. This is.2 Treatment options If pavements are to be constructed over collapsible soils. Geotechnical Design Manual - 2013 Chapter 2 Pavement Subgrade In addition, various ground modification methods can be used to prevent or limit collapse from occurring, or cause the collapse to occur before construction. These methods include partial removal and replacement; densification of the collapsible soil in-place such as by compaction grouting; and pre-wetting of the collapsible soil followed by surcharge loading to cause settlement before construction; conventional compaction with heavy vibratory roller for shallow depths (within 0.3 to 0.6 m); and dynamic or vibratory compaction for deeper compressible soils of more than 0.5 m (combined with inundation with water). More options will generally be available to new construction compared to existing pavements where there are constraints to mitigation options. For depths of collapsible soils greater than 1.5 m, lime pressure and sodium silicate injections could also be helpful, though expensive. 2.3.4 Dispersive soils Dispersive soils are those soils that, when placed in water, have repulsive forces between the clay particles that exceed the attractive forces. This results in the colloidal fraction going into suspension and, in still water, staying in suspension. In moving water, the dispersed particles are carried away or eroded. This obviously has serious implications in earth dam engineering, but it is of less consequence in road (Page-Green 2008) construction, especially when compared to the effects of expansive and collapsible soils. In some places, however, the inclusion of dispersive soils in the subgrade or fill can lead to significant pavement failures through piping, tunnelling and the formation of cavities. It is, therefore, important to identify dispersive soils prior to design (Page-Green 2008). Dispersive soils have not been definitively associated with any specific geological origin but predominantly have a high sodium cation content. They occur mostly as alluvial clays in the form of slope wash, lake bed sediments, loess deposits, and flood plain silts and clays. Early studies indicated that dispersive clays were associated only with soils formed in arid or semi-arid climates and in areas of alkaline soils. Recently, however, the same soils have been found in humid climates and in various geographic locations. In areas of sloping topography where dispersive soils exist, a characteristic pattern of surface erosion is evidenced by jagged, sinuous ridges and deep rapidly forming channels and tunnels. In gently rolling or flat areas there is frequently no surface evidence of dispersive clay, due to an overlying protective layer of silty sand or topsoil from which the dispersive soil particles have been removed. An absence of surface erosion patterns typical of dispersive soils does not necessarily indicate that dispersive soils are not present. Dispersive, slaking and erodible soils are similar in their field appearance (highly eroded, gullied and channelled exposures), but differ significantly in the mechanisms of their actions. Unlike dispersive soils, slaking soils disintegrate in water to silt, sand and gravel sized particles without going into dispersion. The cause of slaking is probably a combination of swelling of clay particles, the generation of high pore air pressures as water is drawn into the voids in the material, and softening of any cementation. Purely erodible soils will not necessarily disintegrate or go into dispersion in water. They tend to lose material as a result of the frictional drag of water flowing over the material exceeding the cohesive forces holding the material together (Page-Green 2008). It is not very important (nor even really possible) to quantify the actual potential effect of dispersive material, as the process is time related and, given enough time, all of the colloidal material could theoretically be dispersed and removed, leading to piping, internal erosion and eventually loss of material on a large scale. However, most studies reported in Ethiopian Roads Authority Page 2-17 Chapter 2 Pavement Subgrade Geotechnical Design Manual – 2013 the literature have shown that failures of pavements built on dispersive clay soils occurred on first wetting. Further damage was associated with the presence of water and cracking by shrinkage, differential settlement, or maintenance deficiencies. Hence it is important to identify the presence of dispersive soils so that the necessary precautions can be taken in an appropriate time. 2.3.4.1 Identification Identification of dispersive soils should start with field investigations to determine if there are any surface indications such as unusual erosional patterns with sub-soil pipes and deep gullies, concurrent with excessive turbidity in any nearby bodies of water. Areas of poor crop production and stunted vegetation may indicate highly saline or carbonate-rich soils, many of which are dispersive. However, dispersive soils can also occur in neutral or acidic soils and can support lush grass growth. Although surface evidence can give a strong indication of dispersive soils, lack of such evidence does not preclude the presence of dispersive clay at depth and further investigation should be undertaken. In many cases, dispersive clays are more related to pedogenic processes or depositional environments and are better identified on soil or agricultural maps if available at the required scale. Figure 2-5: Dispersive potential determined based on percentage sodium and total dissolved solids. From US DOI Bureau of the Interior (1991) Dispersive clays cannot be identified by the standard index tests such as visual classification, grain size analysis, specific gravity, or from Atterberg limits and therefore other laboratory tests have been devised for this purpose. The five laboratory tests most generally performed to identify dispersive clays are the crumb test, the double hydrometer test, the pinhole test, the dissolved salts in the pore water test, and exchangeable sodium percentage (ESP) based tests. Determination of the exchangeable sodium percentage (ESP) has been suggested as the best test, and has been implemented widely, with the addition of results from the pinhole test. Figure 2.3 presents a method of classifying dispersive soils using percent sodium and total dissolved solids (TDS). However, as discussed in the ERA Site Investigation Manual, the crumb test on undisturbed samples is the best first indication for pavement design purposes. Dispersive soils tend to produce a colloidal suspension or cloudiness over the crumb during the test, Page 2-18 Ethiopian Roads Authority Geotechnical Design Manual - 2013 Chapter 2 Pavement Subgrade without the material necessarily disintegrating fully. Disintegration of the crumb in slaking soils is very rapid and forms a heap of silt, sand and gravel. Erodible soils do not necessarily disintegrate in the crumb test as they require a frictional force of moving water to loosen the surface material. Clay soils should be routinely tested for dispersive characteristics during design studies for pavement and hydraulic structures where the clay may be subjected to potential erosion and piping. 2.3.4.2 Treatment options The remedial measures for avoiding dispersive soil damage to road pavements are summarized in Table 2-5 and range from the relatively simple to the more demanding and costly. Precautions include, but are not limited to proper moisture and density control, use of filters and filter drains, selective placement of materials, use of sand-gravel blankets or lime-modified soil on slopes, and chemical treatments of dispersive clays. Avoiding the use of dispersive soils in fills as far as possible and removing and replacing them in the subgrade is the preferred solution. It is important to manage water flows and drainage in the area well. Since the presence of sodium as an exchangeable cation in clays is a major problem, treatment with lime or gypsum will allow the calcium cations to replace sodium and reduce the potential for dispersion. It is also important that the material is compacted at 2 to 3% above optimum moisture content to as high a density as possible (Page Green 2008). Table 2-5: A summary of remedial measures to reduce the effect of dispersive soils. From Soil and water Management (2008) 1. Minimize disturbance to topsoil and vegetation. 2. Choose construction methods that minimize the need for excavation and subsoil 3. 4. 5. 6. 7. exposure. Avoid concentrating water flow over areas that have dispersive topsoil or sub-soils. If possible divert water to areas where the soil is not dispersive. Immediately infill any trenches or holes to prevent collection and ponding of water on subsoil surfaces. Always compact dispersive sub-soils that have been disturbed or excavated. Dispersive soils require above average compaction. Careful control of compaction and water content is important during construction. Top dress the surface of potentially dispersive soils with up to 2% gypsum (if soil pH > 6.5) or up to 4% lime by dry mass of soil (if soil pH <5) or a mixture of both (if soil pH is within the range of 5 to 6.5). Cover dispersive soils with a minimum 100 mm layer of non-dispersive soil prior to revegetation, or the placement of pavement layers. 2.4 Subgrade Treatment Proper treatment of subgrade soils and the preparation of the foundation are important to ensure a long-lasting pavement that does not require excessive maintenance. The subgrade should be treated to form a construction pad or a long-term subsurface layer capable of carrying pavement applied loads. Subgrade soils can be treated using a variety of methods or a combination of them. Techniques that can be used to improve the strength and stiffness of the subgrade and increase pavement performance include moisture control removal and replacement, soil stabilization (modification), and the use of geosynthetics. Ethiopian Roads Authority Page 2-19 may infiltrate through the surface. Capillary action and moisture-vapour movement are also important. Quickly remove moisture that enters the pavement. A major issue in geotechnical design of pavements is especially to prevent the subgrade from becoming saturated or even exposed to constant high moisture levels. These drains intercept the lateral flow of subsurface water beneath the pavement structure. and are covered in the ERA Pavement Design Manual. side slopes and cut sections. Moisture control is therefore often an essential part of pavement design. dispersive soils. The first two approaches involve the upper pavement layers.4.1 Geotechnical Design Manual – 2013 Moisture control It is known that excess moisture has a damaging effect on pavement structures. or could flow laterally from the pavement edges and shoulder ditches. The last approach (removal of moisture) normally needs a subsurface investigation and groundwater study to design the most appropriate drainage system. Moisture. embankments. Deep subsurface drains are usually installed to reduce groundwater levels. This also applies to expansive and to a lesser extent. knowledge of groundwater and its movement are critical to the performance of the pavement as well as stability of the subgrade. It is necessary to drain the ground properly and to allow exposed soils of the subgrade to dry out. Various types of longitudinal roadside drains placed in trenches beneath shoulders at shallower depths can also be used to handle water infiltrating the pavement from above and the sides. Modified from US DOT FHWA (2006) Hence. Page 2-20 Ethiopian Roads Authority . and remove the water that infiltrates the pavement surface. It may seep downward from a higher ground. moisture can enter the pavement from a variety of sources. The movement of vapour is associated with fluctuating temperatures and other climatic conditions. The three main approaches for controlling or reducing the problems caused by moisture on pavements are: • • • Prevent moisture from entering the pavement. Capillary effects are the result of surface tension and the attraction between water and soil.Chapter 2 Pavement Subgrade 2. can have a profound negative effect on both material properties of the subgrade and the overall performance of the pavement. As shown in Figure 2-6. Use materials and design features that are insensitive to the effects of moisture. Groundwater can be especially problematic for pavement subgrades in low-lying areas where inundation is common. in combination with other factors. Figure 2-6: Sources of moisture in pavements. but may not significantly reduce the moisture content of fine grained soils in the subgrade. the use of crushed materials is unlikely to be economically feasible. Reduce subgrade rutting potential of flexible pavements. the selection of appropriate material for replacing poor subgrade soils is a critical issue. In general. including the California Bearing Ratio (CBR) and resilient modulus (MR).2013 Chapter 2 Pavement Subgrade In some cases.5 – 1. Subgrade improvement is often the preferred way of dealing with weak and poorly drained soils compared to increasing the pavement layer thicknesses. Drying should then be accomplished through evaporation of soil moisture at the time of construction. SW and SP) soils. depending upon their intended use.2 Removal and replacement Removal of naturally occurring soil and replacing it with a suitable material is the most obvious method of eliminating many of the subgrade problems.5 m of embankment Ethiopian Roads Authority Page 2-21 . Provide uniform subgrade support over sections with highly variable soil conditions. and are highly dependent on weather and environmental conditions. good quality borrow material. These methods are generally effective only in the top 200 to 300 mm of the subgrade. The thicknesses of granular layers vary. A thick mixed gravel/sand/silt layer may be used as an alternative to soil stabilization for subgrade improvement in areas with large quantities of readily accessible. GP. The type of granular material used is normally a function of material availability and cost. corresponding to AASHTO A-1 and A2 (GW. by reducing the size of soil lumps. Although the high shear strength of crushed stone may be more desirable. combination of granular layer over natural soil). These are typically sand or granular materials with or without limited fines. fine-grained subgrades. Some road agencies use granular materials to replace unsuitable subgrade soils for structural and drainage reasons. Usually. materials with CBR values of 20% or greater which corresponds to MR of approximately 120 MPa can be used (US DOT FHWA (2006A). 2. The objectives and benefits of thick mixed gravel/sand/silt layers for subgrade improvement are to: • • • • • Increase the supporting capacity of weak. To increase the composite subgrade design values (i. Provide a minimum bearing capacity for the design and construction of pavements. and the excavation and replacement solution is extended only to a depth which will reduce the subgrade problem to a tolerable minimum. this is generally not the case. In some cases this approach may be economical if the thickness of the layer to be removed is less than about 1 to 2 m and suitable replacement material is available. subsurface drainage may remove water.4. it is usually necessary to place a minimum of 0. Unfortunately. Hence the required depth of excavation depends upon the suitability of the subgrade soil and the anticipated characteristics of fill that is available nearby. thereby increasing the surface area exposed to evaporation. Several field and laboratory methods are used to characterize the strength and stiffness of granular materials. Often a granular layer is used to provide uniformity and support as a construction platform.Geotechnical Design Manual . Pit-run gravel/sand/silt is the most common. Reduce the seasonal effects of moisture and temperature on subgrade support.e. Disking and tilling of the soil accelerates the drying process. Deflection devices. 2. Most commonly. This means that. Soil stabilization is usually performed for the following reasons: • As a construction platform to dry very wet soils and facilitate compaction of the upper layer. can be used for testing the compacted embankment layer and the constructed pavement surface. before the composite subgrade reaction begins to resemble that of the granular layer. the pavement will be under-designed. For this case. dependent upon the relative difference in moduli. as well as the completed pavement section. Pavement design requires a single subgrade design value. when loaded under the completed pavement section. the modified soil is usually given some structural value in the pavement design process. For this case. it is recommended to characterize the individual material properties by traditional means. the composite reaction of the embankment and soil combination can vary from that of the natural soil to that of the granular layer. The placement of a granular layer over a comparatively weak underlying soil forms a nonhomogeneous subgrade in the vertical direction. and to compare these results to field tests performed over the constructed embankment layers. and the thicknesses of the granular layer. In some instances. The process is often called soil modification when the purpose is to change the physical properties and thereby improve the quality of the subgrade soil. the use of a thick granular layer can allow the dimension of the overlying layers to be reduced without compromising the strength and serviceability of the entire pavement structure. for example CBR or MR. Experience has shown that a good-quality embankment layer must be as high as 1 m or more. To account for non-homogenous subgrades in pavement structural design. The actual composite subgrade response is not known until the embankment (granular) layer is placed in the field. Page 2-22 Ethiopian Roads Authority . It is advisable to use caution when selecting a design subgrade value for a nonhomogenous subgrade.3 Soil stabilization Soil stabilization is a general term that involves the use of mechanical or chemical modifiers to enhance the strength of soils and reduce the change in moisture. • To strengthen a weak soil and restrict the volume change potential of a highly plastic (expansive) or compressible soil. In the case of a non-homogeneous subgrade.Chapter 2 Pavement Subgrade Geotechnical Design Manual – 2013 material. depending on the strength of the granular material relative to that of the underlying soil. or exhibits similar properties. for granular layers of up to 1 m in height. If too high a subgrade design value is selected. the stabilized soil is usually not considered as a structural layer in the pavement design process. This is generally determined through laboratory or field tests.4. Structural design charts found in the ERA Flexible Pavement Design Manual will usually provide the necessary guidance. such as resilient modulus or CBR testing. when the soil mass in the zone of influence of vehicle loads is of the same type. and it may also be different once the upper pavement layers are placed. the composite reaction is a value somewhere between the two extremes. Granular layers less than 0. the composite reaction can be much less than that of the embankment layer itself.5 m thick have minimal impact on the composite subgrade reaction. including the Falling Weight Deflectometer (FWD). Some road agencies use in-situ plate load tests to verify that a minimum composite subgrade modulus has been achieved. Geotechnical Design Manual - 2013 Chapter 2 Pavement Subgrade For further information on soil stabilization, including the use of geotextiles and geogrids, see Appendix A. Ethiopian Roads Authority Page 2-23 Geotechnical Design Manual - 2013 3 Chapter 3 Road Embankments ROAD EMBANKMENTS In road construction, embankment design is treated separately from the sub-grade primarily because embankments are usually constructed of engineered fill imported from other locations. The engineered fill is normally compacted as it is placed. Compaction of the fill is monitored to confirm that it is constructed in accordance with the specification. 3.1 Types of Embankments A road embankment may be constructed from either rock-fill or earth-fill. Fills that comprise both coarse and fine grained materials and are known as earth-rock (composite) embankments. 3.1.1 Rock fill embankments In most cases, rock fill embankments contain more than 75% by volume of large fragments, 100 mm or greater in size. When these materials are placed and compacted, the embankment produced as a result primarily derives its stability from the mechanical interlock of coarser particles. Since standard laboratory compaction tests on coarse materials are of doubtful accuracy, a given amount of compactive effort is normally specified for rock-fill embankments. Special consideration should be given to the type of material that will be used in rock-fill embankments. In some areas, moderately weathered or very soft rocks may be encountered in cuts and used as embankment fills. The use of these materials can result in significant long term settlement and stability problems as the rock degrades. Such rocks should be checked by a slake durability test. If the rock is found to be non-durable, it should be physically broken down and compacted as earth embankment provided the material meets the specification for earth-fill. Shales and mudstones found in sedimentary sequences in northern and eastern Ethiopia are good examples of low durability rocks and special compaction techniques are required if they are to be used as fill. As with all rock-fill, any oversize materials should be removed. Generally, in order to maintain the long-term stability of a rock-fill embankment, 90% of the rock fragments with dimensions greater than 100 mm should have a Point Load strength of 2.0 MPa or greater. In addition, the maximum size of the coarse particles and lift thickness should be specified, as they may vary in accordance with the hardness of rocks in the region. However, in many instances, both the particle size and lift thicknesses are limited to 300 mm. The advantage of rock fills compared with earth embankments is that they can be constructed to steeper side slopes. As a consequence the fill volume is lower than that for a similar height earth embankment, particularly on sloping ground and they can therefore be constructed more quickly. Their disadvantage is that their unit volume is usually more costly than earth, particularly if the rock is not available locally within the project area. Ethiopian Roads Authority Page 3-1 The DCP information can be used to identify locations for sampling and the occurrence of cohesionless layers that could increase the rate of consolidation. including peat. If the project area is in a seismically active zone then the liquefaction potential should be considered. long term total and differential settlements affecting the serviceability of the road. the compressibility index and the coefficient of consolidation.Chapter 3 Road Embankments 3. Page 3-2 Ethiopian Roads Authority . These design issues will usually require the collection of undisturbed soil samples for laboratory strength and consolidation testing.3 Embankments on soft ground If soils underlying an embankment are predominately cohesive. This leads to the possibility that an embankment will deform as the foundation fails under the weight of the embankment and the possibility of progressive failure because of stress–strain incompatibility between the embankment and its foundation (Figure 3-1). Compressibility and shear strength can also be useful measures to determine the stability of earth embankments. peak strengths of the embankment and the foundation soils do not mobilize simultaneously. Embankment fills over soft ground are frequently stronger and stiffer than their foundations. Because of this problem. side-slope stability during construction. This is because most cohesionless soils exhibit good bearing capacity and low compressibility. Grain-size distribution data are also needed.e. Consolidation tests can be conducted to define the preconsolidation pressure. Embankments sometimes have to be built on weak foundation materials or soft ground. 3. such as soft clays and silts and organic materials.1. Embankments on soft ground also have a tendency to spread laterally because of horizontal earth pressures acting within the embankment. then the primary design issues will be bearing capacity. These can result in significant settlement of the embankment during construction which in turn will lead to an increase in the required fill quantity. Settlement or piping may occur as a result. In addition. The vane shear test can provide valuable in-situ strength data. stability analyses performed using peak strengths of soils would overestimate the factor of safety. and long-term settlement. embankments on soft soils are ideally designed so that the increase in stress is relatively small. drainage is an important issue to prevent the loss of shear strength due to saturation. particularly in soft clays. It will usually be necessary to perform triaxial compression tests in the laboratory to determine undrained strengths as well as total stress and effective stress parameters. shallow embankments. These earth pressures cause horizontal shear stresses at the base of the embankment that must be resisted by the shear strengths of foundation soils. It may also be desirable to collect in situ vane shear strength data and conduct DCP tests. Hence the compaction properties of the soil materials (optimum water content and maximum dry density) are very important to the long-term performance of the embankments. i. Often.2 Geotechnical Design Manual – 2013 Earth-fill embankments Earth-fill embankments are typically built by compacting earthen materials such as natural soils.1. Soft soils do not have adequate shear strength and failure may occur as a result. it is necessary to determine the level of groundwater table and its fluctuation characteristics. Cohesionless soils underlying an embankment are not usually a major geotechnical design concern for static loading. Settlements will generally be small and will occur rapidly during placement of the fill. and instability of the embankment. irrespective of the stability of the embankment. Hence. In this case. Sometimes trial embankments are constructed at the beginning of the construction process to check assumptions made during the design. Embankments provide adequate support for roadways if the additional stress from traffic loads and pavement structures does not exceed the shear strength of the soils. any anticipated settlement problem must also be considered. From Abramson et al (2002) Where embankments are required to be built over soft ground. These conditions are most critical when soft cohesive soils are present below the embankment. Attention should be given to the internal stability of the entire embankment foundation interface rather than the embankment or the foundation soils alone. The key geotechnical issues are stability and settlement characteristics of the foundation soils and the bearing capacity at the base. or excessive settlement. 3. The impact of these considerations on stages of construction is also an issue that should be considered during design. the level of groundwater should be determined and its fluctuation monitored. Overstressing the embankment may result in slope failure.Geotechnical Design Manual . This settlement can be short term created as a result of the addition of increased load on the soil beneath the embankment (bearing failure during placement of the fill) or long-term due to consolidation. Some very soft and highly organic materials need to be tested in situ with vane shear apparatus. stability and settlement of the fill should be carefully evaluated. Consideration should be given to removing the soft underlying layer and replacing it with free-draining material prior to the placement of the embankment. Ethiopian Roads Authority Page 3-3 .2 Design Considerations Table 3-1 provides a summary of the engineering properties and field and laboratory tests needed for the design of embankments. The primary design issue is whether the existing foundation soil can support the new embankment loads without undergoing bearing failure. In addition to issues related to side slope stability.2013 Chapter 3 Road Embankments Figure 3-1: Example of stress-strain incompatibility. In addition. as retrieving undisturbed samples from these soils for laboratory testing is difficult. Staged construction may need to be considered whereby the embankment is constructed in a number of stages and excess pore water pressures allowed to dissipate over a period of time before the next lift is constructed. pore-water pressures can redistribute after an earthquake leading to liquefaction-related failures that may occur several minutes after the main earthquake event. etc.) Constructability • • • • • • • • • (soil. however. Consolidation settlement and secondary compression can continue for many years and. Page 3-4 Ethiopian Roads Authority . The long-term stability of embankment slopes should be analysed especially in fine grained soils. as well as differential settlement of the pavement at cut and fill transitions and bridge approaches. depending on the thickness of soils. the amount of settlement can be very high. The duration of loading during a seismic event is usually short. Significant settlement can result in distress to the pavement at the top of the embankment. This load is usually a rare occurrence but may be very important in areas like the rift valley of Ethiopia where earthquakes are much more common. Another load that may occur in embankments is seismic loading. side slope failures and post-liquefaction settlement. This loading occurs after the embankment is constructed to the final grade and excess pore-water pressures have dissipated.Chapter 3 Road Embankments Geotechnical Design Manual – 2013 Table 3-1: Engineering properties and field and laboratory tests for embankment design. rock) Compressibility parameters Shear strength parameters Unit weights Time-rate consolidation parameters Horizontal earth pressure coefficients Interface friction parameters Pullout resistance Geologic mapping including orientation and characteristics of rock discontinuities Shrink/swell and degradation of soils Field testing • • • • • • • • • • • • • Laboratory testing • One dimensional Nuclear density Plate load test Test fill CPT (water content and pore pressure measurement) SPT PMT Dilatometer Vane shear Rock coring (RQD) Geophysical testing Piezometers Settlement plates Slope inclinometers Oedometer • Triaxial tests • Unconfined • • • • • • • • • • • • compression Direct shear tests Grain size distribution Atterberg limits Specific gravity Organic content Moisture-density relationship Hydraulic conductivity Geosynthetic soil testing Shrink/swell Slake durability Unit weight Relative density The second type of load is the long-term operational loading. ground water. Liquefaction could lead to bearing failures. potential settlement will be a design consideration. From Washington State DOT (2013) Engineering parameters Required information for analyses • Settlement • Subsurface profile • • • • • • • • • • (magnitude and rate) Bearing capacity Slope stability Lateral pressure Internal stability Borrow source evaluation (available quantity and quality of borrow soil) Required reinforcement Liquefaction Delineation of soft soil deposits Potential for subsidence (karst. The potential for side slope failure without liquefaction is also a design consideration. mining. When foundation soils are cohesive and not heavily over-consolidated. The primary geotechnical concern during the design earthquake is the potential for liquefaction in foundation soils beneath the embankment. settlement in the range of 30 to 60 mm throughout the design life of the road is considered to be tolerable provided that it is uniform. primary consolidation of the foundation soil.2013 Chapter 3 Road Embankments Embankments under 5 m high in areas of stable ground and with slopes not greater than 1. Immediate settlement of the fill and the foundation soil occur during construction and will not usually have any impact on the future pavement. Information from test-fill monitoring can be used to develop better estimates of soil compressibility and the rate at which settlement will occur. Because primary consolidation and secondary compression can continue long after the embankment is constructed (post construction settlement). they represent the major settlement considerations for embankment design. This procedure is generally used in practice despite the fact that not all of the soil beneath the embankment undergoes one dimensional consolidation. The total settlement in an embankment is a summation of three potential components: immediate settlement of the fill or the foundation soil. The results of these one-dimensional consolidation tests are expressed in an e-log p (void ratio versus the log of pressure) or ε-log p (strain versus the log of pressure) plot. which is called a consolidation curve. Settlement caused by lateral deformation of the foundation soil at the edges of an embankment is not considered here. Therefore. it may be desirable to construct and monitor test fills. Differential settlements that occur along the longitudinal axis of the embankment because of changes in consolidation properties of underlying clays can cause transverse cracking on the surface of the embankment. These embankments can be specified particularly when based on past experience in the same region and on engineering judgment. One-dimensional consolidation tests are often used for the determination of settlement in the laboratory.3 Settlement Analysis Settlement is the amount of vertical deformation that occurs when a load is applied or an embankment is placed over compressible soils. occurs slowly. for cases where there is uncertainty in the settlement estimate using consolidation tests. Post-construction settlement adjacent to bridges can also create deformation to the road surface. especially if these facilities are also supported by adjacent soils that do not settle appreciably. As stated earlier. leading to differential settlements. The settlement may be due to the consolidation of the embankment itself as well as the underlying soils. and does not take place adjacent to a pile supported bridge. Generally.5H:1V generally do not require a detailed geotechnical investigation and analysis. or down drag and lateral squeezing of the foundation. embankments over 5 m high and those constructed over soft ground will usually require a detailed geotechnical analysis. 3. and secondary compression controlled by the composition and structure of the foundation soil skeleton. the analysis of settlement for embankment design focuses on primary consolidation and secondary compression of the foundation soil. On the same basis. Settlement due to consolidation can be estimated from the slope of this curve. Usually.Geotechnical Design Manual . Ethiopian Roads Authority Page 3-5 . post-construction settlement can damage structures located within the embankment. with each layer reflecting changes in soils properties. the soil is said to be normally consolidated. Common correlations for estimating Cc are given in Table 3-2. it is preferable to evaluate a 6 m thick layer as two 3 m thick layers). The total settlement is the sum of the compressions in n soil layers. eo is initial void ratio. The rate at which primary consolidation occurs is also dependent on the rate at which the water is squeezed out of the soil voids. and pc is the maximum past effective vertical stress.Chapter 3 Road Embankments 3. The response of foundation soils to additional loads is also dependent upon their stress history. foundation soils can be classified as normally consolidated. Pre-consolidation pressure in excess of the current vertical effective stress occurs in over-consolidated soils. the more water that can be squeezed out.1 Geotechnical Design Manual – 2013 Primary consolidation The settlement associated with the readjustment of soil particles due to migration of water out of the voids is known as primary consolidation. When the existing load is equal to the historical load. The greater the initial void ratio. Settlement computation starts with the soil profile being divided into layers. over-consolidated. The settlement of an embankment resting on n layers of normally consolidated soils can be computed from Fig. Ho is layer thickness. settlement happens when the weight of the embankment exceeds the previous stress history of the soils. The amount of primary consolidation depends on the initial void ratio of the soil. Usually. Depending upon the magnitude of the existing effective pressure relative to the maximum past effective stress at a given depth. The total settlement is the sum of the settlement from each of the compressible layers. even if no additional load is applied. or under-consolidated.3-2a and using the following Equation 3-1: n S =∑ i p Cc H o log10 f 1 + eo po where Cc is the compression index.3. Soils are considered to be under-consolidated when consolidation under the existing load is still occurring and will continue to occur until primary consolidation is complete.e. po is initial (current) effective vertical stress and pf = po+ ∆p is final effective vertical stress at the centre of layer n. and the greater the primary consolidation. Thick layers with similar properties are also subdivided to improve the analysis since the settlement calculations are based on the stress conditions at the midpoint of the layer (i. Page 3-6 Ethiopian Roads Authority . 27) Inorganic. As mentioned earlier. silt.156eo + 0.0.0. Over-consolidation can be due to the weight of a natural soil deposit that has since been eroded away.2013 Figure 3-2: Typical consolidation curve for normally consolidated soil.75(eo .5Gs(PI/100) All clays Cc= 0. or due to Ethiopian Roads Authority Page 3-7 . In this case the slope of the virgin portion of the consolidation curve is called the modified compression index and is denoted as Ccε as shown in Fig. The assumption is made that the initial and final stress calculated at the centre of each sub-layer is representative of the average stress for the sub-layer. This means that in the past the clay was subjected to a greater stress than now exists. and wn is water content If the water content of a clay layer below the water table is closer to the plastic limit than the liquid limit.0115wn Organic soils.3-2b. where: Ccε= Cc /(1+eo). (a) void ratio versus vertical effective stress and (b) vertical strain versus vertical effective stress. the soil is likely to be over-consolidated.5 m to 3 m thick for pavement design applications. and the material properties are reasonably constant within the sub-layer. The sub-layers are typically 1. From US DOT FHWA (2006B) Sometimes the consolidation data is presented in terms of vertical strain (εv) instead of void ratio. loads due to structures that have been demolished. silty clay Cc = 0. Modified by US DOT FWHA (2006B) from Holtz & Kovacs (1981) Correlation Soil Cc= 0. LL is liquid limit. the weight of a previously placed fill that is removed. PI is plasticity index. Table 3-2: Correlations between Cc & soil index parameters.50) Low plasticity clays Where eo is initial void ratio.0107 Cc = 0.Chapter 3 Road Embankments Geotechnical Design Manual . the total settlement is computed by summing the settlements computed from each subdivided compressible layer within the zone of influence. peat Cc = 0.30(eo. As a result of pre-consolidation. The settlements for the case of n layers of under-consolidated soils are computed by the following Equation 3-3. Fig. pc. is added so that the final stress. From US DOT FHWA (2006B) Under-consolidation is the term used to describe the effective stress state of a soil that has not been fully consolidated under an existing load. pf. the field state of stress will reside on the initially flat portion of the e-log p curve. even if no additional load is applied. the field state of stress will reside entirely on the virgin portion of the consolidation curve. Consolidation settlement due to the existing load will continue to occur under that load until primary consolidation is complete. which corresponds to Fig. 3-4a and Fig. ∆p. This condition is indicated in Fig. 3-4a: n S =∑ i Page 3-8 P H o Cc P log10 o + log10 f 1 + eo Pc Po Ethiopian Roads Authority . where Cr is the recompression index: n S =∑ i P Ho P Cr log10 c + Cc log10 f Po 1 + eo Pc In cases where the foundation soils represent both normally and over-consolidated layers.3-3a. the total settlement is computed by using a combination of the corresponding equations. is greater than the maximum past effective stress. Normally consolidated soils undergo large settlements compared to overconsolidated soils. As a result of under-consolidation.Chapter 3 Road Embankments Geotechnical Design Manual – 2013 desiccation. 3-3a and 3-3b illustrate the case where a load increment. 3-4b by ∆po. The settlements for n layers of overconsolidated soils will be computed from the following Equation 3-2 that corresponds to Fig. The curves shown in Figure 3-3 can be used to determine the settlement of an embankment resting on n layers of over-consolidated soils. Figure 3-3: Typical consolidation curve for over-consolidated soil. (a) void ratio versus vertical effective stress and (b) vertical strain versus vertical effective stress. Figure 3-4: Typical consolidation curve for under-consolidated soils (a) void ratio versus vertical effective stress and (b) vertical strain versus vertical effective stress. From laboratory consolidation test data determine the e-log p curve and estimate the change in void ratio that results from the added weight of the embankment. The total settlement a soil layer is a sum of all sub-layer settlements The detailed methodology for the estimation of consolidation is provided in many text books. over-consolidated or underconsolidated. and horizontal distance from the centre of the load to the point in question. This empirical approach is based on the assumption that the area the load acts over increases geometrically with depth. 3. Ethiopian Roads Authority Page 3-9 . Perhaps the simplest approach is the 2V:1H method. From US DOT FHWA (2006B) The step-by-step procedures for determining the amount of and time for consolidation to occur for a single-stage construction of an embankment on soft ground is outlined below: 1. Create the virgin field consolidation curve by using standard guidelines. and they include subroutines to estimate the increased vertical effective stress caused by the embankment or other loading conditions.Geotechnical Design Manual . 2. The advantage of computer programs is that multiple runs can be made quickly. Use Equations 3-1. Apart from the vertical stress. 3-2 and 3-3 to compute the primary consolidation settlement for each sub-layer of the foundation soils. the inclination of the embankment side slopes. depth below the ground surface. Determine if the soil is normally consolidated. The computations are conducted manually or by using a spreadsheet. Since the same vertical load is spread over a much larger area at depth. the parameters that control stress distribution at depth are the dimension of the embankment. the unit stress decreases.2013 Chapter 3 Road Embankments Several methods are available to estimate the stress distribution at any point in the embankment. There are also many computer programs that can compute settlement. 4. t. The two graphical procedures commonly used for this purpose are the logarithm-of-time (log t) and the square root of time(√?) methods. Because both methods are different approximations of a theory.Chapter 3 Road Embankments 3. the quality of laboratory testing. The step by step procedures to determine rate of settlement are as follows: 1.e. The value of the dimensionless time factor. Tv. the thickness of the layer and the time after loading: Tv H d2 t= Cv Where t is the time (days) needed. Therefore. The ability to quantify both the magnitude and time of settlement depends on field investigations. When confined by a permeable layer on one side and an impermeable boundary on the other. At the end of primary consolidation all excess pore water pressures have dissipated and the average degree of consolidation (U) approaches 100%. The average degree of consolidation at any time. As shown in the equation above. the rate of settlement increases significantly because of the reduced drainage path length. therefore. and type and consistency of the foundation soils. estimating the time of settlement during design is often as important as estimating the magnitude of settlement. Tv. determination of the average drainage path length is an important component of field exploration. Generally. can be defined as the ratio of the settlement at that time (St) to the settlement at the end of primary consolidation (Sf). experience has shown that the rate of settlement is faster than is estimated from calculations. Hd (m) is the longest distance to a drainage boundary. the time of settlement is directly proportional to the distance to a drainage surface squared (Hd2). Generally. The normalized time factor. Cv is the coefficient of consolidation measured in m2/day. the size of the embankment. Tv is a dimensionless time factor. As shown in the following Equation 3-4. Determine the coefficient of consolidation (Cv) from laboratory consolidation test data. for any average degree of consolidation can be taken from tables available in many text books. is used to compute the settlement time for various percentages of settlement due to primary consolidation in order to develop a predicted settlement-time curve.3. reducing the drainage thickness (H) by a factor of two results in a four-fold increase in settlement rate. If the clay deposit has a significant number of sand inter-layers. A part of this faster rate can be attributed to the existence of thin drainage layers. the longest drainage distance of a soil confined by permeable layers on both sides is equal to one-half of the layer thickness. the time for primary consolidation settlement depends on the coefficient of consolidation. This drainage path or the distance the pore water must flow through a compressible layer depends on the permeability of materials below and above it. U= St/Sf).2 Geotechnical Design Manual – 2013 Time for settlement Many projects cannot accept the programming impact associated with waiting for primary consolidation to occur. they do not give the same Page 3-10 Ethiopian Roads Authority . Normally. when excess pore water pressures are zero throughout the consolidating layer (i. the longest drainage distance is equal to the layer thickness itself. 3 Secondary compression The end of primary consolidation is considered as the amount of compression that occurs during the period of time required for excess pore-water pressure to dissipate because of an increase in effective stress. i.3. For example for U=50%. Secondary compression occurs when the soil continues to settle after the excess pore water pressures are dissipated to a negligible level. the value of Tv is 0. o Project the straight-line portions of the primary consolidation and the flatter portion towards the end of the consolidation curve to intersect at T. respectively. i.e. 2. primary consolidation is essentially completed. Determine the average drainage path length (Hd) during field exploration. 3. o Plot the dial readings for sample deformation for a given load increment against time on a semi-log paper.95% primary consolidation.197 from standard tables. which corresponds to 0% consolidation.e. The sample deformation beyond t100 is due to secondary compression. which is at a distance x above point P. o The difference of the dial readings between P and Q is equal to x. Some programs also allow for simulation of multiple layers undergoing simultaneous consolidation.2013 Chapter 3 Road Embankments answers. The dial reading corresponding to this line is d0. 3. Often the √?method gives slightly greater values than the log t method. Locate point R. There are various programs which can calculate the time rate of settlement for various boundary conditions including the effects of staged construction and strip drains in addition to calculating the stresses and settlements. o Draw the horizontal line RS. 50% consolidation.Geotechnical Design Manual . o Plot two points. o Calculate Cv from Equation 3.e. The time corresponding to the point V is t50. The dial reading corresponding to T is d100. Note that t2 = 4 t1. The log t method is shown in Figure 3-5. It can take years for primary settlement to complete and secondary compression continues for decades. o Determine the point V on the consolidation curve which corresponds to a dial reading of (d0+d100)/2 = d50. The occurrence of secondary compression is independent of the stress state and theoretically is a function only of the secondary compression index and time. Establish the time to achieve 90% . 100% primary consolidation. P and Q on the upper portion of the consolidation curve which correspond to time t1 and t2. Ethiopian Roads Authority Page 3-11 .4 for desired U. i. An alternative approach to hand calculation is the use of computerized methods. However. t1 is the time when approximately 90% of primary consolidation has occurred for the actual clay layer being considered. i. Since secondary compression is not a function of the stress state in the soil but rather how the soil breaks down over time. contribution from secondary settlement is small. log time plots of one-dimensional consolidation test shown in Fig 3-5.e. The values of Cα can be determined from the dial reading vs. Ho is layer thickness. From US DOT FHWA (2006B) Secondary compression is normally evident in the settlement-log time plot (Figure 3.Chapter 3 Road Embankments Geotechnical Design Manual – 2013 Figure 3-5: Log t method of determining the coefficient of consolidation. and t2 is the service life of the structure or any other time of interest. it can be large and difficult to predict. For many soils. Cα is the coefficient of secondary compression.5). when the specimen continues to consolidate beyond 100% of primary consolidation. Often the owner must accept the risks and maintenance costs associated with secondary compression if a cost/benefit analysis indicates that mitigation techniques such as using lightweight fills or over-excavating are too costly. in soft soils and particularly in soils with a high peat or organic content. Page 3-12 Ethiopian Roads Authority . detailed numerical procedures for estimating the amount of secondary compression are given in the literature. the contributions from the individual soil layers are summed to estimate the total settlement. though the normal assumption is that it decreases according to the logarithm of time as shown below: Sc = t Cα H o log10 2 1 + eo t1 Where Sc is secondary compression. Secondary compression continues through the life of the embankment. Like the primary consolidation. Generally. beyond t100. eo is the initial void ratio. techniques such as surcharging to pre-induce settlement are sometimes only partially effective at mitigating the effect of secondary compression. it is often not economical to bring the extra fill to the site only to haul it away again.4 Settlement Mitigation Once the amount and rate of settlement are determined from laboratory tests and field measurements. the most commonly used methods to mitigate settlement include acceleration using surcharges and wick drains. This method involves the placement and removal of a fill similar to or greater than the permanent load. 3. it is advantageous to use gravel borrow material to reduce workability issues during the rainy season and wet weather conditions. lightweight fills and removal and replacement. The concept of preloading and its effect on reducing settlement is shown in Figure 3-6. Instead. when fill soils must be handled multiple times (such as with a “rolling” surcharge) for preloading.Geotechnical Design Manual . Also. Mitigation techniques are employed when it is observed that the extent of settlement is beyond the amount that can be tolerated.1 Preloading and Surcharge Preloading is the process of compressing the subsoil prior to placing the permanent load.4. the surcharge is removed after the settlement objectives have been met in order to avoid additional deformation.2013 Chapter 3 Road Embankments 3. Figure 3-6: Concept of pre-loading and its effect on magnitude and time of settlement In preloading. Surcharging is a process which subjects the ground to a higher pressure than that during the service life in order to achieve a higher initial rate of settlement thus reducing long Ethiopian Roads Authority Page 3-13 . the next stage would be to select appropriate techniques that can assist mitigating the short and long-term effects. but this process is expensive and is rarely applied. there is an attempt to reduce potential settlement by compaction of the foundation soil. Often. If the material cannot be moved to another part of the project site for use as site fill or another surcharge. then the surcharge would be designed to achieve 600 mm of settlement or greater under primary consolidation only. the surcharge is constructed to a predetermined height. In embankment construction.Chapter 3 Road Embankments Geotechnical Design Manual – 2013 term deformation. Figure 3-7: Effect of surcharge on magnitude and time of settlement Depending upon the strength of the consolidating layer(s) the surcharge may have to be constructed in stages. The actual settlement occurring during embankment construction is then monitored with geotechnical instrumentation. For example. Page 3-14 Ethiopian Roads Authority . The intent is to use the surcharge to pre-induce the settlement estimated to occur from primary consolidation and secondary compression due to the embankment load. if the estimated primary consolidation under an embankment is 400 mm and secondary compression is estimated as an additional 200 mm over 25 years. Using a surcharge typically may not completely eliminate secondary compression. surcharges can be used to reduce the impact of settlement from secondary compression. When the settlement with surcharge equals the settlement originally estimated. but it has been successfully used elsewhere to reduce its magnitude. then surcharging could be removed. In addition to decreasing the time to reach the target settlement. The actual dimensions of the surcharge and the waiting period for each stage depend on the strength and drainage properties of the foundation soil as well as the initial height of the proposed embankment. Figure 3-7 illustrates the idea behind surcharging and the results achieved. usually between 300 mm and 3 m above the final grade elevation. The surcharge is maintained for a predetermined waiting period (typically 3 to 12 months) based on settlement-time calculations. a large proportion of the fill is left behind after the required settlement is achieved. Unlike preloading. The length of the waiting period can be estimated by using laboratory consolidation test data. a permeable sand blanket. Hence. 0. Other means are needed to address secondary compression in this case.Geotechnical Design Manual . Vertical drains and sand blankets should have high permeability to allow the water squeezed out of the subsoil to travel relatively quickly through them. Surcharging alone may not be effective in reducing settlement time sufficiently since the longest distance to a drainage boundary may be significant. as illustrated in Figure 3-8. which functions as a separator and a filter to keep holes in the plastic core from being plugged by the adjacent soil.6 to 1 m thick. is placed on the ground to permit free movement of water away from the embankment and to create a working platform for installation of the drains.2013 Chapter 3 Road Embankments However. It is known that new fill embankments over soft soils can result in stability problems. Predrilling of dense soil deposits may be required in some cases to reach the design depth.2 Vertical drains Primary consolidation of some highly plastic clays can take many years to be completed. Generally. The time for consolidation is proportional to the square of the length of the longest drainage path. The drains are installed prior to placement of the embankment. the stability of a surcharged embankment must be checked as part of the embankment design to ensure that an adequate short term safety factor exists (see Section 3. vertical drains or wick drains can be used to accelerate the settlement. Ethiopian Roads Authority Page 3-15 . the consolidation time is reduced by a factor of four. Since vertical drains are generally expensive. The significant design and construction consideration for using surcharges is the embankment stability. either with or without surcharge treatment. In such cases. In most applications. They are installed by pushing or vibrating a mandrel into the ground with the wick drain inside. 3. Thus if the length of the drainage path is shortened by 50%. the success of a surcharge to reduce secondary compression may be quite limited. The drains are usually 100 mm wide and about 6. such as removal and replacement. the feasibility of a surcharge solution should always be considered first. wick drains are small prefabricated drains consisting of a plastic core that is wrapped with geotextile.6).4.25 mm thick. The stability of the embankment can be monitored in the field using geotechnical instruments. produced in rolls that can be fed into a mandrel. for highly organic soils or peats where secondary compression is expected to be high. The vertical drains accelerate the settlement rate by reducing the drainage path the water must travel to escape from the soil to half the horizontal distance between them. The unsuitable soils are near the ground surface and do not extend very deeply (removal of unsuitable material beyond the depth of 3 m is not normally economically feasible).3 Removal and replacement Removal and replacement (over-excavation) refer to excavating soft compressible soils from below the embankment and replacing them with higher quality. because of high costs associated with excavating and disposing of unsuitable soils and the difficulty of excavating below the water table. partial or complete removal of compressible foundation material may be necessary. Some of these conditions include the following: • • • • • The area requiring over-excavation is not wide.Chapter 3 Road Embankments Geotechnical Design Manual – 2013 Figure 3-8: Use of vertical drains to accelerate settlement. Where analyses indicate that more foundation settlement would occur than can be tolerated. Temporary dewatering is not required to support or facilitate the excavation. The unsuitable soils can be dumped on site or can be disposed of safely elsewhere close by. Suitable fill materials are readily available to replace the volume of unsuitable soils.4. However. removal and replacement is only justified under certain conditions. less compressible soil (Figure 3-9). Page 3-16 Ethiopian Roads Authority . From NCHRP (1989) reproduced in US DOT FHWA (2006B) 3. If enough down-drag occurs. Figure 3-10: Elements of a bridge approach embankment.Geotechnical Design Manual . Settlements are results of vertical deformation. requiring piles and drilled shafts. From Briaud et al (1997) At bridge approaches. deformation can occur both in the vertical and lateral directions. However. it is better to use select materials and increase compaction requirements to prevent differential settlement. excess materials from a roadway excavation or a convenient borrow site are used to construct bridge approach embankments. the result is that materials surrounding deep foundation systems may cause negative skin friction or a down-drag effect on individual piles or drilled shafts (Fig.5 Bridge Approach Embankments Usually. the axial capacities of piles or shafts may be exceeded. 3-11). As approach embankments and foundation soils settle. Past studies have indicated that this effect can occur with as little as a 10 mm settlement. Ethiopian Roads Authority Page 3-17 .2013 Chapter 3 Road Embankments Figure 3-9: Removal and replacement beneath an embankment 3. because only a small settlement is desirable at abutments. In addition. these slabs can settle and/or rotate. A well-constructed soil embankment will not excessively deform internally.Chapter 3 Road Embankments Geotechnical Design Manual – 2013 Figure 3-11: Settlement and down-drag in bridge abutments and piles. Internal deformation within embankments can be controlled by using fill materials that have the ability to resist the anticipated loads imposed on them. reduce the stability of soils near the slope. External deformation is due to the vertical and lateral deformation of the foundation soils on which the embankment is placed. the reasons for the bump are poor compaction of Page 3-18 Ethiopian Roads Authority . how the slab is connected to the abutment and/or the wing walls. A typical approach embankment cross section is shown in Figure 3-12.g. Furthermore. Special attention should be given to the interface area between the bridge structure and the approach embankment. Often. Lateral squeeze of the foundation soils can occur if the soils are soft and if their thickness is less than the width of the end slope of the embankment. e. Usually. as this is where the bump at the end of the bridge occurs. any large settlement near a bridge structure can lead to the formation of differential settlements in the road surface. The use of a waiting period between the completion of the embankment and the installation of foundations permits settlement to occur prior to driving piles or installing shafts. and potentially lead to migration of fill material and creation of voids or substantial vertical and lateral deformations. Downdrag loads may be considered negligible if the settlement following completion of the waiting period is expected to be less than 10mm. Internal deformation is a result of compression of fill materials.. Due to the deformation of the approach embankment fills. an estimate of the negative skin resistance should be performed. voids may develop under the slab as the approach fill settles. deformation may include both primary consolidation and secondary compression depending on the type of foundation soils. Poor drainage can cause softening of the embankment soils. Generally the problems in bridge approach embankments are classified as internal deformation within the embankment and external deformation in foundation soils. commonly related to drainage problems. and water pressure may act against structural elements or soften the soils with associated reduction in strength. bridge approach slabs are used to provide a transitional road surface between the pavement on the approach embankment and the actual structure of the bridge. Such voids can then fill with water. Once it is determined that down-drag is possible. Depending on the configuration of the approach slab. Modified from US DOT FHWA 2006B Therefore it is important to evaluate potential approach embankment settlements close to the bridge structure and determine if down-drag will occur. Proper compaction can be achieved by optimizing the soil gradation in the interface area to permit compaction to maximum density with minimum effort. Table 3-3 lists specification considerations for selected structural back fill to ensure the construction of a durable and dense backfill. Suggested properties for drainage aggregate are provided in Table 3-4. or screened gravel. under-drain filters can be constructed. sand. The soundness of the drainage aggregate should also be tested.Geotechnical Design Manual . the aggregate should not have a loss exceeding 20% by weight after four cycles of the magnesium sulphate soundness test. Poor compaction is usually caused by restricted access of standard compaction equipment to that area. loss of embankment material due to poor drainage. Ethiopian Roads Authority Page 3-19 . migration of fine soils into drainage material. and. Generally. Figure 3-12: Suggested details of a bridge approach embankment. The drainage aggregate used for under-drain filters should consist of crushed stone.2013 Chapter 3 Road Embankments embankment material near the structure. Modified from US DOT FHWA 2006B Selected structural backfill is usually placed in relatively small quantities and in relatively confined areas to minimize differential settlements. In order to drain water from the embankment. the use of geosynthetic materials to reinforce the abutment backfill and approach area can reduce the differential settlement at the end of the bridge. From United States DOT FHWA (2006B) Sieve Size (mm) 25. consideration may be given to limiting the percent fines to 5%. excess pore pressures can develop and lead to instability of embankment slopes. bridge approach embankments can become saturated as a result of inundation during the occurrence of floods. Small equipment cannot achieve AASHTO T180 densities. Use well graded soil for ease of compaction.85 (No 20) % passing (by weight) 100 30 – 100 0 – 30 0 – 10 0–5 In addition to overall and differential settlement problems. The PI should not exceed 10 to control long-term deformation. 40 (0. For rapid drainage. 200 (0. In areas where selected materials are not available. A minimum of 100% of standard Proctor maximum density is required.00 (No 10) 0. From US DOT FHWA 2006B Consideration Lift thickness Largest particle size Gradation and % fines Plasticity index Durability T99 density control Compatibility Comment Limit to 150 mm to 200 mm.3 (No 3) 2. Page 3-20 Ethiopian Roads Authority . Table 3-4: Suggested gradation for drainage aggregate. poor-durability particles.075 mm) 0 to 15 The limitation on percent fines (particles smaller than No.425 mm) 0 to 70 No. Typical gradation: Sieve Size % passing (by weight) 100 mm 100 No.7 6. 200 sieve) is to prevent piping and allow gravity drainage.4 12. so compaction is possible with small equipment. Particles should not move into voids of adjacent fill or drain material. Limit to less than ¾ of lift thickness. A material with a magnesium sulfate soundness loss exceeding 30 should be rejected. The material should be substantially free of shale or other soft. Such reinforced fills can be designed using the principles of reinforced soil slopes.Chapter 3 Road Embankments Geotechnical Design Manual – 2013 Table 3-3: General considerations for specification of selected structural backfill. When the water level falls rapidly. On the other hand. short-term stability of embankments on cohesive soil is more critical than long-term stability. Shallow. because the foundation soil will gain shear strength as the pore pressures dissipate. failures can also occur when the fill materials at the toe of the embankment are eroded. In general.Geotechnical Design Manual . Embankments over 5 m in height or any embankment on soft soils. blasting or pile driving can also trigger failures of embankment slopes. or those comprised of lightweight fill require more in-depth stability analyses. may be designed based on past precedence and engineering judgment provided there are no known problem soil conditions such as organic and soft soils.5H:1V or flatter side slopes. deep rotational and planar failures that involve both the embankment and the foundation soils are considered to be external. Dynamic forces from earthquakes. Usually such kinds of failures are manifested as sloughing of the surface of the slope. Figure 3-13: Modes of side slope failures in embankments.2013 Chapter 3 Road Embankments 3. In addition. Stability problems in the form of rotational or sliding block failures (Figure 3-13) most often occur when the embankment is built over soft soils such as low strength clays and silts. Internal stability generally results from the selection of poor quality embankment materials and/or improper placement of the embankment fills. as do any embankments with side Ethiopian Roads Authority Page 3-21 . in unstable areas.6 Stability Assessment In addition to settlement. planar slope failures in embankment side slopes are examples of internal instability. From IOWA State (2013) and US DOT FHWA (2006B) Instability of embankments can generally be classified as internal or external. or when the foundation soil is overstressed during or immediately after construction. embankments that are 5 m or less in height with 1. Failure can also occur if the fill materials are not compacted to specification. Usually. the design of embankments should also consider side slope stability and bearing capacity. The resisting force is the total shear strength acting along the slip plane. sometimes. A toe slip circle develops in the embankment and intersects at the toe. Are there potentially liquefiable soils at the site? If so. will likewise require stability analysis. clay or peat? If so. the driving and resisting forces act as follows: • • The force driving movement consists of the embankment weight. Is the embankment temporary or permanent? Factors of safety for temporary fills may be lower than for permanent embankments. testing and analysis are required. the following key issues need to be addressed: • • • • Is the site underlain by soft silt. Are there geometrical and site constraints which need embankment slopes steeper than 1. when the embankment material becomes saturated and failure occurs. In general. The resisting moment is the product of the resisting force times the radius of the circle (LS). Moreover. A slip circle failure in embankments may be a base circle. the adjacent ground rises and the failure surface follows a circular arc of the type shown in Figure 3-14. Generally. at failure. seismic analysis to evaluate this and ground improvement may be needed.5H:1V. more elaborate sampling. Page 3-22 Ethiopian Roads Authority . The driving moment is the product of the weight of the embankment acting through its centre of gravity times the horizontal distance from the centre of gravity to the centre of rotation (Lw). any fill placed near or against a bridge abutment or foundation. depending on the site conditions and the availability of materials. A slip circle develops within the embankment and intersects with the slope. a toe circle.5H:1V? If so. or that can impact a nearby buried or above-ground structure. A discussion on slope stability is also given in Chapter 4. or a slope circle. A base slip circle develops when there is a significant thickness of weak foundation soil. This happens. The base of the failure arc is tangent to the base of the weak layer. Does the new embankment have an impact on nearby structures or bridge abutments? If so. the embankment settles. Methodologies for analysing the stability of embankment slopes are available in many reference books. Prior to commencing a stability analysis.Chapter 3 Road Embankments Geotechnical Design Manual – 2013 slope inclinations steeper than 1. and the arc will have a significant portion of its length in the weak soil. a slope stability assessment may be needed to evaluate the various alternatives. a staged stability analysis may be required. experience and observations of failures of embankments constructed over relatively deep deposits of soft soils have shown that when failure occurs. a friction angle based on residual strength may be appropriate. Since the rule of thumb assumes that there is no influence from groundwater in the embankment. Failure takes place when the factor of safety is less than one. This estimate can aid in directing the drilling and sampling programme either for design or construction review and help to ensure that critical strata are adequately explored and sampled.5 or when groundwater is expected to lie within the slip circle. In the case of over-consolidated fine grained soils. The equation is helpful only very early in the design stage to make a quick preliminary estimation of whether stability may be a problem and if more detailed analyses should be conducted. a total stress analysis using the undrained cohesion value with no friction is generally needed. A more detailed stability analysis is needed when the FS obtained in this way is less than 2. Ethiopian Roads Authority Page 3-23 . then effective stress analysis using a peak friction angle should be used for stability assessment. It can also be used in the field during investigation. If the critical stability is under undrained conditions.2013 Figure 3-14: Typical circular arc failure mechanism. c and γFill are effective stress parameters. such as in sand or gravel. γFill is the unit weight of the fill and HFill is the height (thickness) of the fill. such as in most clays and silts. If in-situ vane shear tests are being carried out as part of the field investigation for a proposed embankment. If the critical stability is under drained conditions.Chapter 3 Road Embankments Geotechnical Design Manual . ?? = ????? ?ℎ??? ???????ℎ ∗ ?? ????????? ?????? = ????ℎ?????? ∗ ?? ??????? ?????? A rule of thumb based on simplified bearing capacity theory can be used to make a preliminary estimate of the factor of safety (FS) against circular arc failure for an embankment built on a clay foundation without presence of free water. the data from vane strength tests on the underlying soils can be used with the equation to estimate the FS in the field. This is as follows: ?? = 6? ????? ∗ ????? where c is the cohesion of the clay foundation soil. This is especially true for soils that exhibit strain softening or are particularly sensitive to shear strain. From US DOT FHWA (2006B) The factor of safety (FS) against failure is equal to the ratio between the resisting and driving moments as given below. 5 m wide and not more than 6m apart vertically.7 Fill Slope Stabilization A variety of techniques are available to mitigate inadequate slope stability for new embankments. these should be at least 1. for side slopes of any road embankment. and vertical drains are used to increase the consolidation process.5. suitable drains should be provided to collect the flow. major retaining structures.5. All bridge approach embankments and those supporting critical structures should have a safety factor of 1. ground improvement. For slopes that would cause greater damage upon failure. depth of ground water table and other necessary parameters are determined by field explorations and/or laboratory testing.3 as determined by the ordinary method of slices (Chapter 4) is sufficient to maintain longterm stability.1 Staged construction Where soft compressible soils are present below a new embankment and when it is not economical to remove and replace these soils with compacted fill. This means that the rate of filling is governed by the increase in soil strength due to consolidation. The stability and degree of consolidation can be related to the gain in strength from the tests and observations of excess pore water pressures. and construction of toe berms and shear keys. 3. Usually the design is carried out using the undrained strength. the embankment can be constructed in stages to increase the strength of soils under the weight of the new fill. 3.6 presents a summary of these solutions to mitigate embankment stability problems. the stability of the embankment can be analysed using standard methods and a factor of safety estimated. General guidelines are given in Table 3-5.Chapter 3 Road Embankments Geotechnical Design Manual – 2013 Once the soil profile. a minimum design safety factor of 1. base reinforcement.7. In general.3 to 1. soil strengths. If an embankment is provided with benches. The method of analysis that should be selected to determine the factor of safety depends on the soil type. These techniques include staged construction to allow the underlying soils to gain strength. and major roadways. Page 3-24 Ethiopian Roads Authority . use of lightweight fill.3. Critical structures are those in which failure would result in property damage. the design safety factor should be increased in the range of 1. Embankments supporting or potentially impacting non-critical structures should have a minimum safety factor of 1. If springs or seepages are found near the embankment. strength characteristics and other parameters. existing embankments or embankment widening projects. such as slopes adjacent to bridge abutments. Table 3. CU consolidated undrained. Use effective strength parameters. CU triaxial test with pore water pressure measurements or CD triaxial test. UU = unconsolidated undrained. • Obtain effective friction angle from charts of standard penetration resistance (SPT) versus friction angle or from direct shear tests.Chapter 3 Road Embankments Geotechnical Design Manual . Use Bishop method with combination of cohesion and angle of internal friction (effective strength parameters from laboratory test). construction. Cohesive • Staged construction Embankments on soft clays: build embankment in stages with waiting periods to take advantage of clay strength gain • due to consolidation. shear strength with depth. Use Bishop method with an effective stress analysis. Some of embankment height. CD = consolidated drained Methods recommended represent minimum requirement. The Use undrained strength clay profile should be divided parameters at Po (Initial into convenient layers and the effective vertical stress) appropriate cohesive shear strength assigned to each layer. Ethiopian Roads Authority Page 3-25 . • Granular • • All types Recommended methods of analysis and remarks Use Bishop method. Consolidation test stress to determine clay data needed to estimate length of strength gain due to waiting periods between consolidation under embankment stages. and settlement devices should be used to monitor pore water Use undrained strength dissipation and parameters at appropriate pressure consolidation during Po for staged height. Use Bishop method at each stage CU triaxial test. • Long-term Embankment on soft clays and clay cut slopes. An angle of internal friction should not be UU or field vane shear used to represent an increase of test or CU triaxial test. Piezometers staged fill heights. More rigorous methods such as Spencer’s and Morgenstern-Price method should be used when the design demands. From US DOT FHWA (2006B) Soil type Type of analysis Source of shear strength parameters Short-term • Embankments on soft clays: • immediate end of construction (cut). Consider samples should be that clay shear strength will consolidated to higher increase with consolidation under than existing in-situ each stage. φ = 0 analysis.2013 Table 3-5: Slope assessment guidelines for the design of embankments and cuts. The embankment soils can be strengthened by incorporating reinforcements within them. A reduction in grade line will decrease the weight of the embankment and will improve the stability of over-stressed soils. care should be exercised in the selection of materials and compaction specifications to ensure that the design unit weight will be achieved for berm construction. Excavation of soft soil and replacement with shear key. Base reinforcement Techniques such as stone columns. The top surface of a berm should be sloped to drain water away from the embankment. and rapid draw-down (as applicable). Reinforcement of embankment soils. The reinforcement permits steeper slopes compared to unreinforced embankments. Page 3-26 Ethiopian Roads Authority . A counterweight berm outside of the centre of rotation provides an additional resisting moment that increases the factor of safety. The embankment should never be completed prior to berm construction since the critical time for shear failure is at the end of embankment construction. The strength of soft soils is often insufficient to support embankments. soil mixing. Also. Lightweight fills. These approaches differ on the basis of loading conditions such as total stress. (flatten slope) Counterweight berms. and grouting can be used to increase resisting forces. Reduce grade line. soil nailing. In some countries. the soft soils are excavated and replaced with granular material that acts as a shear key. Berms should be built concurrently with the embankment. Typical instrumentation consists of slope inclinometers to monitor stability. Computer programs are used to define the height of fill placed during each stage and the rate at which it is placed. construction of the second and subsequent stages commences when the strength of the previous layers is sufficient to maintain stability. shredded rubber tires. From US DOT FHWA (2006B) Relocate highway alignment. lightweight materials such as blast furnace slag. geosynthetics. ground anchors. Different approaches are used to assess the rate of fill placement and the necessary strength gain on various types of foundation soils. this method is practical. effective stress. Use of such materials decreases the driving force as well as settlement. For soils that consolidate relatively fast. along with the time of settlement and the percent consolidation required for stability. such as some silts and silty clays. Slow rate or staged construction Many weak sub-soils will tend to gain strength during the loading process as consolidation occurs and pore water dissipates. Specialty contractors should be considered for these design solutions. Proper instrumentation is desirable to monitor the state of stress in the soil during the loading period to ensure that loading does not proceed so rapidly that a shear failure occurs. Ground improvement Densification of the soil through a special compactor and altering the soil composition may help to minimize embankment slope failure. This option could also be used if weak sub-soils are pre-treated with wick drains. or expanded shale are available. piezometers to measure excess pore water pressure and settlement devices to measure the amount and rate of settlement. In such cases.Chapter 3 Road Embankments Geotechnical Design Manual – 2013 Table 3-6: Design techniques useful for mitigating embankment failure. A shift of the highway centre-line to an area with better soils may be the most economical solution. After the first fill placement. In general. the pore pressure increase beneath the embankment in the soft subsoil is monitored and used to control the rate of embankment construction. this approach may not be feasible if the soil contains a high percentage of organic material and trapped gases. Some roadway embankments may be subjected to water ponding at the base of the slopes during flood events or nearby standing water or lakes. This situation may cause embankment and foundation soils to become saturated.Geotechnical Design Manual . which is the ratio of pore pressure to total overburden stress. Each time fill is added. This situation can create a critical embankment stability condition commonly referred to as a "rapid draw-down" condition (Figure 3-15). the rate of embankment construction is performed using maximum fill heights and intermediate fill construction delays. Ethiopian Roads Authority Page 3-27 . while the soft subsoil and previous fills have already had time to react to the stress increase due to the fills applied earlier. and delay period increments range from 10 to 30 days. Some judgment is needed when interpreting such data and deciding whether or not to reduce or extend the estimated delay period during fill construction. Stability analyses performed to evaluate this situation should model the embankment as being saturated up to the high water elevation. In this approach. the pore pressure increase is not allowed to exceed a critical amount to ensure embankment stability. Figure 3-16 shows the typical result of an analysis using the total stress approach. At the end of the analytical process. field measurements such as the rate of settlement or the rate of pore pressure decrease should be obtained to verify that the design assumptions regarding rate of consolidation are correct. The thickness of the fill and the intermediate delay periods are estimated during design. placement of the next layer of fill can begin. During construction. the fill starts to consolidate. a weighted average of the percent consolidation that has occurred for each stage up to the point in time (or the time for full height) should be used to determine the average percent consolidation of the subsoil due to the total weight of the fill. causing the pore pressure readings to be too high and not drop off as consolidation occurs. Soils may not drain as quickly as the water recedes and may remain saturated for some period after the water returns to its normal lower elevation. pore pressure transducers are typically located at key locations beneath the embankment to capture the pore pressure increase caused by consolidation stress. Also. Typical fill height increments range from 60 cm to 120 cm. To accomplish the pore pressure measurement. and should be performed using effective stress parameters for foundation soils and embankment materials. For this approach. The critical amount is generally controlled by use of the pore pressure ratio.2013 Chapter 3 Road Embankments An analysis based on total stress is often useful to simulate conditions that may exist during or shortly after construction. During these delay periods the fill that was placed is allowed to settle until an adequate amount of consolidation of the foundation soil can occur. Once the desired amount of consolidation has occurred and pore pressure dissipated. as the estimate of the parameters may vary from the actual values in the field. In the effective stress approach. it is best to choose as small a fill height and delay period increment as practical. in that the reinforcement is needed only until the shear strength of the underlying soil has increased sufficiently as a result of consolidation under the weight of the embankment.7. From Washington State DOT (2013) 3. Therefore. Page 3-28 Ethiopian Roads Authority . The base reinforcement can be designed for either temporary or permanent applications.2 Base reinforcement Base reinforcement may be used to increase the factor of safety against slope failure. Most base reinforcement applications are temporary. Base reinforcement typically consists of placing a geotextile or geogrid at the base of an embankment prior to constructing the embankment. Base reinforcement is particularly effective where soft/weak soils are present below a planned embankment location. the base reinforcement does not need to meet the same design requirements as permanent base reinforcement regarding creep and durability.Chapter 3 Road Embankments Geotechnical Design Manual – 2013 Figure 3-15: Effect of flooding and rapid-drawdown on embankment stability Figure 3-16: Concept of calculating the percent consolidation in staged construction. soil modification or replacement.4 Lightweight fills Lightweight fills are other means of improving embankment stability. instead of earth materials. The side slopes of the toe berms are often gentler than the fill embankment side slopes. Where base reinforcement is used. This method will be more difficult if the groundwater level lies above the base of the excavation. as well as reduce settlement. geogrids should be linked with pins and not simply overlapped.7. reinforcement. Situations where they may be required include conditions where the construction schedule does not allow the use of staged construction.6 Toe berms and shear keys Toe berms and shear keys are methods to improve the stability of an embankment by increasing the resistance along potential failure surfaces. All seams in the geotextiles should be sewn and not lapped. drainage. The wick drains in effect significantly reduce the drainage path length. Columns made of stone or chemically stabilized soil increase the stiffness of the foundation and can substantially increase stability and decrease settlement. 3. and other water content controls. In addition to these categories. thereby accelerating the rate of strength gain. 3.7. Likewise. 3. However the effect on stability and long term settlement of the remaining soft material should be considered. If the soft material is much deeper than the practical excavation depth. Base reinforcement materials should be placed in continuous longitudinal strips in the direction of main reinforcement. toe berms are constructed near the toe of the embankment slopes where stability is a concern (Figure 317a).7.2013 Chapter 3 Road Embankments The design of base reinforcement is similar to the design of a reinforced slope in that limit equilibrium slope stability methods are used to determine the strength required to obtain the desired safety factor. cementation.3 Ground improvement Ground improvement can be used to mitigate inadequate slope stability for both new and existing embankments. the use of gravel borrow areas. The use of berms may increase Ethiopian Roads Authority Page 3-29 . Toe berms are typically constructed of granular materials that can be placed quickly. Other ground improvement techniques such as stone columns can also accelerate strength gain in the same way as wick drains. compacted sand or suitable fill) to provide a stable foundation. As implied by the name.7.g. Joints between pieces of geotextile or geogrid in the strength direction (perpendicular to the slope) should be avoided. partial excavation and replacement is also possible. Sometimes partial excavation and replacement of soft material is used with ground treatment techniques to overcome the above problems. may also be appropriate in order to increase the embankment shear strength. basic concepts of soil improvement include densification. low strength materials and unfavourable water content in the soil.5 Removal and replacement As in the case of settlement. Therefore. but the berm itself should be checked for stability. wick drains may be used in combination with staged embankment construction to accelerate strength gain and improve stability and accelerate long term settlement. the very soft compressible cohesive soils are excavated and replaced with better materials (e. do not require much compaction. Most foundation problems occur from high void ratios. but have relatively high shear strength.Geotechnical Design Manual . 3. Shear keys typically range from 1. As with toe berms.17b) and frequently below the toe of the embankment. Toe berms increase the shearing resistance in the following ways: • • • By adding weight. except instead of being adjacent to the toe of the embankment. require minimal compaction.5 m in width and extend 1. but still have high internal shear strength. By creating a longer failure surface. Shear keys are best suited to conditions where they can be embedded into a stronger underlying formation. They are typically backfilled with quarry waste or similar materials that are relatively easy to place below the groundwater. Shear keys function in a manner similar to toe berms. By adding high strength materials for additional resistance along potential failure surfaces that pass through the toe berm.0 m below the ground surface. and thus increasing the shear resistance of granular soils below the toe area of the embankment. the shear key is placed under the fill (Figure 3. Figure 3-17: Use of a counterweight berm (a) and a shear key (b). as the failure surface must pass below the toe berm rather than the embankment and the berm.5 to 4. From US DOT FHWA (2006B) Page 3-30 Ethiopian Roads Authority .Chapter 3 Road Embankments Geotechnical Design Manual – 2013 the magnitude of settlements as a consequence of the increased size of the loaded area.2 to 3. shear keys improve the stability of the embankment by forcing the potential failure surface through the strong shear key material or along a much longer path below the shear key. thus adding more shear resistance. The use of these predominantly granular materials limits the amount of vertical deformation or settlement of the type observed in embankments on soft soils in flat areas. Figure 3-18: Typical construction of embankments in hilly areas. a major consideration during design. therefore. The maximum permissible angle of side-slopes with which it is possible to maintain long-term stability is. From FAO (1998) The main problem of embankments in hilly areas is the overall stability of the fill and the foundation layers. An examination of embankment failures along a number of mountain roads shows that many are caused by: • • Inadequate under-drainage under conditions of pronounced seepage. Ethiopian Roads Authority Page 3-31 . Incomplete removal of vegetation and organic material and lack of benching prior to embankment construction.8 Embankments in Hilly Areas The design of embankments in hilly and mountainous regions often takes a different form from those on flat areas. This is because many embankments in these places are constructed by cut and fill operations. side-long hill slopes often results embankments that are only marginally stable. However. and compacted to form side-long “sliver fills” (Figure 3-18).2013 Chapter 3 Road Embankments 3. poor compaction techniques used on such fills located on inclined. if suitable for use as fill.Geotechnical Design Manual . The procedure is that soil and rock are cut. Unlike relatively homogenous embankments on flat areas. Excessive erosion can lead to the formation of rills and gullies that ultimately affect the side slope stability. Figure 3-19 shows the types of slope failures that are commonly observed in fills and the underlying hill slopes. Figure 3-19: Types of slope instability commonly seen in fills and the underlying hillslope. degree of compaction. as shown in Figure 3-19. soil type. This is especially true when slope conditions are exacerbated by pore water pressure and seepage during the rainy season. Slope failures in fill slopes constructed in flat areas are often in the form of small-scale shallow translational or rotational slides. In addition. they may start to fail. length of slope. In addition. the mechanisms of failure of embankments in hilly terrain are very difficult to analyse using conventional limit Page 3-32 Ethiopian Roads Authority . as well as land-use practices. the types of failure often found in embankments in hilly areas can extend beneath the entire slope upon which the embankment is constructed. rainfall or runoff intensity. Generally. where failure is contained entirely within the embankment side slopes and maximum depth of rupture does not exceed 2 m. Erosion on slopes immediately below the embankment. and vegetation cover. most earth embankments can be prone to erosion problems arising from rainfall runoff and road runoff. the overall stability of a fill slope on a hillside is difficult to quantify. However. slope angle. Slope failures are common where rivers erode side slopes that eventually undermine the stability of fill slopes constructed on the slope above. failure is sometimes associated with the interface between the natural ground and the fill. Presence of pre-existing shear surfaces beneath the embankment the presence of unfavourably orientated planes of weakness in the soil or rock beneath the embankment.Chapter 3 Road Embankments • • • Geotechnical Design Manual – 2013 Construction of embankments on loose spoil material derived from earlier excavations. The amount of erosion is normally a function of runoff source. From MPWT (2008) Sometimes when valley slopes are exposed to high stresses exerted by side-cast spoil. This is especially true when the fill is placed on colluvium. This is because failure surfaces often involve both the fill and underlying hill slope. In addition. the cross-section of embankments in hilly terrain is different from that on flat ground as they are usually constructed only on one side of the road. Table 3-7 shows some slope stabilization techniques appropriate for fill and underlying hill slopes. Hence. Problems also frequently occur when rock strata beneath the fill are dipping parallel to the ground slope 1. Table 3-7: Slope stabilization techniques for embankments on hill slopes.Geotechnical Design Manual . Modified from MPWT (2008) Instability Failure within fill slopes (internal) Failure in fill slopes and underlying hill slopes (external) Failure within underlying hill slopes Stabilization options • • • • • Reduce the slope angle and remove excess material Properly compact fill material Consider the use of a retaining wall Ensure road-side drainage is controlled Bio-engineering is usually important to prevent surface erosion • • • Re-grade the slope and remove excess material Properly compact fill material Before placing fill. these options are site specific and are selected and designed based on the characteristics of the fill and slope materials in the project site. Adversely 1 Or are dipping out of the slope at an angle that is less than the slope angle but greater than the friction angle along joint or bedding/foliation planes Ethiopian Roads Authority Page 3-33 . based on the properties of the material forming the fill. stability and fill side slope angles are often designed on the basis of local experience.2013 Chapter 3 Road Embankments equilibrium methods. prepare benches on the slope to intercept potential planes of failure and to provide a key Consider the use of a retaining wall Ensure road-side drainage is controlled Bio-engineering is usually important to prevent surface erosion • • • • • • • Reduce side slope if sufficient space exists Retaining wall Ensure road-side drainage is controlled Bio-engineering is usually important to prevent surface erosion and increase the resistance of the surface soil. each with significantly different material properties. or where the stratigraphy is such that weak volcanic deposits (such as tuff and ash) underlie layers of stronger material. or where marl and shale are overlain by limestone and sandstone. before constructing a fill in hilly areas. or where the groundwater table is at or very close to the surface. Generally. In many cases. it is necessary to assess the stability condition of the slope against shallow and deep failures. the potential for failure along a deeper surface in the ground beneath should always be considered. Although the weakest layer is often just below the fill and the strength of soils increases with depth. Chapter 3 Road Embankments Geotechnical Design Manual – 2013 orientated rock planes can cause the slope and the fill above to slide, triggered by increased load, or increased pore pressure along the failure plane. Groundwater from different sources can soften the founding material, or cause the fill material to be undermined through seepage erosion. In these situations, fill slopes require surface and sub-surface drainage structures to keep groundwater away from the area. 3.9 Fill-slope Angles and Benches When deciding fill slope angles for design without stability analysis, it is advisable to consider the type of material (rock or soil, and the strength parameters of each) that is going to be used. For example, fills containing high amount of fines may show surface cracking when dry. The design should also consider the stability of existing fill slopes in the surroundings of project sites. Observation of the existing natural slopes should include the vegetation, in particular the types of plants that may indicate wet soil. Indirect relationships, such as subsurface drainage characteristics, may be indicated by vegetative pattern springs and seepage lines in the area should also be checked. Table 3-8 shows preliminary or provisional fill slope batters based on types of materials. In general, for embankments in hilly terrain, where fills are dominantly granular, side slopes are often designed using a slope ratio of 1.5H:1V as shown in Figure 3-20, assuming that the specifications for particle size, drainage and compaction are met. Larger-sized rock blocks (rip-rap) may be placed on the lower side of the fill to reinforce the embankment and drain surface water. Earth fill may be placed on top of rock fill, separated by a cap layer, if rock is scarce or load is an issue. Moreover, smaller sized rock fragments larger than the average size of the fill itself could be placed underneath the fill to drain water away from the embankment and foundation soil. Table 3-8: Preliminary fill slope angles Fill materials <5m Rock fill 1.5H:1V 5-10 m 10-15 m 2H:1V Well graded sand, gravels, and 1.5H:1V – 2H:1V sand or silt mixed with gravels 2H:1V Poorly graded sand - 2H:1V – 3H:1V Sandy clay soils, silty clay soils 2H:1V and stiff to hard clayey soils 3H:1V - Soft clay soils, plastic clays - - 3H:1V In more gently sloping ground, the side-slope can be relaxed to accommodate weaker material or earth fills. The common practice for side-slopes of earth fills is to design them with 2H:1V or lower angle as shown in Figure 3-21. Depending on the type of earth fill, rip-rap could be placed on the downslope side of the fill for additional slope protection. The size of the rock should be large enough that it withstands the tractive force of runoff water. A 300mm thick under-filter may also be needed to prevent potential piping. Environmentally, it is preferable to avoid high fills so that the road is less conspicuous in the landscape. Page 3-34 Ethiopian Roads Authority Geotechnical Design Manual - 2013 Chapter 3 Road Embankments Figure 3-20: Typical side-slopes of a rock fill embankment Figure 3-21: Typical side-slopes of an earth fill embankment The necessity for benching the existing natural slope beneath an embankment and their width and vertical spacing is decided on the basis of the length and inclination of the slope (often > 15%), material properties, groundwater conditions and other environmental factors. Fill benches are horizontal or near-horizontal steps that are constructed by cutting into the natural slope. In high embankments, berms up to 5m width may be constructed to give access to sideslopes for maintenance or for the control of surface drainage (Figure 3-22). They are also important to prevent erosion provided the surface water they collect is properly controlled. Steps or shear keys cut into the hill slope prior to embankment construction (Figure 3-23) increases the resistance to failure along the natural ground-fill interface. Ethiopian Roads Authority Page 3-35 Chapter 3 Road Embankments Geotechnical Design Manual – 2013 Figure 3-22: Fill embankment on a hill-side with outward dipping berms Figure 3-23: Embankment on inward inclined hill-side benches. From Keller and Sherar (2011) Page 3-36 Ethiopian Roads Authority except that they rely on their structural components to mobilize the dead weight of an embankment fill to derive their capacity to resist lateral loads. Retaining walls are generally classified as gravity. Benches can also be constructed both on the hill-side and fill slopes as shown in Figure 3-24 and made to incline outward. Ethiopian Roads Authority Page 3-37 . The gravity wall type includes rigid gravity walls. a geotextile separator may be used to separate the fill from slope materials. they normally dip outwards (Figure 3-22) by up to 5%. Ground anchors are connected directly to the wall structural components whereas the other three types of anchors are connected to the wall structural components through tie rods. The anchors may be ground anchors (tiebacks). The situations where these structures are most frequently used include when fill needs to be confined either because there is not enough space for its construction or where the hillside slope is too steep to form a stable fill slope. From JKR (2010) When berms are utilised. Anchored walls derive their capacity to resist lateral loads by their structural components being restrained by tension elements connected to anchors and additionally by partial embedment of their structural components into existing ground. In this case.2013 Chapter 3 Road Embankments Figure 3-24: Benched fill on a benched hill-side slope.Geotechnical Design Manual . Retaining walls may also be needed at the base of a bridge approach embankment. passive pile anchors. semi-gravity. 3. or pile group anchors. and nongravity cantilevered and anchored.10 Wall-supported Embankments Sometimes. Non-gravity cantilevered walls rely on structural components of the wall partially embedded in foundation materials or hill-side slope to mobilize passive resistance to resist lateral loads. and prefabricated modular gravity walls. mechanically stabilized earth (MSE) walls. passive concrete anchors. retaining walls are needed to support fill on hillside slopes. The purpose of these benches is mainly to drain water from the embankment. In general. Semigravity walls are similar to gravity walls. They may also be essential on road widening projects in urban areas. the direction of bench inclination should be decided by the engineer on site depending on the slope of the hill. and material properties. the type of the fill (earth or rock). Gravity walls derive their capacity to resist lateral loads through the dead weight of the wall. They have relatively narrow base widths. or reinforced concrete and can be used to retain both cut and fill slopes and failed slopes. Due to their rigidity. they should only be used where their foundations can be designed to limit total and differential settlements to acceptable values. Rigid gravity walls may be constructed of stone masonry. and cost. Figure 3-26 presents a schematic diagram of gravity and semi-gravity retaining structures. physical constraints of the site. settlement potential.Chapter 3 Road Embankments Geotechnical Design Manual – 2013 Figure 3-25: Typical use of retaining structures in road embankments Selection of appropriate wall type for embankment support is based on an assessment of the design loading. presence of deleterious soils or groundwater. cross-sectional geometry of the site both existing and planned. constructability. Page 3-38 Ethiopian Roads Authority . desired aesthetics. depth to adequate foundation support. mass concrete. They are most economical to support low height embankments of less than 6 m. maintenance. and are generally not used when deep foundations are required. These types of walls are suitable to stabilize fills but are most Ethiopian Roads Authority Page 3-39 .Geotechnical Design Manual . Often. Semi-gravity walls have relatively narrow base widths. They can accommodate drainage structures and utilities. The most common terminology associated with the design of semi-gravity structures is given in Figure 3-27. and cut slopes. usually consisting of partially embedded soldier piles or continuous sheet piles.2013 Chapter 3 Road Embankments Figure 3-26: Gravity and semi-gravity retaining walls Semi-gravity cantilever. Figure 3-27: Terminology associated with semi-gravity retaining walls Non-gravity cantilevered walls are constructed of vertical structural members. These walls can support embankments. precast concrete or steel piles placed in drilled holes and backfilled with concrete or cast-in-place reinforced concrete. The position of the wall stem relative to the footing can be varied to accommodate right-of-way constraints. and can be supported by both shallow and deep foundations. counterfort and buttress walls are constructed of reinforced concrete. Continuous sheet piles may be constructed with driven precast pre-stressed concrete sheet piles or steel sheet piles. they are most economical at wall heights of up to 10 m. Soldier piles may be constructed with driven steel piles. treated timber. sign structures. 12 Gabion Supported Embankments Gabion walls are constructed from rectangular steel wire mesh baskets that are filled on site with stone or rock to form a gravity retaining structure. cut or fill slopes up to 6m depending on whether they are constructed as a single unit or stepped structures. earthquake loads. and that the wall does not contain uncemented voids. they are not favoured immediately below sealed roads due to the likelihood of movement of the backfill and subsequent pavement cracking. These holes are generally provided at 1-2 m lateral and vertical intervals. are labour-intensive to construct and are usually very cost effective. which allows the gabion wall to deform to an extent without the boxes breaking or significant loss their strength. The permeability of the wall is improved by providing weep-holes. usually rock. Retaining walls should be designed to withstand lateral earth and water pressures. In general.5 . The cross section can be varied to suit site conditions. They are strengthened at the corners by higher gauge wire and mesh diaphragms that divide them into compartments. additional care should be taken to locate the base of the wall on a good foundation. Where gabion walls are nevertheless used to support sealed roads. that the cement mortar conforms to a strength criterion. gabion walls have the following advantages: • • • • Gabions can be easily stacked in different ways. Because of the narrow base width. They are inclined forwards at a slope of 2o – 3o. Because of their inherent flexibility. it is important to ensure that the stone is of good quality and is not significantly weathered. Masonry walls are very rigid and cannot tolerate large settlements. and where adequate founding conditions exist. movement joints should preferably be positioned to reflect the differing wall heights. constructed preferably using a 75mm polythene pipe. and in special cases. The wire should be galvanized. They are especially suited to uneven founding levels. 3.0. so that limited differential movements can be accommodated. If the wall foundation is stepped. The first row of weep holes should be as close to the base of the wall as possible. 3. The baskets usually have a double twisted hexagonal mesh. Mortared masonry walls are useful for the support of natural. in order to avoid potential movements.Chapter 3 Road Embankments Geotechnical Design Manual – 2013 suitable for cut slope support. with internal or external indentation improving stability They can accommodate some movement without significant loss of strength They allow free drainage through the wall. A mortared masonry wall design uses its own weight and base friction to balance the effect of earth pressures. the effects of surcharge loads. Gabion is commonly used for walls of up to 6 m high. The base width to height ratio of mortared masonry walls usually lies between 0. These structures use locally available rock. and sometimes PVC coated for greater durability. they are useful for situations with tight space or right-of-way constraints. Page 3-40 Ethiopian Roads Authority .75:1. the self-weight of the wall.11 Mortared Masonry Walls The most commonly used gravity retaining walls for embankment stabilization are mortared masonry and gabion walls. For high durability. Geotechnical Design Manual . This base width normally occupies about 40 to 60% of the height. In embankments. Figure 3-28: Typical types of gabion walls Gabion walls may be stepped on either the front or back face as shown in Figure 3-28. Ethiopian Roads Authority Page 3-41 . Often. In some cases. Stepping is ideal where space is not an issue as it increases stability. Their high degree of permeability can result in a loss of fines through the wall.2013 • Chapter 3 Road Embankments The boxes can take limited tensile forces to resist differential horizontal movement Their disadvantages include: Gabion walls need a large space to fit the wall base. it is advisable to provide outlet drains from the lowest point of the wall. the bases of gabion walls can be inclined by up to 6o into the slope to increase stability. it is not recommended to construct vertical faced gabion walls as any movement that occurs during backfilling may give the wall an appearance of bulging. and gabions may not be a good solution where space is limited. In order to keep the back-slope as dry as possible. The maximum recommended step at each course is half of the depth of the unit. this can result in settlement behind the wall and on the surface of the road. and ensure that drainage discharge can be visually inspected from these outlets into catch-pits. The design of both types is based on the same principles. Often this problem can be avoided by using a geotextile between the wall and the backfill as shown in Figure 3-28. Further compaction improvements can be obtained in cohesive soils through the use of geosynthetics with in-plane drainage capabilities (e. Often. resulting in a composite structure which can withstand both compressive and tensile forces. and polymer and wire grids are used as reinforcing elements. They are also used to replace conventional retaining walls and repair failed slopes. and often speeds construction. The most prominent use of reinforcement is. however. granular soils are relatively strong under compressive stresses.g. for widening and reconstruction of existing roads. This allows compaction equipment to operate more safely near the edge of the slope. However. When these soils are reinforced. Reinforcement also helps to provide lateral resistance at the edges of a compacted fill. They are used in new construction to steepen side slopes and increase embankment heights thereby reducing fill requirements (Figure 3-29). A geosynthetic reinforced wall face can sometimes be near-vertical depending on internal stability. Metallic strips. non-woven geotextiles) that allow for rapid pore pressure dissipation in the compacted fill. secondary geosynthetic compaction aids placed as intermediate layers between main reinforcements in steepened slopes. as shown in Figure 3-30. geosynthetics may provide a better solution because of reduced cost and ease of construction. The use of reinforced steepened slopes to widen roadways improves mass stability. Reinforced embankments have been used in road construction for many decades. may provide improved face stability and reduce the total cost of the use of geosynthetics.13 Reinforced Embankments When embankments are required to be constructed with steeper than usual slopes where limited right of way and construction constraints exist. eliminates additional rightof-way. Reinforced embankments permit increased height and angle of the embankment side slopes and may also reduce any potential slope failure passing through the foundation. Page 3-42 Ethiopian Roads Authority . In addition. significant tensile strength can be induced to the fill. Reinforced slopes are a form of mechanically stabilized earth that incorporate planar reinforcing elements in the constructed embankment with face inclinations of up to 70 degrees. then there is a need either to reinforce the slope or construct a retaining wall. geosynthetics.Chapter 3 Road Embankments Geotechnical Design Manual – 2013 3. Often. When the length is uniform and equal to the base width of the wall.Geotechnical Design Manual . rural areas where acquiring alternative agricultural land could be a problem. The spacing increases upward from the ground surface to the top of the embankment. In most situations.2013 Chapter 3 Road Embankments Figure 3-29: Application of reinforced slopes in road construction. there are a number of different calculations used to compute the spacing and anchorage lengths. From New York State DOT (2007) The spacing and anchorage length into the slope of secondary geosynthetics is determined on the basis of the geometry. the spacing is about 0. The anchorage length should also be greater than the deepest expected failure surface. In the stabilisation of landslides it may also be possible to re-use the slide debris rather than import high quality backfills. In the case of primary geosynthetic reinforcement. Right-of-way savings can be a substantial benefit. to reinforce the soil mass properly. and the engineering properties of in-situ soils.5 m and the anchorage length is greater than 1. In practice it is common to divide the reinforced mass into zones of constant reinforcement spacing (increasing towards the wall top from zone to zone) (Ortigao and Sayao 2004). The use of vegetated-faced reinforced soil slopes will blend with the environment and provide an aesthetic advantage over retaining wall Ethiopian Roads Authority Page 3-43 . The economic advantages of geosynthetic reinforced soils would normally be the resulting material and right-of-way savings. However. especially in hilly. the result is that the reinforcement spacing is inversely proportional to reinforcement depth. Reinforced embankments may also provide an economical alternative to retaining walls. the types of materials in the fill. the degree of stability required. the base width itself should satisfy all the stability conditions. loading and performance requirements for the design.5 m. this approach often adds construction difficulties. The reason is that this geometry provides a simple means of directly increasing the resistance to failure with the inclusion of reinforcement.Chapter 3 Road Embankments Geotechnical Design Manual – 2013 structures. foundation condition. The internal mode of failure occurs when the slip plane passes through reinforcing elements. availability of materials. The overall design requirements for reinforced embankment slopes are similar to those for unreinforced slopes. experience with a particular system or application and cost. That means the factor of safety must be adequate for both the shortterm and long-term conditions and for all possible modes of failure. environmental conditions. external and compound. aesthetics. Reinforcement may also bring about strength gains in soil layers over the course of time through improved drainage. In the case of the external mode. the failure surface passes behind and underneath the reinforced soil. Figure 3-30: Typical construction of reinforced fills. From NY State DOT (2007) The major factors that influence the selection of a reinforced embankment alternative for any road construction project include geologic and topographic conditions. However. a factor of safety equal to the rotational stability safety factor is applied to the reinforcement. there is a lower risk of long-term stability problems developing in reinforced slopes. The slope stability factor of safety is Page 3-44 Ethiopian Roads Authority . performance criteria. The reinforcement is represented by a concentrated force within the soil mass that intersects the potential failure surface. there are three possible failure modes (Figure 3-31) for reinforced slopes: internal. durability considerations. The tensile capacity of a reinforcement layer is considered as the minimum of its allowable pullout resistance behind the potential failure surface or its long-term allowable design strength. Generally. It is also adaptable by most slope stability computer programs and agrees well with experimental results. Most use rotational limit equilibrium methods. By adding the failure resistance provided by this force to the resistance of the soil. The failure is said to be compound when the failure surface passes behind and through the reinforced soil mass (Figure 3-31). size and nature of the structure. There are several approaches to the design of reinforced slopes. location of the ground water table and piezometric surfaces. compacted lift thickness. The fourth step is concerned with the evaluation of the design parameters for reinforcement. namely. ±2% of optimum moisture content. loading and performance requirements for design.e. strength parameters (cu and ϕu. the identification of the location of previous slip planes and cause of failure. It is defined as the ultimate strength (Tult) divided by the reduction factor (RF) for creep. unit weights γwet and γdry. Figure 3-31: Failure modes for reinforced soil embankments. the retained fill. RF = 7 may be conservatively used for preliminary design and routine. non-critical structures where the minimum test Ethiopian Roads Authority Page 3-45 . For granular backfill. external surcharge loads. there is a need to determine the engineering properties of in situ soils.Geotechnical Design Manual . Cr. In step two. Hence. or c’ and ϕ’). gradation and plasticity index. These include: the foundation and retained soil (i. The performance requirements are related to external. The flow chart presented in Figure 3-32 shows the steps required for design of reinforced soil slopes. or c′ and ϕ′) for each soil layer. overall stability. deep seated. compaction characteristics (γdry. From US DOT FHWA (2001) The first step is to establish the geometric. One of these parameters is the allowable geosynthetic strength (Tal). The geometric and loading requirements are the embankment slope height. shear strength parameters (cu and ϕu. soil beneath and behind reinforced zone). The safety factor for dynamic loading and the magnitude and time rate of post constriction settlement based on project requirements are also important. compound and internal stabilities. installation damage and durability. and internal failures need a factor of safety greater or equal to 1. external (sliding. compound. and ωopt). and traffic barriers. Final design is performed by distributing the reinforcement over the height of the slope and evaluating the external stability of the reinforced section. and for slide repair. and the chemical composition of the soil (pH).2013 Chapter 3 Road Embankments taken from the critical surface requiring the maximum amount of reinforcement.3. The third step involves the determination of the properties of reinforced fill and. consolidation parameters (Cc. if different. Cv and ∆p). slope angle. local bearing failure or lateral squeeze). Page 3-46 Ethiopian Roads Authority .Chapter 3 Road Embankments Geotechnical Design Manual – 2013 requirements are satisfied. From US DOT FHWA (2001) A preliminary design of reinforced embankments can also be made by the use of charts. Figure 3-32: Steps for design of reinforced soil slopes.5 for granular soils. 2 for cohesive soils. The second parameter is the pullout resistance that is modelled with a factor of safety of 1. The use of these charts can be extended for final design in low fills with a slope height of less than 6 m. and a minimum anchorage length (Le) of 1 m is also needed. retaining. They can also be a major source of sediment to local river systems. requiring major reinstatement and/or continued maintenance. bioengineering. piling and deep subsurface drainage. as illustrated in Figures 4-1 and 4-2. Techniques commonly used to control the occurrence of landslides include earthworks. Other options that need to be considered during design include avoidance through alignment selection. depending upon circumstances. Stabilization methods that need more substantial engineering works can involve anchoring.2013 4 Chapter 4 Roadside Slopes ROADSIDE SLOPES Unstable natural slopes and road cuts often create a considerable problem to road users in Ethiopia during the rainy season. Figure 4-1: Terms used commonly to define a road and associated slopes Ethiopian Roads Authority Page 4-1 . and surface drainage.Geotechnical Design Manual . These unstable slopes or landslides typically occur where a natural slope is over-steep or where cut slopes in weathered rock and soil encounter groundwater. Unless stabilized early. Slope stabilization and landslide control usually utilises a number of methods to mitigate the causes of failure. failed natural slopes and road cuts may enlarge and can affect large areas. roadside slopes are defined as those slopes that are either cut or fill slopes. together with measures to improve the stability of the slope. In this chapter. or adjacent natural slopes. both within and outside of the Right of Way. realignment or perhaps even tunnelling. but which can influence the stability of the road. Figure 4-3 presents an illustration of these different terms in the case of a toppling failure. These inactive slopes may be "dormant". Often. or they may be "stabilized" when the factors causing the movement have been removed or reduced naturally or artificially. Page 4-2 Ethiopian Roads Authority . As a result. even with a complete geotechnical study. A "relict" landslide is an inactive landslide which developed under climatic or slope conditions considered to be different from those that exist at present. Natural Slopes Many roads in Ethiopia are constructed in mountainous or hilly areas.1. Inactive landslides or slope failures are those that have failed in the past but for which there is no evidence of movement currently or in recent times. Table 4-1 describes different types of natural slopes and associated slope stability problems.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Figure 4-2: Natural and cut slopes adjacent to a road 4. Natural slope failures are commonly classified as actively unstable or previously unstable based on their history of failure. slopes that have been stable for many years may fail because of changes in stress conditions or the effects of road construction on drainage. the susceptibility of natural slopes to failure is little understood in advance of construction. when the causes of failure remain potentially recurrent and movement may occur again. and remain stable unless disturbed by toe erosion or road construction earthworks. Modified from Hearn and Hunt (2011) Facet Topographic features Rounded relief Sharp relief Ridge top Irregular relief Asymmetric relief Ridge lines generally Slope > 40º Slope = 25° . however.2013 Chapter 4 Roadside Slopes Table 4-1: Slope stability problems associated with natural slopes.Geotechnical Design Manual . are often the most problematic for road construction and maintenance. the engineering implication of a slope failure will vary according to whether it is a first-time failure or reactivated failure. First-time failures have an immediate effect on roads in their path but may not represent a long-term maintenance problem because the failed mass may come to rest at an angle significantly lower than that from which it failed. Reactivated slope failures.35° Continuous rock slopes with constant dip Valley sides Small farms or crop fields Elongated mid-slope benches Vegetated areas Slopes at margins of rivers Valley floor Forested slopes behind river terraces Typical stability problems Deeply weathered soils likely Landslides possible Rock outcrop possible Costly and difficult rock excavation Rock falls and slides possible Difficult alignment along ridge top Landslides possible Joint controlled slopes govern stability of alignment and cut slope Landslides possible May be subject to intense erosion Landslides possible Probably underlain by rock Landslides unlikely Colluvium or landslide material Landslides likely Joint sets control long-term stability May be problematic for excavation Rock slides possible Drainage problems likely May be colluvial in origin Potentially unstable Ancient river terraces or rock benches Stable or easy for road construction Landslides unlikely Possibly areas of wet ground Landslides possible Possibly actively unstable Very difficult for road alignment Possibly old periodically active instability Slope failures on active natural slopes may involve two or more mechanisms. occurring either at different places on the slope. Ethiopian Roads Authority Page 4-3 . at different depths or at different times due to changes in ground conditions once initial failures have occurred. On these slopes. e. In an advancing landslide the rupture surface extends in the direction of movement. In a retrogressive landslide the rupture surface extends in the direction opposite to the movement of the displaced material. An example of this is shown in Figure 412 later.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Figure 4-3: Illustration of the terms used to describe stages of slope failure. upslope. Most landslides that expand progressively in area are retrogressive. a landslide is said to be diminishing. i. This is relatively uncommon. Figure 4-4 illustrates these terms. The materials most likely to exhibit retrogressive failure are often clays and shale with some cohesive strength that allow steep back scarps to form that then become subject to failure. In a widening landslide the rupture surface extends into either or both flanks. When the volume of displaced materials is decreasing downward. From Cruden and Varnes (1996) Reactivation can be advancing or retrogressive. Page 4-4 Ethiopian Roads Authority . the alignment of the road and the design will usually require the excavation of slopes to accommodate or widen the roadway. earthquake. The shear strength along existing slip surfaces is lower than the peak for the materials involved. material weathering. Other influences (e. external loading and toe erosion) can also trigger slope failure. blasting. measures should be considered to increase stability. 4. Most natural slopes are in a state of equilibrium under normal climatic conditions. and is referred to as residual strength.Geotechnical Design Manual . Ethiopian Roads Authority Page 4-5 . but they frequently become unstable when either the ground conditions exposed during excavation are different from those envisaged during design or when steeper slopes have to be constructed due to space constraints. rainfall. In such cases. This equilibrium is often disturbed by slope excavation.2013 Chapter 4 Roadside Slopes Figure 4-4: Graphic description of the evolution and extent of slope failure. Cut slopes should be designed to be stable for the anticipated groundwater regime. From Cruden and Varnes (1996) In the case of inactive landslides.g. the knowledge that old slip surfaces exist may make it easier to understand and predict the behaviour of the slopes during road construction.2. Hence cut slopes should be designed to have enough resistance against the worst credible combination of conditions that may develop after excavation. Cut Slopes During road construction in mountainous areas and along valley sides. A box cut is required where the outside of the road formation is unable to daylight on the adjacent slope. as a result of topography and vertical alignment. such as on a steep hill crest. Often. the practice is that cut slopes need to be as steep as possible. the safe angle should be determined from an overall evaluation of the predominant soils encountered. In the event of uncertainty. should as a rule be transported and used elsewhere along the road rather than side-cast as spoil. slopes often contain a mix of materials and are heterogeneous. percent or degree. the inclinations of rock strata. the degree of weathering. In many cases. full cut. The information needed for the design of cut slopes includes the nature and strength of the materials that will be excavated. The selection of cross-section in any situation is dependent on topographical and geological conditions. accompanied by the relevant stabilization and protection measures. and a balance or partial balance of cut and fill (Figure 45). The steepness or inclination of cut slopes is normally expressed using a slope ratio. it is necessary to select a conservative angle of inclination.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Cut slope design requires the determination of the angle at which excavated slopes will remain stable for the assumed or observed groundwater conditions that will apply. as with all cuts. It is normally used where the ground must be cut through to avoid an overly steep road grade. The choice of cross-section dictates the geometric requirements for cut slope design. and the extent of joints or any other potential weaknesses. Figure 4-6 shows an example of a box cut in Ethiopia. In general. making it virtually impossible to determine the average or effective shear strength of the slope forming materials by laboratory tests. In these situations. it is difficult to adopt variable cut slope angles at a location where the soil type changes within short distances laterally or vertically. such as expensive retaining structures. The methods to use in the design of an individual cut slope will vary according to the conditions at the site. the groundwater conditions. Page 4-6 Ethiopian Roads Authority . it becomes more difficult to design safe cut slopes to steeper angles without the use of other measures. for low volume roads. In such cases. This involves the examination of soils and rock from surface exposures and studies of the performance of existing natural slopes and cut slopes in the area. stability analyses will help very little in determining the safe angle of cut. geometric requirements. and at a steeper angle than fills since they constitute relatively undisturbed in-situ soil or rock. are especially necessary when there are Right-of-Way or property line constraints. and the cut slope design must be accompanied by the application of judgment and experience. Steep cuts. the slope ratio is the ratio between the horizontal distance and the elevational change written in the form of H:V. In Ethiopia. The material excavated from a box cut. and cost. and options include box cut. In areas where there is a high water table condition or erosion and sliding is apparent. Geotechnical Design Manual . balancing is achieved along the centreline of the road. the excavated material is either used elsewhere along the alignment as fill or is properly disposed of in designated safe and environmentally secure disposal sites. This type of design is normally called a ‘cut and fill’ approach whereby materials cut from a hill slope are used to build an adjacent fill for the purpose of supporting part of the road. All cut material used as fill should be compacted to specification and not side-cast. other than those constructed in flat terrain. assuming the excavated material is suitable for use as fill. it is important to avoid side-casting of waste material and especially to prevent it from entering streams and other watercourses. In practice this is rarely the case as ground elevation with respect to the centreline can change abruptly and balanced cut and fill is normally used to describe the situation whereby the balance can be achieved within a short distance along the alignment.2013 Chapter 4 Roadside Slopes Figure 4-5: Types of cross-section design. During cut and fill construction. Modified from Keller and Sherar (2011) Most roads are constructed partially in cut and partially in fill. Theoretically. If the amount of material from excavations roughly matches the amount of material needed to make fill slopes and embankments. Ethiopian Roads Authority Page 4-7 . Full cut road construction is typically found in terrain with side slopes of 40 degrees and above. then the process is called a ‘balanced cut and fill’ operation. For full cuts. Similarly. The height and width of benches depends on the characteristics of the slope materials.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Figure 4-6: Example of a box cut Full cut cross-sections usually result in deep cut slopes. silty clays and other low strength soils may also be unsuitable for forming deep cut slopes. Benched cut slopes are usually deployed in rock and weathered rock and should not be used in weak soils.e. including soft or highly fractured/closely jointed sedimentary rocks may not be suitable for full-cut designs. Figure 4-8 shows a typical example of bench construction. As a rule. Cut slopes in these materials can result in significant slope instability. An example of a full cut cross-section is shown in Figure 4-7. i. In such situations fill slopes and retaining walls may need to be considered or the alignment modified accordingly. Page 4-8 Ethiopian Roads Authority . excavation should be performed from the upper part of the slope to the lower part to maintain stability. excavated in steps in order to enhance drainage control and intercept falling debris. such as colluvium. Often cut slopes are benched. and is usually specified in design and earthworks manuals. Weak rocks and rock masses with adverse rock structure. Water control is key in benched cut slopes and drainage from above must be transported laterally using ditches at the toe of each cut section and discharged into adjacent streams or via chutes and cascades into culverts. "topple". can be classified on the basis of the type of movement and material involved. The type of movement describes the internal mechanics of the landslide mass during displacement. both on natural slopes and in earthworks slopes. Landslides. "spread".2013 Chapter 4 Roadside Slopes Figure 4-7: Full cut cross-section Figure 4-8: Schematic profile of cut slope benches 4. debris or earth down a slope. Types of movement categories include "fall". "flow" and "creep". "slide". It results from the failure of materials which make up a natural slope or cut or fill slope and is driven by the force of gravity.Geotechnical Design Manual . Landslides are often described using two Ethiopian Roads Authority Page 4-9 .3. The material in a landslide mass is either a rock or soil (or both). Landslides A landslide is the movement of rock. Landslides can also be divided into shallow and deep-seated based on the depth of the slip plane. In Ethiopia. Methods to investigate a landslide are described in the ERA Site Investigation Manual. water content and the gradient of the slope. Hence. the movement of land slide material can vary from abrupt collapses to slow gradual slides and at rates which range from the almost undetectable to the extremely rapid. Often. Often. this period may extend into October and November as severe and localized rainfall can occur during this time. For example. Although these types of landslides are not usually a threat to people they can cause damage to roads and other infrastructure. Page 4-10 Ethiopian Roads Authority . these types of landslides involve the soil mantle Deep-seated landslides are those slides in which the slide plane is more likely to be within weathered rock. classifying landslides in practice is often very difficult. debris flow. The distance travelled by landslide material can also range significantly from a few centimetres to many kilometres depending on the volume. since both the type and rate of movement can change with motion. with some occurring very locally and impacting a very small area. Further discussion on shallow and deep-seated landslides is presented in the Site Investigation Manual of the Ethiopian Road Authority (ERA).1. this is most likely to happen during the wet season between July and the end of September. Types of landslides Table 4-2 summarises the different types of landslides. They can also vary in their extent. road section or hill slope while others affect much larger slopes and drainage channels beneath them.3. Landslides may also form a complex failure mechanism containing both rock and soil and encompassing more than one type of movement.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 terms that refer to material and movement type (for example: rock fall. In the southern part of the country. Slides with a sliding depth of less than 3-5 m are considered to be shallow. These are usually deeper than 5m. etc). While this classification system is useful as a general guide. deep-seated landslide or debris flow. Extremely slow moving landslides might move only millimetres a year and the movement can be active over many years. A brief description of the different types of landslides is given below. Sudden and rapid events are the most dangerous because of a lack of warning and the speed at which material can travel as well as the force of its resulting impact. landslides may range from a single boulder in a rock fall or topple to tens of millions of cubic metres of material in a large. there is often a need to survey the area and understand the history of ground movement before finalizing its classification. a landslide which is described as a flow often initiates through sliding along discrete shear surfaces. In terms of quantity. landslides often occur after periods of prolonged and heavy rainfall. Moreover. 4. 2. In some cases creep failures may represent a deep-seated (or viscous) flow. Modified from Cruden and Varnes (1996) Type of material Type of movement Rock Fall Rock fall Topple Rock topple Rock slide Rock slump Rock spread Rock flow Rock creep Slide Planar Rotational Spread Flow Creep Soil Debris Earth Debris fall -- Debris slide Debris slump --Debris flow --- -Earth (mud) slide Soil slump Soil spread Earth (mud) flow Soil creep A fall is a form of movement in which the mass in motion travels most of the distance through the air after detachment. Flows can be slowmoving. These failures are the most common. translational (the slip surface is more or less planar) or wedge (the slip surface is formed by two or more intersecting surfaces). Often. Translational slides occur in rock masses with discontinuity planes that dip unfavourably or in granular soils. A slide is a slope movement by which the material is displaced more or less coherently along well-defined shear surfaces. or rapid to very rapid in granular soils and failed rock masses. Slides can be rotational (the slip surface is curved). they start as a slide on steep slopes. Causes of landslides The susceptibility of a slope to the occurrence of landslides is often related to an increase in shear stress (driving force) or a reduction in shear strength (resisting force).3. Often. analogous to the movements in liquids. A fall starts with the detachment of material from a steep slope along a surface in which little or no shear displacement takes place. Creep is a long-term movement of non-increasing velocity without well-defined sliding surfaces. but does not involve a complete separation at the base of the failure A spread is characterized by the lateral extension of a more rigid mass over an underlying ground surface or through a deforming underlying material in which the controlling basal shear surface is often not well-defined. 4. The processes that increase the shear stress or decrease the shear strength in a slope are normally termed as landslide preparatory or triggering factors.Chapter 4 Roadside Slopes Geotechnical Design Manual . A topple is very similar to a fall in many aspects. Rotational slides are relatively infrequent and occur in cohesive soils or very weak rock masses with little or no structural control. A flow is characterized by internal deformation in which the individual particles travel separately within the mass. if in fine-grained soils. Toppling occurs as a result of forces that cause an over-turning moment about a point below the centre of gravity of the mass. either single or multiple. Preparatory Ethiopian Roads Authority Page 4-11 . it includes movement by bouncing.2013 Table 4-2: Simple classification of landslide types. a number of factors contribute to the occurrence of a landslide both on natural slopes and cut slopes.g. Triggering factors include rainfall. toe erosion and manmade activities. External causes are responsible for an increase in shear stress through an increase in the height and steepness of the slope. Table 4-4 shows a number of common natural and artificial preparatory and triggering factors. • Weak rocks with adversely orientated discontinuities characterized by low shear strength such as bedding planes and joints. Page 4-12 Ethiopian Roads Authority . An increase in pore pressure or a reduction of cohesion due to weathering are considered as internal causes of landslides. Increased loading: • Natural accumulations of water. However. it is usual to find that only one factor triggers the actual movement. • Shocks and vibrations (blasting). such as those caused by fill slope construction. External causes Removal of slope support: • Undercutting by water (waves and stream incision). The causes of landslides on natural slopes can be divided into external and internal. • Fine-grained soils which are subject to strength loss due to weathering. talus. Table 4-3: Common landslide causal factors. for example due to road cuts or river scour and down cutting. Table 4-3 lists some of the more common internal and external causes of landslides. Often. Internal causes are those which occur without any change in the external conditions of the slope. • Man-made pressures (e. fill and buildings). spoil dumping and earthquake loading. snow. • Washing out of soil. Triggering factors are events that finally initiate movement. Thus geological and topographical parameters are usually regarded as landslide preparatory factors. Transient effects: • Earthquakes and tremors. • Man-made excavations. Weathering: • Physical and chemical weathering of soils causing loss of strength (apparent cohesion and friction). Pore-water pressure: • High pore-water pressures causing a reduction in effective shear strength. They are associated with a loss of the shear strength of materials. Modified from Nettleton et al (2005) Internal causes Materials: • Soils subject to strength loss on contact with water or as a result of stress relief (strain softening).Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 (preconditioning) factors make the slope increasingly susceptible to failure without actually initiating movement. and any structural load increases. This triggering factor may be natural or artificial. earthquakes. Topographical causal factors may include steep hillsides which can promote gravity induced slope failures and concave slope facets which can hold moisture for long periods of time. mechanical weathering • Shrink and swell of expansive soils. lithological contacts) • Contrast in permeability and its effects on groundwater and pore-pressures • Contrast in stiffness (stiff. thus leading to lower effective stress and reduced strength.e. blasting. Geological causes may be jointed rocks exposed on rock slopes and in cliffs (Figure 4-9). i. Ethiopian Roads Authority Page 4-13 .2013 Table 4-4: Natural and artificial causes of landslides Natural causes Ground conditions: • Plastic and otherwise weak material • Sensitive and collapsible material • Weathered and sheared material • Jointed or fissured material • Adversely orientated structural discontinuities (including joints faults. Weathering may for example form a mantle of weak soils overlying harder rocks which provide an interface or potential shear surface along which landslides can initiate. etc • Drawdown (of reservoirs) • Irrigation • Defective maintenance of drainage systems • Side casting of uncompacted spoil Table 4-5 presents another way of dividing landslide causes based on geology. etc • Overloading slopes • Mining and quarrying activities • Vibrations from heavy traffic. or colluvium overlying rock head surfaces. dense material over less dense and weaker material) • Geomorphological processes: • Tectonic and volcanic uplift • Fluvial erosion of the slope toe • Subsoil erosion (solution. piping) • Natural loading of the slope by accumulation of material from above • Undercutting of cliffs and river banks Physical processes: • Intense. short period rainfall • Prolonged high precipitation • Rapid drawdown following floods • Increase in pore water pressure • Earthquake loading • Freeze-thaw. topography. or changes to. Artificial causes • Removal of vegetation • Interference with. natural drainage • Modification of slopes during the construction of roads. railways. buildings. flexural shears.Chapter 4 Roadside Slopes Geotechnical Design Manual . climate and hydrology. seepages and springs. The water may increase the weight on a slope thereby increasing the total stress or driving force. Some landslides are triggered by a rise in groundwater level during or soon after heavy rain. • Irregular depressions or undrained swampy areas on slopes. it may increase pore pressures within a slope or decrease the coefficient of friction on a potential sliding surface. weathering grade. a perched water table is created in unconsolidated soils that lie above an underlying impermeable rock stratum (Figure 4-10). The effect of groundwater on the initiation of landslides can occur in several ways. Climatic causes • Periods of prolonged and/or intense rainfall that could lead to saturation of the slope • Anomalous high rainfall. Perched water of this nature is common in the highlands of Ethiopia where closely jointed basaltic rock masses are present in association with weak volcano-clastic rocks. susceptible to erosion. Another contribution of groundwater to the occurrence of landslides is through the development of a seepage force. A common observation in many areas of Ethiopia is that a road cut or a natural slope may be perfectly stable during the dry season but may fail after the rains begin.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Table 4-5: Geological. particularly if this could cause toe erosion during periods of flood or high flow • The presence of a drainage course at or above the crest of the slope • Any indications of a high or temporarily perched water table within the slope. Alternatively. topographical and hydrological causal factors Geological causes • Rock type. • The persistence of joints and clay filling • The sequence of the underlying strata. contributing to the driving force. A seepage force is a drag force that moving water exerts on each soil particle in its path. This leads to a decrease in shear strength. Topographical causes • Steep slopes • Un-vegetated steep slopes. particularly if this includes weak or impermeable layers • The presence of colluvium and unconsolidated materials. The concept of the seepage force may be visualized by noting how easily portions of a coarse-textured soil may be dislodged from a road cut when the soil is transmitting a relatively high volume of groundwater. Groundwater can also cause clays to hydrate and expand and make them susceptible to failure. Often. particularly if bedrock is exposed or at a shallow depth beneath the surface. Page 4-14 Ethiopian Roads Authority . jointing and fracture patterns • Presence of faults or shear zones • The direction and angle of dip and joints in underlying bedrock compared to the angle and orientation to the slope.g. This seasonal change in stability is due mainly to the rapid increase in the level of groundwater within shallow perched aquifers. e. Hydrological causes • Periods of prolonged and/or intense rainfall that could lead to saturation of the slope • Anomalously high rainfall • The presence of a river or stream at the base of the slope. Geotechnical Design Manual . These include failures that affect the cut slope only. or through the road in the case of deep-seated failures. Ethiopian Roads Authority Page 4-15 .3. and those that are confined to the natural slope. Landslides which occur below the road can involve fill slopes as well as the underlying natural slope material. those which involve the cut slope and natural slope together. Landslides above the road can occur in the cut slope or can involve movement of the natural slope above. Figure 4-11illustrates landslides commonly observed above a road. Roadside landslides Landslides can occur either above or below the road. These types of failures are discussed below.3.2013 Chapter 4 Roadside Slopes Figure 4-9: Adversely jointed rock mass that can fail if joints become undercut by excavation Figure 4-10: Perched water table and the formation of landslides on road cuts 4. These failures are discussed in Chapter 3. Figure 4-13 shows a landslide developed in a deep cut slope formed in colluvium. ultimately affecting large areas of slope. Sometimes. Figure 4-12: Retrogressive landslide developed on a natural slope Most cut slope failures are shallow. From Hearn and Hunt (2011) An example of a retrogressive landslide developed on a natural slope is given in Figure 412. This landslide initiated in the weaker soils of the upper portion of the cut slope and travelled as a flow to block the road. even small. While most develop in the cut slope and propagate upslope (retrogression) some may initiate in the natural hill slope and spread downwards (advancing). Failures that involve the cut slope and the natural slope above are often shallow but can be retrogressive.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Figure 4-11: Landslides above a road. however. The reduction of support in the cut and the percolation of water from above through tension cracks are often responsible for failures that encompass the cut Page 4-16 Ethiopian Roads Authority . shallow failures can develop into major failures and result in frequent road closures. This is mainly due to inadequate compaction of the fill. Figure 4-14 shows a landslide which affected both the cut and the natural slope at the above. The materials most likely to exhibit retrogressive failures that encompass both natural and cut slopes are fissured clays where positive pore pressures are allowed to develop within the discontinuities.2013 Chapter 4 Roadside Slopes and natural slopes. Figure 4-13: Landslide on a cut slope Figure 4-14: Landslide affecting both the cut and natural slope above Fill slope failure commonly occurs either within the fill itself or at the fill/natural ground interface. or due to the development of a perched Ethiopian Roads Authority Page 4-17 . Knowledge of the presence of slip surfaces in natural slopes makes it easier to understand and predict the performance of cut slopes within previously failed material. leading to saturation by surface infiltration and shallow failure.Geotechnical Design Manual . Usually. hydrological. and the presence of a water source above the road that is hydraulically connected with a spring below the road.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 groundwater table resulting in a preferential slip plane at the fill/natural ground interface. benching of the natural ground prior to filling. with or without affecting the cut and natural slopes above. landslides may occur across the width of a road as shown in Figure 4-16. such as the Blue Nile. Page 4-18 Ethiopian Roads Authority . Figure 4-15: Failure of a fill slope Deep-seated landslides are common where elevational differences are large. and topographical conditions favour their development.3. should prevent such failures from occurring. reactivation of much larger movements of which the one affecting the road is only a part. a major triggering factor in the initiation and acceleration of these movements is toe erosion.4. Figure 4-15 depicts a fill slope failure that has extended to the road centreline. such as in deep gorges. These types of failure are most frequent in areas where the entire road formation is constructed on colluvium and weathered rock. or on fill. and the prior provision of drainage where seepages are seen to occur at the interface. Roadbed landslides Occasionally. 4. where the stratigraphical. In the latter case. irrigation or agricultural activity in the surrounding area can aggravate the problem through improper and uncontrolled drainage.2013 Chapter 4 Roadside Slopes Figure 4-16: Example of a failure affecting the entire road This type of ground movement also occurs where runoff percolates through road layers and soaks the sub-grade because of a drainage problem or the absence of sufficient crossdrains.Geotechnical Design Manual . In many cases. Features that indicate the occurrence of landslides within and above a road are summarized in Table 4-6. Ethiopian Roads Authority Page 4-19 . Stream erosion below the road can also worsen the situation. May indicate additional fences unstable area May indicate unstable slope above the road Blocked drainage May contribute water and loss of stability May cause distress May indicate unstable slope Loose debris on above the road slope. The selection of appropriate remedial measures depends on engineering feasibility. most remedial works usually involve a combination of two or more methods. compression May indicate additional ridges in slope unstable area Tilted features. Detailed discussions on some of the methods listed in Table 4-7 are given in Sections 4. the remedial measures must provide a reduction in the driving forces. While one remedial measure may be sufficient to minimize the effect of a landslide.3. The study and identification of landslides is normally carried out using the methods discussed in the ERA Site Investigation Manual. an increase in the resisting forces.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Table 4-6: Indications of slope instability at or above a road Things to look for Heave in pavement. or both.5. and environmental acceptability. Page 4-20 Ethiopian Roads Authority . Table 4-7 contains a list of the most common methods that can be used to stabilize or remedy the effects of landslides. stability Properly drain evaluated Avoid Monitor and record locations disturbing until and changes stability Properly drain evaluated Monitor and record locations and changes. Properly drain Monitor and record locations and changes Unblock drain Avoid Monitor and record locations disturbing until stability and changes evaluated 4. Landslide stabilisation Once a landslide is identified on a natural.6. cut or fill slope. Stabilise through Avoid cutting earthworks. May indicate unstable slope such as trees. economic viability. retaining walls and/or drainage Surface cracking May indicate unstable slope in surrounding above the road area May indicate additional unstable area May indicate unstable slope Springs above the road May contribute water and loss of stability May indicate unstable slope Bulging or above the road. Pavement elevation rising Implication Course of action May indicate unstable slope above the road and circular failure Monitor and record locations and changes.5 and 4. walls. Properly drain Monitor and record locations and changes. May indicate additional unstable area Things to avoid Avoid Monitor and record locations disturbing until and changes. above the road poles. • Reinforced earth retaining structures • Retention nets for rock slopes Drainage: • Surface drains to divert water from • Shear key. Generally. landslides may occur suddenly as the slope is being excavated. In cohesive soils. The shear strength parameters for these stability analyses are determined from laboratory tests and can be corroborated from empirical correlations.Geotechnical Design Manual . In areas where long lengths of road are affected by shallow instability. saturated slopes may be susceptible to debris slides and flow slides. From Sassa and Canuti (2008) Modification of slope geometry: Retaining structures: • Removing material from the area adjacent • Gravity retaining walls to the crest of the slope(with possible • Crib walls replacement by lightweight fill) • Gabion walls • Adding material to the area at the base of • Passive piles.4. When failure is confined to large landslides. In granular soils. flowing into the slide area Internal slope reinforcement: • Shallow or deep trench drains filled with • Rock bolts free-draining materials (coarse granular • Micropiles fills and geosynthetics) • Soil nailing • Buttress counterforts of coarse-grained • Anchors materials • Grouting • Vertical (large diameter) wells with • Stone columns gravity draining • Freezing • Sub-horizontal drains • Bioengineering. galleries or adits • Dewatering by pumping • Drainage by siphoning. 4. Soil slope cuts Soil slopes contain a variety of materials that may range from loose colluvial deposits to cohesive residual clays. the stability of cut slopes consisting of granular soils depends on the slope angle. Normally. the determination of cut slope angles based on experience is a more practical approach.2013 Chapter 4 Roadside Slopes Table 4-7: Common landslide remedial measures. • Drainage tunnels. the angle of internal friction. the inherent properties and structure of the parent rocks in the case of residual soils. Depending upon the rate of reduction in factor of safety. piers and caissons the slope(counterweight berm or fill) • Cast-in situ reinforced concrete walls • Reducing the overall slope angle. and pore pressures. and any secondary precipitates or cementing materials. Ethiopian Roads Authority Page 4-21 . wetting and drying (swelling and shrinkage) can result in the development of shrinkage cracks that allow water ingress. the processes of consolidation that these materials have undergone throughout their depositional history in the case of sediments. thus exacerbating the situation. the safe angles of excavation can be determined using simple slope analyses. The stability of these slopes in a road cut is usually a function of groundwater and drainage regime. or after the slope has been standing for some time. the unit weight of soil. the size and composition of the constituting materials. soil slopes that can stand for some time after excavation are those which contain appreciable amounts of clay with associated negative pore pressures. Shallow failures and ravelling of surface materials are a common feature of Ethiopian roads. and pore pressures. increasing driving forces. over the long term. and its magnitude depends on the rate of dissipation of this pore pressure. The loss in strength is attributed to reduction of negative pore pressure after excavation. explains why clayey cut slopes sometimes fail a long time after initial excavation. This increase is accompanied by swelling. therefore. raises pore pressures. The presence of groundwater within or just below a proposed cut will affect the slope angle required to achieve and maintain stability. on the ultimate groundwater level in the slope. Generally. The initial shear strength is often equal to the undrained shear strength on the assumption that no drainage occurs during excavation. which results in reduced shear strength. Figure 4-17 shows the general variations of shear strength. and causes a corresponding reduction in effective stress and shear strength. slope angle. In situ shear strength is a direct function of the maximum past overburden pressure for cuts in over-consolidated clays. the factor of safety decreases over time until an unstable condition is reached. The higher this pressure is. the pore pressure within a clay cut slope increases over time. load and factor of safety (stability) with time for a clay cut slope. the greater the shear strength. Long-term cut slope stability is also dependent on seepage forces and.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 In cohesive soil slopes. for the most part. its strength no longer depends on the prior loading and will decrease with time. Thus. Figure 4-17: Stability condition of a clay cut slope over time From Bishop and Bjerrum (1960) reproduced in Abramson et al (2002) Page 4-22 Ethiopian Roads Authority . This. It also adds weight to the soil mass. pore pressure. After excavation. stability is a function of consolidation history. soil density. This drawdown usually occurs rapidly in cut slopes made up of granular materials but is usually much slower in clay. Hence it is important to identify seepage within proposed cut slopes so that adequate drainage designs are prepared in advance. the groundwater surface will usually drop slowly to a stable zone at a variable depth below the new cut surface. In contrast to embankment slopes. However. groundwater adds to seepage forces. will be required. stability is largely independent of height. u. the stability of cut slopes in clay soils during design should be evaluated for both the end-of-construction and the long-term conditions.2013 Chapter 4 Roadside Slopes 4. when increase in pore water pressure and swelling and weakening of soils is complete. Ethiopian Roads Authority Page 4-23 . The short-term condition is computed using total stress methods (undrained condition). cuts where high groundwater or seepage forces are likely. the long-term condition is usually the more critical. As a result. performing accurate analyses of stability using effective stress procedures is often difficult. drained strength (c' and φ'). If the materials in which the cut is made are so highly permeable that these changes occur completely as excavation and construction proceeds. depending upon the risk they pose. For major roads and highways. In principle. In principle. For effective stress analyses. and cuts in old landslides or in areas susceptible to landslides should also be analysed using standard geotechnical approaches. For dry granular soils in which the anticipated mode of potential failure is planar. deep cuts and cuts with irregular geometry and varying stratigraphy (especially with weak layers). Cut slopes greater than 5 m in height may require geotechnical analysis. cuts involving soils with questionable strength. and therefore slope angle becomes the parameter of most interest. For purely cohesive soils. Design considerations Cuts slopes in soils up to a height of about 3 to5 m are generally designed based on past experience with similar materials in the surrounding area. it is virtually impossible to determine accurately what excess pore pressures will result from excavation. and saturated soils. Cuts along mountain roads in Ethiopia are usually in rock.1. the height of the cut becomes a critical design parameter. As shown in Figure 4-17. The strength parameters φ = 0 and cu (undrained strength) are used for total stress analysis assuming that the soil behaviour is exclusively cohesive. granular soil or in situ weathered residual soil. as will mainly be the case for Ethiopian conditions. Any cut slope where failure would result in large rehabilitation costs or threaten public safety should normally be designed using rigorous techniques. however. the long-term condition should be analysed using effective stress methods assuming that excess pore pressures dissipate completely during that time.Geotechnical Design Manual . the stability of a cut slope in clay soil decreases with time after excavation as pore pressures increase and the soils within the slope swell and become weaker. The major parameters in relation to design of cut slopes are the slope angle and height of the cut and its constituent materials and water condition. For soils with both cohesion and friction. it is advisable to analyse stability using effective stress methods since the strengths of soils are governed by effective stresses under both undrained and drained conditions. slope stability is dependent on both slope angle and the height of the cut. Also critical to the proper design of cut slopes is the groundwater level (pore pressure) and its fluctuation. If the cut is designed to be permanent. the critical condition for stability of cut slopes is normally the long-term condition. In practice. Observations and experience in the performance of cut slopes in the region provides knowledge of stable cut slope angles and common stability issues affecting large cuttings in soil. the end-ofconstruction and the long-term conditions are the same.4. In many cases it would be acceptable to assume c' = 0. Because of this. In addition. along with pore pressure. The advantages of steep slopes is that they result in less Right-of-Way width. less excavated material. the use of the cut slope angles shown in Table 4-8 or other customized values should target a design factor of safety of 1. even for long-term cases. The reason for using a factor of safety greater than one is to take into account the variation of soil properties in slopes and the uncertainty over longer term groundwater conditions.4.1 may be used. Their disadvantages are that they are difficult to vegetate and are more prone to failure. and can be especially important in places where the heterogeneity of the soil and rock layers makes any kind of stability analysis very difficult. then the slope angles should be flatter than those given in this table. If near surface groundwater or seepage water exists or is expected to occur during the design life of the slope. the cut slope angle should be varied. as would be the case for low trafficked roads. Figure 4-18: Typical cut slope ratios in most soil types Generally. profiles and benches Table 4-8 shows cut slope angles (H:V) recommended for preliminary design purposes. to take advantage of stronger materials.3. If temporary road closures and debris clearance can be tolerated. Cut slope angles. Page 4-24 Ethiopian Roads Authority . These values are fairly close to the long-term equilibrium conditions. then steeper slopes with a factor of safety of 1. less spoil disposal.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 4. or appropriate drainage measures should be taken. and a shorter length of slope exposed to erosion. This factor of safety is considered as a minimum value for static roadside slope stability. They can normally be used for relatively dry slopes. A schematic illustration of these slope ratios is shown in Figure 4-18.2. where possible. When two different soil layers are present. 10 10 . or to contain falling debris from one bench to another. sub-angular cobbles and gravels in a fine matrix).75:1 1.5:1 2:1 1. multi-sloped or benched (Figure 4-19). Keeping the slope dry is extremely important. Benched slopes are designed when there is a need to slow down and intercept surface runoff.2013 Table 4-8 Soil cut slope ratios (H:V) for preliminary design purposes Type of Material Cutslope total height (m) 3–6 6 . 1:1 1. Keep a constant slope.Chapter 4 Roadside Slopes Geotechnical Design Manual . Appropriate drainage and vegetation is necessary. This issue and whether higher factors of safety are needed. should be decided on a case by case basis depending on the consequences of failure. Singlesloped profiles are usually cut in dense soils with enough resistance against failure. Multi-sloped profiles are cut where an excavation encounters soil overlying rock or where the stratigraphy consists of two or more soil or rock layers with different strength characteristics. gravels and sands in a fine matrix) Loose to medium dense transported soils (boulders. Ethiopian Roads Authority Page 4-25 . past experience with similar soils.5:1 2:1 Cover the slope with grass and other suitable plants and keep the slope dry. Note: Slope angles are indicative and require site-specific assessment.5:1 2:1 Reduce the upper portion to 1:1 to limit gully formation or widening.15 Residual clay soils 1:1 1:1 Heavy. Roadside cut slopes are generally not designed for seismic conditions unless slope failure could impact major structures. There is no fixed rule regarding the appropriate dimension of a bench. Granular soils with some clays Dense transported soils (subangular cobbles. or talus 2:1 --------- Remark Consider benching when the slope height is above 6 m. Good practice might be to provide a bench width equal to one third of the height of the cut immediately above it. Cut slope profiles can be single-sloped. plastic clay soils 1.5:1 2:1 0. as would be the case for an excavation in jointed rock. Vegetation cover is highly recommended. and uncertainties in the analyses. Their height is often limited to 6 m. Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Figure 4-19: Types of cut-slope profiles For benches to perform properly. Page 4-26 Ethiopian Roads Authority . Generally. Talus (scree) on the other hand is a term commonly used to describe broken rock fragments derived from rock cliffs and deposited at their angle of repose in a cone or an apron below the exposed face. Most failures in colluvium are planar. Benches should be well maintained and cleaned regularly to ensure that they will not create a problem by blocking drainage.3. Care should be taken to avoid undercutting planes of weakness or over-steepening weak soil or rock layers. Retrogression is also common in these soils. temporarily perched water tables. they consist of a heterogeneous mixture of soils and rock fragments ranging in size from clay particles to rock boulders. Among the possible causes of movement is the reduction in shear strength at the interface between the colluvium and the residual soil/bedrock interface due to the increase in pore pressures. Reactivation is sudden and can occur anytime during heavy rain or if support is removed in excavations. or a uniform slope selected instead. and can be rendered unstable by heavy rainfall. they should be designed with an inward slope of 3-5% and have a longitudinal gradient.4. The benched slope should be designed accordingly. The composition of colluvial deposits varies according to the nature of the bedrock sources and the climate under which weathering and transport occurred. in terms of drainage and collecting falling debris. 4. It has become widely misused to include heterogeneous transported soils of mixed origin and mixed sizes to the extent that any transported soil on a slope is now acceptably referred to as colluvium. Colluvium and talus deposits are often only marginally stable. and potential for renewed movement. Colluvium and talus slopes Colluvium is a term used to describe a fine-grained slope deposit. or other external factors. although slumps and flows can also be encountered. Both colluvium and talus deposits are characterized by high permeability and compressibility. these soils have not been disturbed and can exhibit an additional cohesion associated with the residual rock structure. They are especially common on the hill sides of deep gorges such as those of the Blue Nile basin. Often.5:1) for a cut height of less than 10m as shown in Table 4-8. More frequently. The pore pressure distribution can be represented by the pore pressure ratio (ru).Geotechnical Design Manual . 4. the pore pressure distribution may best be represented by a groundwater or perched water table. On most mountain hillsides. Figure 4-21 illustrates the formation of residual soils and the associated effect of erosion and consolidation. Road cuts in colluvium and talus deposits are expected to be stable if they have a slope of 33o (1. Figure 4-20 shows an example of a road cut on a colluvial slope along the Gundewein to Mekhane Selam road in the Blue Nile Gorge. Residual soils Residual soils are formed by in-situ weathering of rocks. which is the ratio of the water pressure to the weight per unit area of the overburden at any point within the soil mass. which increases their strength. If a relatively thin colluvial cover overlies bedrock or a residual soil layer on inclined ground. Reliable representation of pore pressure distribution is critical in the stability analysis of colluvial slopes.4. although the results can be misleading if large boulders are present. Ethiopian Roads Authority Page 4-27 . good indication of the strength of colluvium is obtained by pit excavation and Standard Penetration Tests. Unlike colluvium.4.2013 Chapter 4 Roadside Slopes Figure 4-20: Road cut on a colluvial slope The shear strength of colluvium is generally governed by the matrix of granular and finegrained fractions. colluvium is treated as a cohesionless material. They often occur on flat ridge tops and structural benches where in situ weathering has predominated over erosion and transportation. it is advisable to remove it during cutting if it is practical to do so. and usually (though not always) contain a high proportion of clays. Colluvial deposits frequently occur in the highlands of Ethiopia. residual soils (Weathering Grade IV and V) may present special problems with respect to slope stability and erosion. Such soils may contain relict structures from the original bedrock that act as planes of weakness and their strength properties may vary significantly over short distances due to variations in weathering grade. Page 4-28 Ethiopian Roads Authority . again due to relict structural variations in the soil mass and variations in weathering grade and rock head level. Groundwater also tends to be complex within in situ weathered soils. Because of this. Generally. these soils can often be cut steeply for appreciable heights. it may be difficult to determine design shear strength parameters from laboratory tests.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Figure 4-21: Simplified illustration of the formation of residual soils Depending on rock type and climate. Because of the cohesive nature of the fine grained component of Weathering Grade V-VI materials and the maintenance of soil suctions when slopes are kept largely dry. Figure 4-22 shows a 1:1 road cut slope through residual red clays. Weathering Grade V materials are often highly prone to erosion. In this case. slopes formed in in situ weathered soils should be designed using both total and effective stress approaches to assess short-term strength and also long-term stability. shear strength parameters should be determined by back-analyzing slope failures and by using empirical design procedures based on local experience. failure can occur either during or sometime after the rainfall ends. however. any percolation from the top of cuttings can lead to softening and slope failure. even with no increase in vertical pressure. Figure 4-23: Failure in black cotton soil due to infiltration of water 4. they experience much larger settlements.Geotechnical Design Manual . On slopes. Collapsible soils Collapsible soils undergo a reduction in volume upon wetting. Collapsible soils are present in the central and southern part of the Ethiopian Rift Valley in the form of silty loess-type deposits. Such soils have considerable strength and stiffness in their dry natural state and can withstand a large applied vertical stress with a small compression. Pore pressures in residual soil slopes often react quickly to heavy rainfall and any infiltration can eliminate soil tensions and increase positive pore pressure by raising either perched water or groundwater tables.2013 Chapter 4 Roadside Slopes Figure 4-22: Relatively steep 1:1 cut slope in residual clay In Ethiopia. These soils contain a large amount of void space. They have usually a low dry density and low moisture content. When they are wet. They can stand at steep slopes (0.4. as illustrated in Figure 4-23.5.5:1) Ethiopian Roads Authority Page 4-29 . residual soils are found in many parts of the country usually associated with basaltic rocks that can weather easily. Along roads. and particles are held together by the clay component. 4. may undergo large rapid deformations that can result in slope failures. Figure 4-24: Cut through weathered rock and residual soils In Ethiopia. The amount of erosion that occurs on a slope is a function of soil type. and southern part of the country. including especially areas around Assossa and many places in western Gojam. known as laterite. All cut slopes should be designed with adequate drainage to limit erosion and piping as much as possible. Latosols Latosols (often referred to incorrectly as laterites) form a group comprising a wide variety of red. However. from medium dense soils to rock-like material. Slope failures in collapsible soils can occur as either shallow planar slides or minor slumps. Page 4-30 Ethiopian Roads Authority . the soil loses its strength and because of the open internal structure. They vary significantly in density and texture.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 indefinitely provided the moisture content remains low. as long as they remain dry and protected from surface erosion and piping through drainage control. brown. irreversible hardening occurs. and yellow. They are characterized by the presence of iron and aluminium oxides or hydroxides responsible for the red colour of the soil. slope angle. Often. rainfall intensity. length of slope. For cuts in collapsible soils. producing a laterite material suitable for use as a building or road stone. When some of these soils (plinthites) are exposed to the air.6. and vegetative cover. upon wetting. lateritic soils are distributed in the north western. western. fine grained in situ weathered tropical soils (Weathering Grade IV and VI) as well as nodular gravels (concretions) and cemented soils (ferricretes and bauxites). in situ weathered and other sensitive soils. Surface erosion and subsurface piping are most common problems on slope cuts containing collapsible. consideration should be given to preventing earthwork activity during the wet season. depending upon weathering history. The stiff nature of some in situ weathered tropical soils allows steep cuts to be formed to a height of 3 to 6 m. strength under field conditions tends to be much higher than is indicated by the laboratory tests.4. These slopes can remain stable for long periods of time. 5. The role of discontinuities The term discontinuity is used for all structural breaks in geological materials with negligible or zero tensile strength. Intact rock made up of minerals susceptible to weathering is usually weak. and discontinuity orientation and friction angle usually control rock slope stability. It is often difficult to reproduce field conditions in the laboratory. the strength of intact rock blocks. persistence. spacing. joints. infillings.Geotechnical Design Manual . If the orientations of the discontinuity surfaces are inclined out of the slope (daylight). Rock Cut Slopes Roads in mountainous areas often require rock excavation. Table 4-10 summarizes the characteristics of these discontinuities. the degree and extent of the rock mass weathering. Orientation is measured in the field using dip and dip direction (or strike) as shown in Figure 4-25. They include bedding planes.2013 Chapter 4 Roadside Slopes 4. It depends on the physical properties of the constituent minerals and the bond and cementation that exist. foliations and shear zones. faults.1. It is an important parameter because it defines the potential volume of the failure mass. they can have a significantly negative influence on the stability of slopes. Others composed of minerals such as quartz are relatively strong. in situ tests such as point load or Schmidt Hammer rebound tests may be needed to estimate intact rock strengths and correlate them with unconfined compression values. The orientation of discontinuities with respect to applied loads can be critical to deformation or stability. 4. Persistence refers to the lateral extent of a discontinuity as is shown in Figure 4-25. and roughness. Seepage pressures are also controlled by discontinuity orientation and connectivity. and the groundwater conditions. rock cut slope stability is influenced by a number of factors such as the type of rock. Intact rock strength is the strength of a block of rock or specimen that is not affected by discontinuities. Ethiopian Roads Authority Page 4-31 . aperture. The influence and shear strength of discontinuities in a rock mass are controlled by their orientation. Discontinuities present in a rock mass are the most significant factors affecting rock slope stability and the primary controlling parameters for rock cut slope design. Unlike soil slopes. the characteristics of discontinuities. Hence.5. In most rock masses (except where discontinuities are widely spaced) intact rock strength usually exceeds the strength of discontinuities. Recording trace lengths to describe joint persistence is useful in determining the stability and ultimate cutting angles of deep excavations. They are planar fractures formed to relieve stresses. Joints are developed to some degree in almost all rocks. or any combination of these. It also controls the mode of failure. When a set can be distinguished from parallel or sub-parallel joints.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Table 4-9: Types of discontinuities and their characteristics. forming joints that mark distinct terrains. striations. they are distinct physical discontinuities. Foliation is a structural property exhibited only in metamorphic rock types. They may occur at the interface between different rock types at various spacings within a single rock unit. Discontinuities associated with cleavage are likely to be smooth and continuous. mylonite. They may be persistent and generally extend over greater areas than any other type of discontinuity. Faulting often produces continuous or persistent planes of weakness. they are likely to be a major factor controlling slope stability. Faults can provide the shearing or release surfaces for several modes of failure. A structural break where differential movement has occurred along a surface or zone of geological failure. Spacing is a required input to several rock mass classification systems. Page 4-32 Ethiopian Roads Authority . From GWP Consultants (2008) Discontinuity Bedding planes Joints Faults Foliation Shear zones Descriptions Arising from the deposition of sediments in layers. across which there has been little or no movement. the true or apparent spacing can be measured as illustrated in Figure 4-25. Faulting can occur in any rock. Within the rocks affected. Faults occur less frequently than joints and may have undergone substantial displacements. Spacing affects block size and geometry in the rock mass. Spacing is the distance between two discontinuities of the same set measured normal to the discontinuity surface. Slate. The spacing of adjacent joints largely controls the size of individual blocks of intact rock. crushed and often weathered material (termed ‘gouge’). crystalline metamorphic rock and tightly folded sedimentary rocks show closely spaced laminations which are not directly related to bedding features. slicken sides. They often occur at the edges of tectonic blocks. gouge. Jointing plays some part in the majority of slope failures in rock masses since intact rock is generally stronger than the discontinuities. Fault zones may develop in which the fault occurs as a series of displacement surfaces in an area of distorted. In some rock types. Shear zones involve volumes of rock deformed by shear stress under brittle-ductile or ductile conditions. The nature and inclination of bedding is always of prime importance when considering slope stability in sedimentary rocks. characterized by polished surfaces. movements along bedding planes may have developed weakened shear zones. breccia. Hence. and planar in metre scale. The shear strength and any associated slope failure are also controlled by the relative positioning of these asperities. The shape and roughness of a discontinuity constitute its surface characteristics. degree of alteration. The degree of fracturing of a rock mass is described by fracture density. since even small steps or undulations. smooth. Fracture frequency on the other Ethiopian Roads Authority Page 4-33 . or rough.2013 Chapter 4 Roadside Slopes Figure 4-25: Description of different discontinuity parameters Aperture is the openness or separation within a given discontinuity and is measured normal to the discontinuity surfaces. strongly influence the potential for sliding along that surface.Geotechnical Design Manual . Roughness is a measure of the surface unevenness and waviness of the discontinuity relative to its mean plane. This effect is most pronounced when the asperities are oriented perpendicularly to the direction of sliding. known as asperities. their type or composition. and seepage characteristics. It can be a small scale surface irregularity or unevenness. Descriptions of infillings are site specific but must address the thickness of fillings. undulating. and hardness. deformability. and slickensided in centimetre scale. Discontinuity strength is rarely the same in all directions. fracture frequency and rock quality designation (RQD). or a large scale undulation or waviness. Roughness affects the shear strength of the discontinuity directly and is an important parameter for stability analyses. while movement through two fully interlocked joints is very unlikely. Secondary minerals such as quartz and calcite may provide significant cohesion to the rock mass if present as infillings. Materials that fill the aperture of discontinuities are termed as infillings. The presence of infillings affects the permeability and shear strength of a discontinuity. Aperture affects the strength. Fracture density is based on the spacing between all natural fractures in an exposure or in cores from boreholes. failure can occur where waves and undulations are in point contact. It can also be described as stepped. Figure 4-26 illustrates the system of determining RQD from rock cores and outcrops. RQD is a measure of the degree rock fracturing.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 hand is defined as the number of discontinuities per metre length. Core recovered mostly in lengths from 100 to 300 mm with most lengths about 200 mm. It is defined as the percentage of rock cores that have a length equal or greater than 100 mm over the total drill length. Page 4-34 Ethiopian Roads Authority . highly weathered. and jointed rocks yield lower RQD values. The length of cores is measured along the centreline of the core. Lengths average from 30 to 100 mm with fragmented intervals. Core recovered mostly in lengths less than 100 mm.11. ??? = ∑ ?????ℎ??????? ≥ 100 ?? × 100% ???????????? If cores are broken by handling or the drilling process. or core recovered mostly in lengths greater than 1 m. Core recovered mostly in lengths from 300 mm to 1 m with few scattered lengths less than 300 mm or greater than 1 m. sheared. Core recovered mostly as chips and fragments with a few scattered short core lengths. soft. Common descriptions of fracture density are summarized in Table 4. fractured. Modified from US DOI Bureau of Reclamation (1998) Fractured density Unfractured Slightly fractured Moderately fractured Highly fractured Extremely fractured Description No observed fractures. Table 4-10: Fracture density. the broken pieces are fitted together and counted as one piece (provided that they form the requisite length of 100 mm). but it should be noted that core recovery is also dependent upon the strength of the rock mass and the method of drilling. Weathering is important in rock slope design because it may be the primary criterion for determining cut slope angle. directional weathering along permeable joints. Each weathering grade boundary marks sharp or gradational changes in geotechnical properties. Examples include differential weathering within a single rock unit. compressibility. absorption. Porosity.Geotechnical Design Manual . The role of weathering Rock weathering takes place by mechanical and chemical processes and involves the disintegration and decomposition of a rock mass. although zones of differential weathering can occur and may modify the weathering profile. it affects the physical and mechanical properties of rock masses. differential weathering of contact zones associated with thermal effects such as interflow zones within volcanic rocks. Weathering effects generally decrease with depth.5. Weathering in rock masses often starts along discontinuity surfaces. apparently due to relatively higher permeability along discontinuities. along which weathering penetrate more deeply into the rock mass. and bedding planes.2. depth. and differential weathering because of topographic effects. density. faults. Hence. method and ease of excavation. differential weathering due to compositional or textural differences.2013 Chapter 4 Roadside Slopes Figure 4-26: Graphical illustration of the rock quality designation (RQD) 4. Weathering generally is indicated by changes in the colour and texture of the body of the Ethiopian Roads Authority Page 4-35 . The degree of weathering is governed by the interaction between the rock mass and different internal and external factors such as geology and climate. as it advances deep into rock masses through joints and bedding planes. shear and compressive strengths. and resistance to erosion are the major engineering parameters influenced by weathering. and use of excavated materials. A low porosity closed jointed rock mass may experience a rapid rise in groundwater level of several metres as a result of only a few millimetres of rain since infiltrating water is concentrated in a small number of fractures with low aperture. which can contribute to slope failure.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 rock. thus reducing the shear strength. orientation and persistence of discontinuities on groundwater levels. The ERA Site Investigation Manual describes weathering classification in the field. Water pressure within discontinuities that run roughly parallel to a slope also increases the driving forces.3. high angle faults or steeply dipping strata act as barriers. Water levels may be significantly different on either side of these barriers. there is a high degree of connection of voids and groundwater levels vary only gradually over large areas. the presence of water in both intact rock and discontinuities can promote weathering thus reducing shear strengths. which are closely spaced. Where a rock mass has many discontinuity sets.5. Seemingly erratic groundwater levels can also develop when geological dykes and sills. and condition of fracture fillings and surfaces. The role of groundwater Groundwater occupying the discontinuities within a rock mass can significantly reduce the stability of a rock slope. in rock masses with only a few sets of discontinuities and where the discontinuity spacing is large. the flow can erode material below the seepage level. and physical properties such as hardness. Water pressure within a discontinuity reduces the effective normal stress acting on the plane. Further. or can erode infilling materials. water pressures can vary appreciably from one fracture to the next. Surface recharge can have a marked effect on groundwater levels in rock slopes. the groundwater may behave much as it would in a soil. 4. Page 4-36 Ethiopian Roads Authority . However. grain boundary conditions. Figure 4-27 illustrates the effect of type. Where groundwater intercepts the slope face. colour. particularly those rock types with low slake durability. Determination of the permeability of the rock strata is important because the discharge of water from slopes along a highway can necessitate the requirement for increased maintenance as the result of pavement deterioration and the need for higher capacity drainage systems. and the presence of artesian pressures.2013 Chapter 4 Roadside Slopes Figure 4-27: Effect of discontinuity characteristics on groundwater level Determination of groundwater levels and pressures includes measurements of the elevation of the groundwater surface and variations of this elevation based on seasonal fluctuation. Water pressure in rock slopes reduces the stability of the slope by reducing the available shear strength of potential failure surfaces.28 shows an example of a rock slide initiated by groundwater seepage. Also important is the location of perched water tables. Ethiopian Roads Authority Page 4-37 . increasing pore pressures and resulting in an increased rock fall potential and the potential for more large scale failures. causes materials to lose strength over time. Changes in moisture content of the rock. the location of aquifers. Thermal expansion or freezing of groundwater causes ice wedging and may effectively block drainage of discontinuities in the rock mass. Figure 4.Geotechnical Design Manual . Planar failure is governed by a single discontinuity surface dipping out of the slope face (Figure 4-29). A single block with potential for sliding along a single plane represents the simplest of planar sliding.5. Page 4-38 Ethiopian Roads Authority . wedge.4. and circular.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Figure 4-28: Rock slide initiated by groundwater seepage out of the slope 4. toppling. Failure will occur if the joint plane intersects the slope and dips at an angle greater than the angle of internal friction (ᵠ) of the joint surface. The mechanism is kinematically possible in cases where at least one joint set strikes approximately parallel to the slope strike and dips toward the excavation slope. The modes of failure common in rock cuts can be divided into planar (translational). Mode of failures The mode of rock slope failure is primarily controlled by the orientation and spacing of discontinuities within the rock mass as well as the orientation of the excavation and the angle of inclination of the slope. Wedge failure involves a failure mass defined by two discontinuities with a line of intersection that is inclined out of the slope (Figure 4-29).2013 Chapter 4 Roadside Slopes Figure 4-29: Modes of failure in rock slopes. In addition. The parallel joints may or may not be continuous. In the case of continuous parallel joints. Ethiopian Roads Authority Page 4-39 .Geotechnical Design Manual . at least one joint plane must intersect the slope plane to result in failure. a second set of joints is necessary to act as release joints. this mode of failure requires that the dip angle of at least one joint-intersect is greater than the friction angle of the joint surfaces and that the line of joint intersection intersects the plane of the slope. However. Modified from GWP Consultants (2008) Planar failure along stepped planes is possible in cases where a series of closely spaced parallel joints strike approximately parallel to the excavation slope and dip toward the excavation slope. These release joints must also strike more or less parallel to the slope and the magnitude and direction of the joint dip angle must be such that the joint plane does not intersect the slope plane. or in weak rock such as marls and shale. however. If frequent cross joints are present. This needs to be contrasted. In the absence of cross jointing. the layers can topple as rigid columns. and because the cut height is generally less and the excavation footprint smaller. Toppling failure involves slabs or columns of rock defined by discontinuities that dip steeply into the slope face.5. Although conceptually possible. such as constructability. potential environmental impacts.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Multiple wedges can be formed by the intersection of four or more sets of discontinuities. Layer separation may be rapid or gradual. Circular failures are commonly associated with soil slopes. 4. highly fractured rock masses.30. the sliding failure of a multiple wedge system rarely occurs because of the potential for kinematic constraint. require a smaller volume of rock excavation.5. each layer tends to bend down-slope under its own weight thus generating flexural cracks. may be perceived as a more environmentally sensitive design. It involves overturning or rotation of rock layers. Steeper cut slope angles result in lower construction costs. but they may also occur in highly weathered and decomposed rock masses. In practice. toppling is usually initiated by layer separation with movement in the direction of the excavation. Closely spaced. Page 4-40 Ethiopian Roads Authority . with the increased risk of slope failure. In either case. Rapid separation is associated with block weight and/or stress relief while gradual separation is linked with environmental processes such as thermal expansion and freeze/thaw cycles. economics. require less Right-of-Way. An example of toppling failure in a layered and vertically jointed limestone is given in Figure 4. and the accepted level of risk. characteristics of discontinuities and groundwater conditions. the design process is a balance between stability and other considerations. steeply dipping discontinuity sets that dip away from the slope surface are necessary prerequisites for toppling. Design considerations Successful rock slope design demands a sound and complete understanding of the combined influence of intact rock strengths. other factors such as the potential for rock fall and aspects of drainage. ease of construction and maintenance should also be considered. Table 4-12 gives a summary of the factors that need to be considered during design. provision would need to be made for rock fall containment. The factor of safety is primarily dependent on the geometry of the potential slip plane (mode of failure) with respect to the cut slope orientation and profile and the shear strength along this plane. although in steep terrain there is a limit to the area that can be made available for this purpose and a compromise may be required. Superimposing a slightly steeper cut onto an already steep natural slope may present problems associated with difficult access to begin the excavation. Weathering. Over a period of time the rock fall quantity should ordinarily reduce. a ‘sliver cut’ may be unsafe to excavate because the narrow work area provides inadequate room for construction equipment. Rock falls from cut slopes are generally more dominant in the early years after excavation when the rock slope has been freshly cut.2013 Chapter 4 Roadside Slopes Figure 4-30: Example of toppling failure Usually. land take (Right of Way). a rock slope is assessed to be stable or unstable on the basis of the factor of safety. In addition. The natural establishment of vegetation also tends to slow any rock fall after a period of time. Furthermore. As in the case of soil slopes. Alternatively. rock cuts made steeper than their discontinuities may require expensive measures to maintain stability. relaxation and rainfall can lead to rock slope failures exposing fresh rock and inducing further rock falls. Ethiopian Roads Authority Page 4-41 .Geotechnical Design Manual . Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Table 4-11: Aspects to be considered during rock cut slope design. blasting) often weakens the rock mass. particularly at bench locations if they are considered in the design. Benches if provided should be as wide as possible (preferably at least 2 min fresh rock) in order to contain small rock falls and debris and allow access for maintenance. drainage ditches should be provided along benches. The option of providing a catch ditch or a rock fall barrier at road level may not necessarily take up significantly more space than the provision of benches on the slope. with consequent reduction of erosion and infiltration. Controlled techniques (e. possible bouncing of rock fall should be assessed. If stability is assured. the rock face should be mapped in detail by experienced engineering geologists. The classification of the slope and the formation of design units are generally based on slope material properties.g. Bench drainage ditches would reduce the velocity and volume of runoff on the slope. the potential risk of rock fall can be difficult to assess. and weathering. The maintenance of benches is such that they should be regularly cleaned. especially during rainy seasons. including provision of safe access should be considered and agreed during design. The construction process (e. The practice of relying on generalised assumptions about strength of discontinuities and groundwater may not adequately cater for local zones in the rock mass. the degree of weathering.g. and fault zones should be given special attention in the design of rock cuts. Unless it is considered that there is no potential for surface erosion. For large rock slopes. a catch ditch or fence should be provided along the toe of the cut to contain falling rocks. Generally. Page 4-42 Ethiopian Roads Authority . Benches should be provided between batters of significantly different gradients. Benches constructed without drainage provision may encourage infiltration of surface water. Stability Potential rock fall Drainage Land take (Right of Way) Construction Vegetation Maintenance The overall cut slope profile (slope angle and possible use of benches) should be determined to ensure stability. The slope gradient and the topsoil should support vegetation growth. and the influence of groundwater. Where benches are provided. This should be considered as an alternative. It starts with adequate exploration of the project site within which each rock slope along the road is divided into design units. In areas where failure can lead to major damage to properties and structures or significant road blockage. pre-splitting. discontinuity conditions. bedding planes and sheet joints. If practicable and economically feasible. trimming) should be employed to minimize damage. Benches are generally not necessary in massive hard rock from a stability point of view. Aspects of maintenance inspections and works. hand scaling. weak weathered seams. Benches can facilitate access for maintenance. groundwater condition. Major discontinuities such as adversely dipping persistent joints. this should be considered. taking into account the stratigraphy and characteristics of discontinuities in the rock mass. the stratigraphy of the slope. leading to reduced stability A rock slope without benches can minimize land take. the design of rock cut slopes is a progressive process. 6. may decrease slope stability significantly by increasing the feasibility of movement along joint sets. Ethiopian Roads Authority Page 4-43 .5H:1V or less) may be needed.2013 Chapter 4 Roadside Slopes A design unit is defined as a portion of a roadside slope that can be cut at a consistent angle with or without benches. material properties. infillings. However.Geotechnical Design Manual . a suitable cut slope for a design unit can also be decided by inspecting existing cuts in similar materials along the proposed route or adjacent alignments. persistence. spacing. Once design units are formed. when the geological conditions affecting slope stability are favourable and constructability is not a controlling factor. and groundwater seepage requiring the same drainage measures. more stable slope (0. discontinuity characteristics. etc) in the entire unit should be allowed for in the design. any design option in rocks should target a minimum factor of safety of 1. even slight. The difference in discontinuity characteristics (orientation. Table 4-13 provides cut slope angles developed from the condition of rock cuts in Ethiopia and considering the type of rocks in the country. In such cases. Safe angles of cut and benches In a procedure that does not involve detailed rock slope stability analyses. 4. Like soil slopes. In general. groundwater conditions and the degree of weathering should also be the same. a steep cut (0. It is important to note that changes to the orientations of cut slopes. Alternatively. Other factors being equal. the economic advantages afforded by a steeper slope can be lost and a flatter. The degree of weathering should also be the same throughout. rock quality designation (RQD) or fracture frequency can be used in the field to assess the overall stability of a design unit and the safe angle of excavation.3. the safest and most economical design which best accommodates the site geology and project-specific constraints is prepared. It may contain a single rock type or different rock types of similar intact rock strength.5. This outcome might require rock fall mitigation and protection measures to compensate. new cuts can be formed at the same slope as stable existing cuts.25H:1V) may be possible. 50:1 Remark Remove hanging blocks before cutting.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Table 4-12: Indicative maximum cut slope angles (H:V) for rock slopes (without discontinuity control) Type of Material Fractured and slightly to moderately weathered basalt Fractured and weathered basalt in blocks. 1. drainage systems are needed to keep the slope dry. For discontinuity-controlled slopes. thickly bedded limestone with intercalations Cut-slope height <10m 10-15m 15-20m 0. with poorly defined stratification 0. cobbles and gravels of all sizes in a matrix of silt or clay Weathered tuff. When the rock is fresh and no significant weathering is observed.50:1 1.25:1 Slightly weathered and fractured mudstone and 1.25:1 1.75:1 1. If differential weathering is observed or anticipated. poorly cemented sandstone.00:1 marl Slightly weathered. 2.25:1 0. volcanic breccias or agglomerate Fractured. similar cut slope angles may be used for one or more design units. slightly weathered.25:1 0.50:1 0.00:1 Controlled or trim blasting is recommended so as not to damage the rock behind the cut face. Hence.00:1 1. the use of variable cut slope angles is advisable.50:1 Remove weathered and unconsolidated portion at low angles.00:1 mudstone 0. benches and drainage are needed at different heights.25:1 1.50:1 1.00:1 Apply controlled excavation. Critical factors to consider when fixing bench dimension are summarized in Table 4-13. Benches may also be required.00:1 Shale has a characteristic of being deformed when wet. vertically or laterally. Drainage is important to keep the slope dry. each will need to be assessed on a case by case basis. 1.00:1 1.00:1 1. Page 4-44 Ethiopian Roads Authority . Reinforcement works and erosion controlling measures may be needed.25:1 Weathered. highly friable shale and jointed 1.50:1 1. This minimizes the formation of overhangs 1. Installing a bench drain along the contact between competent and incompetent rock units where groundwater is anticipated is advisable. Figure 4. Ripping can be done in close proximity to populated areas where noise and vibration from breaking and blasting are restricted. The height of benches is dictated generally by stability issues (including rock fall and safe working arrangements for scaling the face).Geotechnical Design Manual . • • • • • • • • Benches on rock slopes should be at least 2 m wide and sloped in wards to retain falling blocks and to facilitate drainage For incompetent materials thicker than 3 m. Ethiopian Roads Authority Page 4-45 .31 shows the situations when these activities are applied during rock excavation. benches must have suitable drainage measures included in the design. ripping. breaking or blasting.2013 Chapter 4 Roadside Slopes Table 4-13: Factors controlling bench dimension on rock slopes. Access for maintenance equipment should be provided to the lowest (foot) bench. engineering judgment should be used to determine the sitespecific bench size required. Ripping is a process of breaking up rock and soil with a large tooth or teeth attached to the back of a bulldozer. Engineering judgment must be used to determine site-specific minimum thickness of a weathered rock unit or a weak layer bed that requires benching. the following points should be considered during the design of benched cut slopes. For ease of working. Benches should always be maintained and regularly cleaned. and if feasible to all higher benches. Ripping is also less dangerous and requires fewer precautions. In relatively unweathered rock. or slope.7. Benches must be adjusted during construction to follow changes in the character of discontinuities and bedding surfaces. The bench face angle. Modified from GWP Consultants (2008) Bench width Bench height Bench inclination The optimum width of a bench depends on geotechnical parameters and the purpose of the bench itself. benches should be made wider as necessary based on specific conditions. it is advisable to limit the height of benches to 6m. 4.5. the configuration of benches must consider drainage issues. Generally. Methods of rock excavation Rock excavation for road cuts is normally carried out by techniques of digging. In all cases. Where permeable formations overlie impermeable ones (including areas of fractured flow). debris removal) and this should be accommodated in the design. a minimum bench width of 2 m is recommended. Ripping is generally preferred over breaking and blasting because it is considerably less expensive. Digging is used in high weathered and relatively weak rocks. depends on stability and rock properties (including permeability and weathering). For inter-layered rocks. Benches usually require access for maintenance (ditch cleaning. ripping is limited to weak and fractured rock. and blast-hole diameter and pattern. an excavator can be used to construct the designed slope.5. a hydraulic hammer can be used in most rock types. In hard rock with few discontinuities. Page 4-46 Ethiopian Roads Authority . spacing. The meaning of some of these terms is graphically described in Figure 4-32. 4. From Pettifer and Fookes (1994) However. which in itself is controlled by factors such as side burden.1 Blast design The degree of rock blasting depends on blast design. Once rock is loosened by ripping. sub-drilling. light blasting is sometimes performed before ripping or breaking. Blasting is accomplished by discharging an explosive placed in a borehole (confined) or in mud capping boulders (unconfined). Like a ripper. A confined charge uses high gas energy. In areas where the amount of rock needed to be excavated is high. although when shaping of a slope face is needed. it works best in weak and moderately to highly fractured rock. Breaking is carried out with a hydraulic hammer (also known as a breaker or hoe ram) fitted to an excavator. whereas an unconfined charge works by shock energy output. rock blasting may be used. stemming (collar distance).7.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Figure 4-31: General guide to select a method of rock excavation. delays. hole depth. Existing discontinuities in the rock act as presplit lines. It is used to break up rock in areas where blasting is prohibited due to environmental or other constraints. minimizing hammer induced scars and fractures while creating a slope face that appears to be naturally weathered. Spacing that is too close causes crushing and ponding between holes. Hole depths greater than four times the side burden are also undesirable. measured perpendicular to the side burden. Figure 4-32: Blast design parameters. and excessive ground vibrations. as poorly controlled blasting will result. In road construction. but it is common practice to use drill cuttings because of availability and economics. Spacing is defined as the distance between adjacent blast holes. Too large a side burden will produce inadequate fragmentation.5 and 4. because of the short. crushed gravel works best as stemming.2013 Chapter 4 Roadside Slopes The side burden is defined as the distance from a blast hole to the nearest free face of the rock mass at the instant of detonation. Well-graded. Spacing that is too wide causes inadequate fracturing between holes. Sub-drilling is the distance drilled below the floor level to ensure that the full face of rock is removed. This zone is usually filled with an inert material to give some confinement to the explosive gases and to reduce air-blast. Modified from US DOI Bureau of Reclamation (1998) Ethiopian Roads Authority Page 4-47 .0. thick shape of the side burden.Geotechnical Design Manual . the side burden and the blast-hole depth (or bench height) must be reasonably compatible. gives coarse and uneven fragmentation. toe problems. A stemming that is too short results in excessive violence in the form of airblast and fly-rock and may cause back-break (breaking beyond the desired limiting wall). Stemming that is too long creates large blocks in the upper part of the rock mass. Spacing is calculated as a function of the side burden and also depends on the timing between holes. A hole depth less than 1. The rule of thumb for bench blasting is that the hole depth-to-side burden ratio should be between 1. An insufficient side burden will cause excessive air-blast and fly-rock. Stemming is the distance from the top of the explosive charge to the collar of the blast hole. sub-drilling is generally limited to 10% or less of the bench height (H). and is accompanied by humps on the rock face In any blast design. toe problems.5 times the side burden causes excessive air blast and fly rock and. 7. The density of rock is a major factor to determine how much explosive is needed to displace a given volume (powder factor). the perimeter holes of a blast should be aligned with principal joint sets. the density and tensile strength of which is in between those of basalt and sandstone.5. the presence of a pronounced horizontal foliation. In a bed of hard material greater than 1 m thick. Open bedding planes. Jointing is probably the most significant parameter in blasting design. and vibration indicate a hard rock that will be difficult to break. Denser rocks such as basalt require smaller ratios (higher powder factors). This produces a more stable excavation. especially where the jointing is persistent. accompanied by a lack of cuttings or return of water or air. bedding plane. Blasting in limestone usually results in fine fragmentation and dust. Fast penetration and a quiet drill indicate a weaker. Bedding has a pronounced effect on both the fragmentation and the stability of the excavation perimeter. Lighter materials such as some sandstone can be blasted with higher ratios (lower powder factors). often results in a very blocky blasted mass because the joint planes tend to isolate large blocks in place. Lack of cuttings or return water may also indicate the presence of an open bedding plane. Figure 4-33 shows the effect of blasting in limestone. Where possible. excessive drill noise. Total lack of resistance to penetration. more broken zone of rock. it is often beneficial to load an explosive of higher density than is used in the remainder of the hole. or joint can be conveniently used for the bench floor. joint or fissure.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 4. A detailed drill log that contains information on drilling resistance and the depth of various geological boundaries and discontinuities is required to effectively design a blast. Close jointing usually results in good fragmentation. open joints. indicates that the drill has encountered a void. Widely spaced joints. changing the powder factor. Slow penetration. Rows of holes perpendicular to a primary joint set produce an unstable perimeter. Page 4-48 Ethiopian Roads Authority .2 The role of geology Rock mass properties are critical variables affecting the design and results of a blast. or beds of weaker materials should be treated as zones of weakness. Aside burden-to-charge diameter ratio of 25 to 30 is used for average density rocks. The burden-to-charge (hole) diameter ratio varies with rock density. In bench blasting. Alternate zones of hard and weak rock exposed for the same explosive energy usually result in unacceptably blocky fragmentation and unnecessary displacement of the hard rock. Alternatively. This allows a better powder distribution in the rock mass. However. and created some overhangs. In this case. when joints dip into the excavation wall. the top of the powder column should be checked frequently as loading proceeds. Ethiopian Roads Authority Page 4-49 . Displacement can also occur if there is a void between the bedding in hard rock. Figure 4-34 shows an example of a thickly bedded rock mass displaced outward by about 1 m because of excessive blasting. advancing the opening perpendicular to dipping beds may be a compromise. When charging the blast-hole. In this case blasting has also widened vertical joints and solution cavities. and shears pose serious problems to blasting because explosive energy always seeks the path of least resistance. inert stemming materials rather than explosives should be loaded through these weak zones and voids. A void exists if the column fails to rise as expected. If the presence and exact depth of voids is in doubt. Blast hole cut-offs (part of a column of explosives not fired) caused by differential movement along joints may also occur. undetected voids and zones of weakness such as solution cavities. However. The explosive charges should always be concentrated in the hard strata and stemming should be added through weak zones and voids. the stability of the slope is enhanced.2013 Chapter 4 Roadside Slopes Figure 4-33: Blasting in limestone Steeply dipping joint sets that daylight in the cut face can cause stability problems and difficulty in breaking the toe of the excavation. The presence of weak rocks can be known by observing adjacent exposures or information from boreholes. “mud” seams. In this situation. the inclination can still create toe problems because the toe rock tends to break along bedding or foliation planes. the best approach is to use smaller blast holes with smaller blast pattern dimensions. resulting in poor fragmentation. By contrast.Geotechnical Design Manual . it is possible to use techniques of controlled blasting. Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Figure 4-34: Rock displacement as a result of uncontrolled blasting 4.5.7.3 Controlled blasting Depending on the purpose of rock excavation and the degree of stability required, two types of blasting are commonly used for road construction. These are bulk (massive or heavy) blasting and controlled blasting. Bulk blasting uses large explosive charges that are designed to fragment a large amount of side burden. Hence, it typically creates radial fractures around the blast hole and back-breaks (fractures that extend into the final slope face), which reduce the strength of the remaining rock mass and increase its susceptibility to slope ravelling and rock fall. Bulk blasting should only be used for road aggregate production where stability is not a concern. Controlled blasting is used for removing material along the final slope face without disturbing the general stability of the rock mass. It creates less back-break than bulk blasting because it removes less side burden and uses more tightly spaced drill holes with lighter charges. In some cases, it is also used before bulk blasting to create an artificial fracture along the final cut slope, which will prevent the radial cracks caused by the latter from penetrating back into the finished face. There are different types of controlled blasting techniques. The difference between these techniques is most importantly in the amount of side burden they remove and the type of powder they use. The most common techniques are pre-splitting blasting, smooth blasting, and cushion blasting. A summary of the advantages and limitations of these methods is given in Table 4-15. Page 4-50 Ethiopian Roads Authority Geotechnical Design Manual - 2013 Chapter 4 Roadside Slopes Table 4-14: Controlled blasting techniques. From US DOT FHWA CFLHD (2011) Procedure Pre-splitting Smooth blasting Cushion blasting Description Advantages Protects the final cut by Presplit holes are blasted. producing a fracture plane Procedure uses small along the final slope face. diameter holes at close Can produce steeper cuts spacing and lightly loaded with less maintenance with distributed charges. issues. Performs well in hard competent rock Limitations The small diameter borings limit the blasting depth to shallow depths (15 m). Borehole traces are present for entire length of boring. Does not perform well in highly fractured, weak rock. The small diameter boring Produces smooth, stable limits blasting to shallow Smooth blast holes are slope. Can be done on depths (15 m). Borehole blasted after main blasts. slopes years after initial traces are present for much of Procedure uses small construction. Drill hole the boring length. Does not diameter holes at close traces are less apparent than protect the slope from damage spacing and lightly loaded pre-splitting. Performs best caused by main blasting. with distributed charges. in hard, competent rock. Does not perform well in highly fractured, weak rock. Cushion blasting is done Reduces the amount of Radial fractures are more after main blasts. Larger radial fracturing around the abundant than presplit and drill holes are used with borehole and also reduces smooth blasting. Slope face is small diameter, lightly borehole traces. The large more prone to ravelling. loaded distributed loads. diameter holes allow Borehole traces still apparent Space around the blasting depths up to 30 m. in hard, competent rock. explosive is filled with Produces a ragged final crushed rock to cushion slope face. Performs well in the explosive force. all rock types Pre-splitting is used before bulk blasting to protect the final rock face from damage caused by the latter. Pre-splitting creates a fracture plane along the final slope face, which prevents the radial cracks created by bulk blasting from penetrating into the finished face. Without pre-splitting, bulk blasting damage can extend up to a considerable depth into the final slope face. Pre-splitting also allows for steeper and more stable cuts than any other blasting procedure. In massively bedded, competent rock, such as the limestone shown in Figure 434, a properly charged presplit blast will contain drill hole half cast for almost the entire length of the blast line and will have no back-break because the energy from the blast will travel uniformly, thus creating a continuous fracture between holes. Pre-splitting requires relatively small drill holes, from 50 to 100mm in diameter because the goal is to create discrete fractures, not massive breaking. However, because the small hole diameter allows the drill bit to deviate from the anticipated line more readily than a larger drill diameter, the maximum depth of pre-splitting is usually about 15 m. For this reason pre-splitting is used only for relatively small blasting operations. Because of these limitations, pre-splitting is most often used on slopes steeper than 1H:1V, which helps the drillers to maintain adequate hole alignment at depth. Pre-splitting performs best in competent, hard to extremely hard rock. It is most difficult in highly fractured, weathered, and/or weak rocks, where there is a requirement for the use of closely spaced drill holes and/or uncharged guide holes. Table 4-16 gives a summary of the parameters used to carry out pre-splitting. Theoretically, the side burden for presplit blasting is unlimited. But in reality, variations in geology that are not visible on the outer Ethiopian Roads Authority Page 4-51 Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 face of the slope can limit that burden. Thus, the condition of the rock in the interior of the slope needs to be ascertained before determining the blasting design. In any case, a minimum of 10 m of side burden is recommended for any presplit blasting procedure. Table 4-15: Parameters used for drilling in presplit, smooth and cushion blasting. From US DOT FHWA CFLHD (2011) Blasting method Pre-splitting Smooth blasting Cushion blasting Hole (mm) 38-44 50-64 75-90 100 38-44 diameter Spacing (m) Side Burden (m) 0.3-0.46 0.46-0.6 0.6-1.0 0.6-1.2 0.6 --------1 Explosive charge (kg/m) 0.03-0.1 0.03-0.1 0.05-0.23 0.23-0.34 0.05-0.55 50 0.75 1.06 0.05-0.55 50-64 75-90 100-115 127-140 152-165 1.0 1.2 1.5 1.8 2.1 1.2 1.5 1.8 2.1 2.7 0.03-0.1 0.05-0.2 0.1-0.3 0.3-0.45 0.45-0.7 Smooth blasting, also called contour blasting or perimeter blasting, can be used before bulk blasting as an alternative to pre-splitting. It is also used after bulk blasting, either as an entirely different event or as the last delay of the bulk blast. Smooth blasting uses drill holes with roughly the same diameter and depth as those used in pre-splitting, spaced slightly further apart and loaded with a slightly larger charge density. If the side burden is adequately reduced, smooth blasting produces a more ragged slope face with minimal back-break. Smooth-blasted slopes may require more maintenance than presplit slopes due to increased radial fractures from the controlled blasting and overall fracturing from bulk blasting. Smooth blasting is best preformed in hard, competent rock, although it can be used in weak or highly fractured rock by increasing the spacing of the drill holes and/or adding uncharged guide holes to the pattern. The advantages of reducing over-break usually outweigh the cost of the additional perimeter or guide holes. Smooth slope blasting can be used on a variety of cut slope angles and is effective in developing contoured slopes with benches or other slope variations. The spacings and side burdens commonly used to drill for smooth blasting operation are given in Table 4-16. Cushion blasting, sometimes referred to as trim blasting, uses a row of lightly loaded “buffer” holes filled with crushed stone over the entire depth of the hole, which reduce the impact on blasting holes and protect the surrounding rock mass from the shock caused by the blast, thus minimizing the stress and fractures in the finished slope face. The maximum diameter for cushion holes used in road projects is typically 75 mm. The drill steel used to advance these smaller holes tends to drift at depth, meaning the maximum depth is usually held to 12 m. Cushion blasting creates some back-break, which can make a slope more prone to ravelling and rock fall. Page 4-52 Ethiopian Roads Authority To study the effect of groundwater conditions. A factor of safety of 1. In road construction. These include conventional limit equilibrium methods. Both in initial stability (first-time failure) and back analyses procedures.6.Geotechnical Design Manual . • • • The strength and density of soil materials and the configuration of soil layers. The groundwater table and/or soil moisture condition.2013 Chapter 4 Roadside Slopes Cushion blasting is more demanding than presplit or smooth blasting because the hole spacing. To enable the redesign of failed slopes and the planning and design of preventive and remedial measures (Chowdury et al 2009). Suggested blast parameters for conducting cushion blasting are summarized in Table 4-16. and stability charts. Soil Slope Stability Analyses Soil slope stability analysis is an iterative process through which traditionally a factor of safety (FS) is computed and potential sliding surfaces are determined. There are different methods that can be used to analyze the stability of soil slopes. probabilistic approaches. A slope stability analysis in soil commonly requires the following parameters to be determined: The depth and configuration of the failure surface in the slope section. sensitivity analysis is sometimes performed by varying the water table according to drainage measures and the strength parameters to assess their effects on factor of safety. burden. To evaluate the possible occurrence of landslides. numerical (finite element and finite difference) analyses. cut slopes and embankments. and charge density must be carefully chosen and continually reassessed in order to minimize back-break. the primary purpose of slope stability analysis is the contribution to the safe design of cut and embankment slopes. if the pre-failure topography can be reasonably deduced. Cushion blasting produces better results than smooth blasting or pre-splitting in poorly lithified. To assess the stability of cut and embankment slopes under short-term (often during construction) and long-term conditions. For failed slopes. the strength parameters and the groundwater condition. To assess the stability of existing landslides and natural slopes.0 is assumed and. and the influence of environmental factors. back analysis is undertaken to determine the condition of the slope at the time of failure. seismic loadings and other factors on natural slopes. It is concerned with identifying critical soil properties that govern the stability of these slopes. 4. Some of the objectives of slope stability analyses are summarized as follows: • • • • • • To understand the development of natural slopes and the processes responsible for modifying these slopes. It can also be more time consuming because more drilling is required and charges take more time to load. Ethiopian Roads Authority Page 4-53 . then the unknown parameters are reduced to the failure surface location. moderately to highly fractured and weathered rocks. An accurate cross-section for analysis. cohesion. they can be used to obtain reasonably accurate answers for the analysis of a variety of short-term and long-term conditions if irregular slopes are approximated by simple slopes. i. Users are strongly encouraged to refer to these books. moments. Although the charts assume simple slopes and uniform soil conditions. and pore water pressures are assigned specific values. Often two-dimensional sections are analyzed and plane strain conditions are assumed. The stability of slopes can be analyzed quickly using stability charts. 4. A comparison is made between forces. Probabilistic approaches consider the magnitudes of uncertainties related to shear strengths and other parameters involved in computing the factors of safety. Limit equilibrium methods Most limit equilibrium methods are based on the principles of statics. They assume the validity of Coulomb's failure criterion along an assumed failure surface and utilize the Mohr‐Coulomb expression to determine the shear strength along the sliding surface. With the advance in technology. The method is particularly useful for soil-structure interaction problems. Most of the methods of stability analysis currently in use for slope design fall in this category. providing a means of evaluating the degree of uncertainty associated with the computed factor of safety. In probabilistic methods. and those that resist instability.1. vertical forces.e.6. external loads. or stresses tending to cause instability of the mass. their use allows addressing issues beyond those tackled by deterministic methods. the shear strength. The finite element method can be used to compute stresses and displacements in earth structures. Use of the finite element method for stability problems needs software developed for this purpose. The procedure in limit equilibrium methods is that a free body of the slope is considered to be acted upon by known or assumed forces. This manual focuses on limit equilibrium methods. the possibility that these parameters may vary is considered. The shear stress at which a soil fails in shear is defined as the shear strength of the soil. slope geometry. The requirements for static equilibrium of the soil mass are used to compute a Page 4-54 Ethiopian Roads Authority . In the traditional (deterministic) approaches. and friction angle are used (Duncan et al 1987). summation of moments. Shear stresses induced on the assumed failure surface by the body and external forces are compared with the available shear strength of the material.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 The conventional limit equilibrium methods of soil slope stability analysis in geotechnical practice investigate the equilibrium of a soil mass tending to move down slope under the influence of gravity. These methods assume that the shear strengths of the materials along the potential failure surface are governed by linear (Mohr-Coulomb) or nonlinear relationships between shear strength and the normal stress on the failure surface. the use of relatively complex finite difference computer programs for stability analyses is also becoming common to refine results obtained from traditional methods. but the computed stresses in a slope can be used to compute a factor of safety. and horizontal forces. in which structural members interact with a soil mass. The stability of a slope cannot be determined directly from finite element analyses. Although probabilistic techniques may not be required for routine design purposes. Discussions on numerical and probabilistic approaches are available in many geotechnical engineering reference books. and carefully determined average values of unit weight. with one Ethiopian Roads Authority Page 4-55 . the soil mass above the assumed slip surface is divided into vertical slices for purposes of convenience in the analysis. Some methods are simple. For slopes composed of uniform cohesionless soils (c' = 0). The infinite slope method assumes that the slope is of infinite longitudinal extent and that sliding occurs along a plane surface parallel to the face of the slope.2013 Chapter 4 Roadside Slopes factor of safety with respect to shear strength. the critical slip surface will be parallel to the slope. and are suitable for preliminary design while others are rigorous and should be favoured for evaluation of final designs.The slope is said to have reached limit equilibrium when the FS is equal to 1. Each of these methods may result in different values of FS because they employ different assumptions to make the problem statically determinate. The factor of safety (FS) is defined as the ratio of the available shear resistance to that required for equilibrium. Figure 4-35 shows some other means of defining the FS using force and moment equilibrium.0. and some of the methods do not satisfy all conditions of equilibrium mentioned in Figure 4. Limit equilibrium analyses assume the FS is the same along the entire slip surface. Limit equilibrium methods do not account for the load deformation characteristics of the materials. The Infinite slope method is a special case of the force equilibrium procedure. An FS of greater than 1.0 indicates the available strength exceeds the required resistance and that the slope is stable with respect to sliding along the assumed slip surface. In most limit equilibrium methods.35. The slope will be unstable if the FS is less than 1. Figure 4-35: Equilibrium conditions used to define the factor of safety Table 4-17 presents different types of limit equilibrium method useful for soil slope stability analyses.0.Geotechnical Design Manual . it is rarely used for final design. the simplified Bishop method is rarely used in designs that include provision for seismic forces. This method uses non-circular failure surfaces and. The main difference between the two methods is that Spencer solves for the inter-slice angle. The ordinary method of slices relies on solving moment equilibrium conditions and is applicable to circular failure surfaces only. Since horizontal equilibrium is not satisfied. Therefore. This method assumes that horizontal forces are not only perpendicular to the vertical sides of the slice. The method assumes that the inter-slice forces are parallel and act on a certain angle from the horizontal. Page 4-56 Ethiopian Roads Authority . two equations are available (horizontal and vertical force equilibrium). The simplified Bishop method was developed by Bishop in 1955. The Morgenstern and Price method is very similar to the Spencer approach. The method was originally developed to determine the stability of circular failure surfaces. This angle is one of the unknowns in this method. the horizontal forces are assumed to cancel each other out during mobilization. The main assumption required in using this method is the inclination of the horizontal forces on the given slice. but can now be used for non-circular failure surfaces as well. the first approximation of the angle should be the slope angle. The Morgenstern and Price method provides added flexibility using the inter-slice angle assumptions. With only one slice. Hence. This method is relatively simple and can be solved by hand calculators. the simplified Bishop methods can only be used on circular failure surfaces. This method does not solve either vertical or horizontal force equilibrium conditions. The other unknown is the factor of safety that is solved through an iterative process by using computer programs. Like the ordinary method of slices. while the Morgenstern and Price method solves for the scaling parameter that is used as a function that describes the slice boundary conditions. It solves two of the equilibrium equations. may be solved graphically. but are equal and opposite. Simplified Janbu is a force equilibrium method in which the moment equilibrium is either ignored or assumed to be zero. The inclination of the horizontal forces acting on a slice may be either the slope angle or the average slope angle if multiple slopes are involved. therefore. Therefore. moment and vertical.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 slice. and two unknowns must be evaluated (the factor of safety and the normal force on the base of the slice). the simplified Janbu solves both the horizontal and vertical forces. Like any other force equilibrium methods. Spencer’s method solves all three conditions of equilibrium and is therefore termed a complete limit equilibrium method. The normal force on the base of the slice is calculated by summing forces in a direction perpendicular to the bottom of the slice. Simplified methods assume the resultant of the vertical forces is zero on each slice. Non-circular Simplified procedure satisfies force equilibrium in both the horizontal and vertical directions. Circular Forces on the sides of the slice are neglected. horizontal equilibrium of forces is not satisfied (not suitable to analyze the effect of earthquakes for example). Factor of Safety is Recommendation Suitable for plane failures over long slopes. The method does not satisfy equilibrium of forces in either the vertical or horizontal directions. Realistic shear surfaces can be used. Useful when calculation must be done by hand using a calculator. Computer programs readily available. plane surface parallel to the slope. Does not consider the forces on the sides of the slice. Final design should be checked using Spencer or Morgenstern-Price methods. Simplified method compares well with other advanced methods.Chapter 4 Roadside Slopes Geotechnical Design Manual . but not moment equilibrium. Moments are summed about the centre of the circle to compute FS. The interslice forces are horizontal. Circular failure surface may not be representative of all failures. The method also may be used to overcome problems that may develop near the toe of steep shear surfaces. especially for those with a thin layer of weathered soil over rock. Very efficient and relatively common. Computed values are for homogeneous materials (can give large errors in slopes composed of more than one material). Useful where circular failure surface is assumed. The method is recommended for use in hand calculations where noncircular slip surfaces are being analyzed Page 4-57 . Circular Considers force and moment equilibrium for each slice. Generalized procedure considers force and moment equilibrium on each slice. Routine analysis can be easily handled by a Ethiopian Roads Authority Advantages Limitations Failure surface assumptions are always an approximation. Suitable for shallow landslides.2013 Table 4-16: Commonly used limit equilibrium methods Method Infinite slope Ordinary methods of slices Bishop Janbu Failure surface Assumptions Planar (Straight line) Assumes a slope of infinite longitudinal Relatively simple for manual extent and that sliding occurs along a calculation. Rigorous method assumes values for the vertical forces on the sides of each slice until all equations are satisfied. Implemented in force and moment equilibrium. It can simulate internal Considers forces and moments on each shearing. Requires computer software to perform the calculations. considered as a benchmark. Often slice like Janbu generalized procedure.Chapter 4 Roadside Slopes Spencer Geotechnical Design Manual – 2013 Non-circular (Rigorous) Morgenstern. Ethiopian Roads Authority . statically complete Satisfies fully the requirements for both solution.Non-circular Price (Rigorous) Page 4-58 programmable calculator or by hand. Computer help is necessary. Most useful for back analyzing failed slopes and to refine analyses performed by simple methods. It should also be used as a check on final designs where the slope stability computations were performed by simpler methods. many computer programs. Handles any geometry and loads. Handles any geometry and loads. Computer programs available. underestimated in simplified procedure. Gives The side forces are parallel (all side forces are inclined at the same angle). This is particularly important in profiles where the mode of failure is governed by geological factors. Back analysis of similar existing failures may also be an issue to consider. most of these schemes are designed to locate one slip surface with a minimum factor of safety. φ. An understanding of the possible modes of failure is crucial to the successful application of the result of the analysis. Depending on drainage allowed. If this is not possible then a sample parallel check using another program is recommended. Computer programs provide a means for efficient and rapid detailed analysis of a wide variety of slope geometry and load conditions. circular failures do not generally occur and shallow non-circular analysis would be appropriate. shear strength properties. In addition. a number of separate searches should be conducted to confirm that the lowest factor of safety has been calculated. The choice Ethiopian Roads Authority Page 4-59 . unconsolidatedundrained (UU). The user should be able to determine if the critical slip surface is passing through the relevant material.6. for example drainage condition. if possible by hand or spreadsheet methods. and pore water pressures at the bottom of slices. and consolidated-drained (CD). The use of computer programs Many of the limit equilibrium methods discussed above are at present implemented in most slope stability computer programs.2013 Chapter 4 Roadside Slopes 4. Automatic searches are controlled largely by the data that the user inputs into the software.2. The results of automatic searches are dependent on the starting location for the search and any constraints that are imposed on how the slip surface is moved. The program output should be checked to ensure that results are reasonable and consistent. Important items to check include the weights of slices. It is often recommended to check results from computer programs. In such cases. 4. three types of test are possible. The schemes may not be able to locate more than one local minimum. The analytical program being used must be compatible with the critical elements of the slope problem to be investigated. Regardless of the software used. Issues that should be remembered when using any computer program are: • • • • • • A thorough knowledge of the capabilities of the software and knowledge of the theory of limit equilibrium slope stability analysis methods is important to determine if the software is appropriate for any given situation. Any search scheme employed in computer programs is restricted to investigating a finite number of slip surfaces. Failures of colluvium over bedrock or failures in weathered rock most frequently occur along the surfaces dictated by structure. c' and φ') are normally determined using laboratory tests. In cases where the accuracy of parameters is in doubt.3. loading condition. Determination of shear strength parameters The shear strength parameters (c. The software analyzes a failure geometry that reasonably reflects the actual condition. or layering of materials within the soil-rock mass. The use of reliable and verified slope stability analysis software is essential in cases where conditions are complex.Geotechnical Design Manual .6. consolidated-undrained (CU). it is appropriate to undertake a sensitivity analysis to determine the effects on factor of safety of variations in these parameters. Appropriate shear strength and pore water parameters must be used for the analyses. above which all slope materials are dry. Select trial slip surfaces and compute factors of safety. Consider the possibility of artesian pressures and perched water tables by examining geological details. drained conditions in the field. A reasonable representation of groundwater conditions can be obtained by combining observed water tables with knowledge of subsurface variations in geology and soil permeability. or rapid drawdown). Effective strength parameters from these tests should be used to analyze cohesive soil cut slopes and to evaluate long term effects. This usually involves laboratory testing. consider the type of materials. whether reliable estimates of pore pressures can be made in advance. or prior shear zones (Chowdury et al 2009). strength parameters may be estimated using in situ tests such as the Standard Penetration Test (SPT). particularly if field testing was performed during the dry season. Attempt to visualize the probable shape of the slip surface or surfaces. Consolidateddrained tests procedures are also used to measure the residual shear strength of clays using a direct shear test. In particular. Cohesive soil shear strength parameters should be obtained from undisturbed soil samples using consolidated-undrained (CU) triaxial tests with pore pressure measurement if portions of the proposed slope are saturated or might become saturated in the future. the natural moisture content of the soil at the time of testing must be determined since this will affect the shear strength parameters. particularly cohesive soils. Consolidated-drained (CD) loading procedures are used to determine the effective stress shear strength parameters of freely draining soils. Establish the geometry of the slope. Unconsolidated undrained (UU) triaxial tests can be used to obtain undrained shear strength parameters for short term stability analysis. including the ground surface and subsurface boundaries between various materials. Special attention must be given to the Page 4-60 Ethiopian Roads Authority . and whether pore pressures are to be monitored in the field (Chowdury et al 2009). steady-state seepage (long term). 4. The assumption of a single groundwater level. Make decisions concerning the use of effective or total stress types of analysis. Select loading conditions for analysis. Stability analysis procedures • • • • • Determine soil strength parameters. Correlations between strength parameters and other soil properties such as grain size and plasticity may also assist in selecting approximate values. Rely only on the residual strength along parts of assumed slip surfaces that correspond to existing. It should be noted that for unsaturated soils. or when it is determined that total stress (strength) parameters are sufficient. Distinguish clearly between first-time landslides and possible renewed movements. Establish seepage and groundwater conditions. Consideration should be given to future changes in moisture content.6.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 depends on the state of stability at the time of testing (end-of-construction (short term). is an oversimplification in most situations. or if the interest is a preliminary design only.4. If laboratory test data are not available. These soils will drain with relatively short testing times and the consolidated-drained loading procedure comes closest to representing the loading for long-term. whether analysis is for short-term or long-term conditions. The analysis involves a comparison of the orientations of the dominant discontinuity sets with the orientation of the cut slope. Where a rock mass is highly fractured by randomly orientated discontinuities or composed of very weak rock. Table 3-6 in Chapter 3 summarizes the types of analysis. or toppling. Compare the computed factor of safety with experienced-based criteria The design assumptions should be verified during construction. Ethiopian Roads Authority Page 4-61 . In embankments. The advantages and limitations of conventional rock slope analyses are summarized in Table 4-18. 4. Failures tend to occur as discrete blocks. nonhomogeneity. failure is kinematically possible. This may require repeating the above steps and modifying the design if conditions are found that do not match previous assumptions Following construction. Where discrete blocks are formed and where the failure surfaces that bound these blocks dip out of the slope at an angle steeper than the shear strength along the discontinuity. Therefore. In homogeneous soil slopes without discontinuities. tension cracks. sources of shear strength data and the type of limit equilibrium methods suitable for cohesive and granular soils.7. A stereonet is used to display the discontinuity and slope data in this analysis. zones of different materials. Repeat selecting slip surfaces until the “critical” slip surface has been located.2013 • • • • Chapter 4 Roadside Slopes existence of major discontinuities. therefore. and second by slope stability analyses to determine the factor of safety. existing slip surfaces. Failure modes typically fall within one of three categories: plane failure. the mode of failure may be circular as in a soil slope. This analysis establishes the possible failure modes of the blocks that comprise the slope. Kinematic analysis A kinematic analysis is the first step in evaluating slope stability. The analysis determines if the orientations (dip and dip direction) of the various discontinuities will interact with the cut slope orientation and inclination to form discrete blocks with the potential to fail without regard to any forces that may be involved.1. Actual groundwater levels based on pore water pressure measurements should be compared with those assumed during design. represents the most likely failure mechanism.Geotechnical Design Manual . Rock slope stability analyses The stability of hard rock slopes is normally controlled by discontinuities (joint and joint sets) within the rock. the performance of the constructed slope should be monitored. kinematic analysis of the discontinuities is performed first to determine the most likely mode of failure. wedge failure. 4. give consideration to method of construction.7. stratification. and nature of foundation in order to visualize the probable shape of the slip surfaces (Chowdury et al 2009). The critical slip surface is the one that has the lowest factor of safety and which. assume a slip surface of circular shape unless local experience dictates otherwise. and open joints. many methods available all with varying assumptions. Analyses are undertaken to provide either a factor of safety or. these methods are the most commonly adopted in rock engineering. reinforcement and/or groundwater profiles.2.Chapter 4 Roadside Slopes Geotechnical Design Manual – 2013 Table 4-17: Advantages and limitations of conventional rock slope analyses. can be combined with statistical techniques to indicate probability of failure. factor of safety gives no indication of instability mechanisms. discontinuity shear strength characteristics. through back-analysis. Limitations Only really suitable for preliminary design or design of non-critical slopes.). toppling. critical discontinuities must be identified. From Eberhardt (2003). All limit equilibrium techniques share a common approach based on a comparison of resisting forces/moments mobilized and the disturbing forces/moments. etc. Wide variety of commercially available software for different failure modes (planar. groundwater conditions. Limit Equilibrium Representative geometry and material characteristics. Relatively simple to use. modified from Coggan et al (1998) Analysis method Critical parameters Advantages Kinematic (using stereographic interpretation) Critical slope and discontinuity geometry. though many failures involve complex deformation and fracturing. primarily evaluates critical orientations. support and reinforcement characteristics. wedge. and the assumptions adopted in order to achieve a determinate solution (Eberhardt 2003). can analyse factor of safety sensitivity to changes in slope geometry and material properties. links are possible with limit equilibrium methods. Methods may only vary with respect to the failure mechanism (e. strains and intact failure not considered.7. soil or rock mass shear strength parameters (cohesion and friction). translational or rotational sliding). probabilistic analysis requires well-defined input data to allow meaningful evaluation. neglecting other important joint properties.g. more advanced codes allow for multiple materials. Limit equilibrium analyses Limit equilibrium analyses are routinely used in the analysis of rock slope stabilities where movements occur on distinct failure surfaces. may allow identification and analysis of critical keyblocks using block theory. Mostly deterministic producing single factor of safety (but increased use of probabilistic analysis). Page 4-62 Ethiopian Roads Authority . 4. a range of shear strength parameters at failure. gives initial indication of failure potential. In general. representative shear strength characteristics. need to be used with representative discontinuity or joint shear strength data. ) may need to be developed. Depending on the project stage and the extent of information needed. preliminary and final geotechnical evaluations as they relate to the project.1 Preliminary Level Geotechnical Report A preliminary level geotechnical report is typically used to provide geotechnical input in the early stage of project development and reconnaissance studies (pre-feasibility. For preliminary level design.2013 5 Chapter 5 Geotechnical Report and Checklist GEOTECHNICAL REPORT AND CHECKLIST A geotechnical report is a document used to communicate the site conditions and design and construction recommendations to the design and construction personnel. etc. and project background. A brief summary of the regional and site geology. A preliminary level geotechnical report should contain the following contents: • • • • A general description of the project. 5. and for structures such as bridges. and possibly even preliminary design stages). rock. feasibility. bridges. The primary audience for the report are highway designers and construction supervisors and contractors. This is commonly the case on roads passing through difficult terrain where alignment and/or grade changes may be necessary because of stability issues or excavation problems. Site investigations for transportation projects provide specific information on subsurface soil. soil profiles for key project features (e. This information is normally compiled in a site investigation report.). A preliminary level geotechnical report is prepared based on a minimum of a desk study review of existing geotechnical data for the site. For preliminary design reports in which new borings have been obtained. A description of the project soil and rock conditions. The amount of detail included will depend on the nature of the project. project elements. rock-fall.g. and water conditions. as well as some detailed analyses to characterize key elements of the design. and construction planning reviews. The geotechnical report summarizes all of the pertinent information that will be used in road design. For preliminary design of more complex road projects with potentially unusual subsurface conditions. geotechnical reports can be divided into preliminary and final level reports as discussed below. bridge foundation scour. and to estimate preliminary costs. a geotechnical reconnaissance of the project site and a limited subsurface exploration is often sufficient.Early submittal of geotechnical information and recommendations or engineering evaluation of preliminary data may be necessary to establish basic design concepts or design criteria. It may also be prepared for rapid assessment or emergency repair needs (during the occurrence of landslides. Geotechnical design for a preliminary level report is typically based on engineering judgement and experience at the site or similar sites. and generally consist of feasibility assessment and identification of geologic hazards. a geotechnical reconnaissance of the site should be conducted in addition to the desk study review to assess the site conditions. A summary of the field exploration and laboratory testing conducted. It provides findings from the field explorations. Ethiopian Roads Authority Page 5-1 . It also contains design (and construction) recommendations for the alignment and Right of-Way issues. etc. results in design and construction recommendations that should be presented in the project’s geotechnical report. to assess potential alternatives. retaining walls. laboratory testing.Geotechnical Design Manual . Interpretation of the site investigation information by a geotechnical engineer. if known. soft ground. any field data obtained. If multiple groundwater level readings were obtained over time. bedrock and soil geological units. potential magnitude of shaking. existing test hole logs. soil profiles developed. 5. Regional and site seismicity for major bridges. liquefaction. any influences on the groundwater levels observed. historical activity. are to be constructed.). the dates and depths measured. frequency. perched water tables. etc. etc. grade. In road construction. Also a brief description of the method used to obtain groundwater levels (open standpipe. and any photographs. previous or current geologic reconnaissance results.Chapter 5 Geotechnical Report and Checklist • • • Geotechnical Design Manual – 2013 A summary of geological hazards identified that may affect the project design (e. as well as a summary of the number and types of tests that were conducted. potential source zones. project elements. rock-fall. final alignment.g. etc. A summary of the laboratory testing conducted with the description of the methods and standards used. The development of a final geotechnical report will not normally be completed until the design has progressed to the point where specific recommendations can be made for all of the geotechnical aspects of the work. geological maps. potential seasonal variations. as-built bridge or other structure layouts. A description of any field instrumentation (e. a detailed geological assessment of the site. A summary of geotechnical conditions that briefly describes the subsurface and groundwater conditions for key areas of the project where foundations. piezometers) installed and its purpose should be included. A summary of the site data available from project or site records (e.). debris flows. and location of nearby faults.2 Final Level Geotechnical Report A final level geotechnical report is developed for final design and construction review purposes. and project background.g.). A summary of the field exploration conducted. A summary of the conceptual or preliminary geotechnical recommendations. This section should describe the site stress history and depositional/erosional history. or as a minimum the range of depths measured and the dates the highest and lowest water level readings were obtained. vibratory piezometer. Appendices that include any boring logs and laboratory test data obtained. A description of the soil/rock units encountered at the project site. etc. pneumatic piezometer. with a description of the methods and standards used as well as a summary of the number and types of explorations that were conducted. expansive soils. Page 5-2 Ethiopian Roads Authority . Regional and local geology. This document should also describe the impact of these subsurface conditions on construction. complete subsurface investigation and laboratory programmes. if applicable.g. It is prepared based on a desk study review of existing geotechnical data. and geometry will usually have been selected prior to issuance of the final geotechnical report. fills. etc. and direction and gradient of groundwater. seismic hazards. artesian pressures. cuts. landslides. and detailed analyses and interpretations. Project site surface conditions and topographic assessments. groundwater conditions including the identification of any confined aquifers. final construction records for previous construction activity at the site.. A final geotechnical report should contain the following contents: • • • • • • • • A general description of the project. Interpretive information contained in these illustrations should be kept to a minimum. For other wall situations. if any. and fills.e. and cuts with adverse geological structure. and comparison of those results to any laboratory test data.). soil profiles should be provided for features of significant length. subsurface profiles should always be provided for soil nail walls. a circular failure model should not be used to analyze a cut in rock where discontinuities will control the stability. As such. cut design. steep cuts. the results of those analyses. Proposed cuts and excavations should be considered in terms of temporary (shortterm) and long term and stability analyses performed for those that have a potential for failure. landslides. the following data is needed: o The analytical approach and assumptions used. and non-gravity cantilever walls. fills. o Values of the design parameters. o A description of any back-analyses conducted. The potential for variability in the stratification must be discussed in the report. the foundation. The level of analysis should be consistent with the consequences of slope failure. and all other walls in which there is more than one boring along the length of the wall. The stability analyses used must be appropriate for the slope conditions.). Subsurface cross-sections must always be provided for landslides. The location and extent of the geologic hazards should be described. If stereographic analyses are used. For retaining walls. and landslides. viaducts.Geotechnical Design Manual . the boring logs (including SPT values. compaction characteristics and potential problems with the materials to be used in the embankments. Embankment design recommendations should include the slope Ethiopian Roads Authority Page 5-3 . Global and local stability conditions should be analysed as appropriate. rock-fall. A summary of geological hazards identified and their impact on the project design (e. and for cuts. along with the input data and any assumptions made. For the analysis of unstable slopes (including existing settlement areas). and subsurface drainage should include analysis of settlement. judgement may be applied to decide whether or not a subsurface profile is needed.g. etc. and the location of any water level(s). structures. liquefaction.2013 • • • • • • Chapter 5 Geotechnical Report and Checklist The descriptions of soil and rock conditions illustrated with subsurface profiles (i. Special attention is required for very high cuts and fills. parallel to road centreline) and cross-sections (i. What appears to be the same soil or rock unit in adjacent borings should not be connected together with stratification lines unless that stratification is reasonably certain. soil/rock units. anchored walls. o Any definition of acceptable factors of safety or discussion of acceptable risk of failure. For cuts. subsidence. cuts. and other significant structures. Geotechnical recommendations for earthwork (fill design. The design of embankment features such as fill slope angles. and walls that are large enough to warrant multiple borings to define the underlying geology. perpendicular to roadway centreline) of the key project features. etc. soft ground or expansive soils. fills. the stereo-nets should be appended and the results of the analyses summarised. as appropriate. slope stability. debris flows. The method of analysis should be stated. groundwater conditions. usability of on-site materials as fill). the profile or crosssection will contain the existing and proposed ground line. A subsurface profile for bridges. A subsurface profile or crosssection is defined as an illustration that shows the spatial distribution of the soil and rock units encountered in the borings and probes. where multiple borings along the length of the feature are present. For example.e. including sketches of local sources and regional location maps. and lateral earth pressure parameters. Such recommendations should include any special measures to produce a stable rock slope such as rock bolting/dowelling as well as any recommendations to prevent erosion and undermining of intact blocks of rock. the quality of materials and their suitability for the different road structures. controlled rate of embankment construction. minimum width for overall stability. the approximate quantity of material available described. etc. Recommendations on aggregate and borrow materials. extreme event limit state bearing. bearing resistance..g. and design parameters for determining earth pressures should be provided. foundation design requirements (for strength limit state: the ultimate bearing resistance and depth. the recommended wall/reinforced slope options. recommendations on minimum width for external and overall stability. and estimated quantity. landslides. disadvantages. as appropriate. and the need for and extent of removal of any unsuitable materials below.. feasible methods of rock removal and rock excavation. The limits of the material source relative to the proposed alignment should be defined. For anchored walls. and any special design requirements). Cut design recommendations should contain the slope angle required for stability. for aeration to reduce the moisture content. foundation options considered. foundation type and design requirements (for strength limit state: ultimate bearing resistance. and design earth pressure distribution. bearing resistance and settlement. This section should provide a discussion on mitigation options. lateral and uplift resistance if deep foundations have been selected. uplift and lateral resistance if deep foundations have been selected). and the effect of environmental factors on their usability. strength. including a discussion of the advantages. Geotechnical recommendations for bridges and hydraulic structures. and detailed recommendations regarding the most feasible methods for mitigating the unstable slopes. and lateral and uplift resistance. any measures that need to be taken to provide a stable embankment (geosynthetic reinforcement. and material excavation characteristics. cut slope and other on-site materials should be identified as to their feasibility for use as fill. for service limit state: settlement limited bearing.. and any other special measures required to produce a stable slope. achievable anchor capacity. etc. rock-fall areas. if any. design considerations for scour when applicable.). and the need for controlled blasting or any other special techniques that may be necessary.g. soil reinforcement spacing. In addition. and soil nail walls). debris flows. the amount of overburden to be stripped. For reinforced slopes requiring internal stability design (e. and length requirements. Geotechnical recommendations for stabilization of unstable slopes (e. Geotechnical recommendations for retaining walls and reinforced slopes with a discussion on considered wall/reinforced slope options. with a discussion on the type of fill material for which they could be utilised. design acceleration coefficient. seepage and piping control and erosion control measures needed. settlement. embankment settlement magnitude and rate.g.). and dimensions to meet external stability requirements are needed. and risks associated with each option. no-load zone dimensions. Geotechnical recommendations for rock slopes and rock excavation. geosynthetic walls. For other retaining walls. light-weight materials. wick drains. embedment depth. for service Page 5-4 Ethiopian Roads Authority . the need. internal and external slope drainage requirements.Chapter 5 Geotechnical Report and Checklist • • • • • Geotechnical Design Manual – 2013 required for stability. seismic design parameters and recommendations (e. embedment depth. resurfacing or rehabilitation • Project limits of the various construction operations • Proposed alignment changes • Specific geometric design features (number of lanes. design charts for foundation bearing and uplift. Project scope: • Proposed construction. design detail figures. including a boring log legend for each type of log. shoring needs and potential installation difficulties. laboratory test data obtained. A sample outline with contents that need to be covered in a geotechnical report for road construction is given below. design acceleration coefficient. widths. 5. locations. all boring logs used for the project design (includes older borings as well as new borings).Geotechnical Design Manual . temporary slopes. Long-term or construction monitoring needs which should include recommendations on the types of instrumentation required to evaluate long-term performance or to control construction. potential foundation installation problems. Project background information: • Site topography • Type and amount of relief • Land use • Seismicity • Water courses. Address issues of construction staging. uplift. etc. subsurface profiles and typical crosssections that illustrate subsurface stratigraphy at key locations. etc. Typical appendices should include layouts showing boring locations relative to the project features and stationing.) type. 1. culverts. and soil spring values). Construction considerations.. description of any proposed rehabilitation 2. liquefaction mitigation requirements. bypass lanes) • Proposed structures (bridges. and any special design requirements). Appendices. and the zone of influence for each instrument. number. earth pressures on abutments and walls in buried structures. and special provisions needed. earthwork constructability issues.3 General Geotechnical Report Outline The objective of using any standard outline to prepare a report is to provide a degree of uniformity in the presentation of geotechnical data. interchanges. extreme event limit state bearing. reconstruction. while retaining enough latitude in the contents to reflect the individuality of each project.2013 • • • Chapter 5 Geotechnical Report and Checklist limit state: settlement limited bearing. dewatering. ponding areas and wetland locations 3. seismic design parameters and recommendations (e.g. design considerations for scour if applicable. instrumentation measurement results. turn lanes. Surface geology and features of interest • General soil types • Extent and uniformity • Bedrock features Ethiopian Roads Authority Page 5-5 . and lateral resistance. soil profile type for response spectra development. and recommendations regarding bridge approach slabs. Modulus. 200 Sieve.construction considerations Page 5-6 Ethiopian Roads Authority . Rock excavation • Recommended techniques of excavation (blasting. Atterberg Limits. gradations. maximum densities. typical section) • Road.) • Description of excavated material and their use 8.Chapter 5 Geotechnical Report and Checklist Geotechnical Design Manual – 2013 • Rock type/formation/extent • Weathering conditions • General subsurface conditions (depth to bedrock. etc. quantity.) • Drilling information (field logs and observations) • Soil and bedrock profile. depth. percent passing No. ripping. etc. Groundwater conditions • Levels (including types of piezometers) • Springs • Nearby wells 5. Landslides and unstable areas • Description of landslides • Causes of landslides • Stability analyses • Correction measures 10. CBR. Laboratory test data • Optimum moisture contents.) test results • Geophysical information • Instrumentation 4. etc. textural classifications. drainage. Shallow and deep foundations (walls and bridges). use) 9. Proctor compaction results. refusals. etc. CPT. DCP. topsoil horizons • In-situ (SPT. 6. Aggregate and borrow sources • Possible sources of aggregate • Materials available (quality. Subgrade conditions • Sub-cuts (length. Retaining walls and reinforced slopes • Design requirements • Earth pressures • Foundation analysis 11. backfill material. widened sections • Turn lanes and curvatures Embankments and road cuts • Fill-slope analyses • Cut-slope analyses • The use of excavated materials for fill 7. References 5. Ethiopian Roads Authority Page 5-7 . the geotechnical engineer should summarize the responses and use the result to determine if additional follow-up actions are appropriate. The advantage of using this checklist is to ensure that pertinent data are not forgotten or overlooked. etc.Geotechnical Design Manual .4 Checklist The checklist given in Table 5-1 covers important information that should be presented in geotechnical reports. Upon completing the checklist.2013 Chapter 5 Geotechnical Report and Checklist Appendices • • • • • • Project location map Borehole logs and descriptions Subsurface profiles Laboratory results Photos. and to provide an overview of aspects that need to be investigated further. and specifications? Is a design CBR value (or an equivalent of it) provided? If drainage or groundwater is an issue with the proposed subgrade. are recommendations made to accelerate the settlement before the bridge abutment is constructed (waiting period. Atterberg limits. has an appropriate drainage system (pipe.. have the stations and lateral extent of the problem areas been defined? Has a method (change alignment. using stabilizing counter-berms. buildings. excavate and replace weak subsoil. station limits. have sufficient analyses been completed to evaluate consolidation at locations representative of the most critical conditions? Have the total settlement and the time of consolidation analyses indicated acceptable values at all locations for the scope of the embankment work? If total settlement or time of consolidation is unacceptable. etc. performed on selected representative samples to verify field visual soil identification? Are laboratory test results such as shear strength. and lateral limits for the planned removal been provided? Do subsurface investigations and existing moisture contents for the proposed subgrade soils indicate the need for subgrade stabilization? If stabilization is needed. geotextile fabric reinforcement.. including depth. surcharge. or wick drains)? Has the effect of any foundation soil consolidation (including differential settlement) been evaluated with regard to adjacent structures (e.) been recommended? Embankments: If soil conditions and project requirements warrant. consolidation. etc. lightweight fill.) been chosen as a solution to the settlement issues? Based on accepted design practices.Chapter 5 Geotechnical Report and Checklist Geotechnical Design Manual – 2013 Table 5-1: Checklist of important information in geotechnical reports Tasks to be completed for geotechnical report Y N Laboratory test data: Were lab soil classification tests such as natural moisture content. included and/or summarized? Subgrade: Has the subsurface investigation adequately characterized the soil or rock? Are there weak or problem soils at the subgrade level and have soft-spots been properly identified throughout the road alignment? If these materials are to be removed and replaced. under drains. gradation. bridges. were there any attempts to evaluate the effectiveness of the chosen solution(s)? Has an economic analysis been performed to evaluate the cost benefits of the recommended solution compared to other methods? For bridge approach embankments. culverts. depth. have the station limits. have settlement and stability issues been addressed? Have the consolidation properties of the foundation soils been determined? Has the total expected embankment settlement and the time of consolidation been estimated? If differing foundation soil and/or loading conditions occur throughout the embankment area. etc.g. is the detail of this treatment shown on the report. utilities) which may also undergo settlement due to the consolidation of the surrounding Page 5-8 Ethiopian Roads Authority . lateral extent. lower grade. and 1.S. side slopes. type.30 for short and long term conditions. and specific locations of the proposed instruments? If piezometers will be used. of 1. have the stations and lateral extent of the problem areas been defined? Has a method been chosen as a solution to the stability issues? Has an economic analysis been performed to evaluate the cost benefits of the recommended solution compared to others? Has the effect of the solution being used been evaluated with regard to structures (e. was this layer considered as a possible failure zone? Have erosion protection measures been addressed for back slopes. are detailed recommendations provided on the number. and specific drainage structures? Are there evidences of springs and excessively wet areas? Did the design consider additional drainage in the cut slope (springs / seeps) and roadway base? Are recommended slope designs and blasting specifications provided for rock slopes? Are rock slope designs based on orientation of major rock joints and weathering conditions rather than “template” design procedures (such as designing all rock slopes at 0. was not met or exceeded. has the critical groundwater table been determined and the appropriate information included in the reports Are water-bearing zones properly identified and their impact addressed? Cuts and excavations: Does drilling provide continuous stratigraphic sections for the range of elevations that represent proposed cut slope areas? Do the cut slopes have a minimum stability F. erosion protection measures for back slopes.. buildings. as determined by the calculations. and ditches. bridges.10 for rapid drawdown (flood condition).2013 Chapter 5 Geotechnical Report and Checklist soil? Has the total (short term) and effective (long term) shear strength of the foundation soils been determined? Have calculations been performed to determine the F.S. are station to station recommendations included for fill slope design? If the F.S. for the given stability conditions: 1. for stability? Are the following Factor of Safety (F.g. 1. side slopes.30 If there is a potentially unstable soil or rock layer within the cut slopes.Geotechnical Design Manual . utilities) which may be subject to unusual stresses or require special construction considerations? If geotechnical instrumentation is proposed to monitor fill stability and settlement.50 for embankment supporting bridge abutments? When differing soil or loading conditions occur throughout the embankment area.S.) criteria achieved or exceeded. culverts. and ditches (including riprap recommendations or special slope treatments)? Is the usage of excavated soils and rocks properly addressed? Are station to station recommendations included for excavation limits of unsuitable materials.25:1)? Is the effect of blast-induced vibrations on adjacent structures and slopes evaluated? Was controlled blasting considered in the design? Landslides: Has a site investigation been conducted to define the limits and depth of the landslide? Is a site plan and scaled cross-section provided showing ground surface conditions both Ethiopian Roads Authority Page 5-9 . has the influence of groundwater been taken into account with regards to soil unit weights and active pressures? Are standard methods used to determine the lateral earth pressures? Was an economic analysis performed to evaluate the cost benefits of the chosen wall type compared to others? Does the design lateral earth pressure include the effects of soil backfill strength.S. has the source of recharge been identified? Is a stability analysis carried out to determine the F. and global stability? If special drainage details are needed behind and/or beneath the wall. = 1. gabion. and surcharge loads? Are all the required F.S.3) Overturning (F. values meet or exceed the minimum: Bearing Capacity (F. slope geometry. reinforced soil. head scarp. = 3.S.50)? If poor foundation soils are present. tieback.)? Are the soil strength parameters used to compute the design factor of safety for overturning. sliding. were the cause and effect of any existing rock-fall conditions determined? Were bedding and jointing of the bedrock formations identified as a significant factor affecting the slope stability? Was there any groundwater monitoring programme to identify the phreatic surface through the landslide area? Is the landslide failure plane and mode of failure determined from field observations or instrumentation? If groundwater (static or flowing) significantly influences the stability of the landslide. shown on these cross sections and profiles? In the case of rock-falls. = 1. for the given stability conditions? Is a landslide correction or stabilization method determined? Has a cost comparison been performed to evaluate the effectiveness of stabilization alternatives? Is long-term monitoring necessary? Retaining walls: Is/are the most suitable and cost-effective wall type(s) selected for the project site conditions? Are reasons given for the choice and/or exclusion of certain wall types (gravity.S.0). maintenance work.S. sliding. inadequate bearing capacity. computed? Do the F.S.S. and toe bulge. for stability? Is a Factor of Safety (F.0). and past corrective measures? Has a site specific geotechnical investigation been performed to investigate the landslide area? Has the vertical and lateral extent of defined landslide conditions been included on cross sections and profile sheets? Are detailed slide features. are recommended details provided in the geotechnical report? Page 5-10 Ethiopian Roads Authority . has a solution been determined with respect to excessive settlement.) of 1. etc. including movement history. External Stability (F. Sliding (minimum F. and external slope stability? Are the groundwater level and proper loading conditions known? If applicable. = 2.3 achieved or exceeded. including location of ground surface cracks.Chapter 5 Geotechnical Report and Checklist Geotechnical Design Manual – 2013 before and after failure? Was the history of the landslide area studied. cantilever.S. as determined by the calculations. need for sheeting or shoring)? Has the effect of the wall design and construction procedure been determined and accounted for on the construction schedule of the road? Are all the necessary notes. special provisions. and details for the construction of the wall system included in the geotechnical report? Ethiopian Roads Authority Page 5-11 . specifications.Geotechnical Design Manual .2013 Chapter 5 Geotechnical Report and Checklist Are excavating requirements covered (safe slopes for open excavations. . (ed). SB. Course No G04-005 Chowdury. DC.D. DC.. Washington. Lee. Guide for Design of Pavement Structures. (1998). (2011). West Conshohocken. Hearn. American Association of State Highway and Transportation Officials. Holtz. Guide to Stabilisation in Roadworks. GWP Consultants. Washington D. Watershed management field manual. CED Engineering. Slope Design. In: Landslides. Prentice-Hall Inc. Appendix 4-4 Guyer.J. Rock Slope Stability Analysis – Utilization of Advanced Numerical Techniques. Flentje. DJ. Boulder. 437-501. (1998). Quarry Design Handbook. Austroads. Standard Test Methods for One-Dimensional Swell or Collapse of Cohesive Soils. RL eds. W. N.. P and Bhattacharya. DOI: 10..J. Cruden. Briaud. and Hunt. Bishop. Settlement of Bridge Approaches. J. Route corridor and alignment selection. Sharma. Eaglewoods Cliffs. P. 135-144.D. London. Slope Stability and Stabilisation Methods. Slope Engineering for Mountain Roads. Ethiopian Roads Authority Page 6-1 . (1998). G.M.S. Evaluation of Techniques for Quarry Slope Stability Assessment.2013 6 Chapter 5 Geotechnical Report and Checklist REFERENCES AND BIBLIOGRAPHY References AASHTO (1993). Institute of Mining and Metallurgy. Geotechnical Slope Analysis. UK. D.org Abramson. T.. The Design and Construction of Residential Slabs-on-Ground. R. LW. (2001).. Austroads (1998). Ch 6 in: Road Construction Techniques 13/5. James. E. PA. USA. pre-publication draft.C. (1997). S and Boyce. Proc ASCE Conf on Shear Strength of Cohesive Soils. Australia. Stead. NCHRP Synthesis 234. TS. BRAB (1978). Geological Society of London. www. Earth and Ocean Sciences. Vancouver. CRC Press. J. An Engineering SoundBite. J Wiley.J. FAO. Canada. Colorado. In: Hearn. GM. (2011). (1981).Geotechnical Design Manual . State of the Art. An Introduction to Geotechnical Engineering. Engineering Geology Special Publication No 24. The relevance of the triaxial test to the solution of stability problems. and Eyre. Washington. (2003). and Kovacs. RW and Hoffman. ASTM International. 1960. (2009). TRB/NRC Turner. L. C1. 2003. Geosynthetic Engineering: Geosynthetic Separators. Spec report 247. AK and Schiuster. Sydney. Coggan.astm. JL. ASTM 4546. AW and Bjerrum. Eberhardt. (1996) Landslide types and processes. Transportation Research Board/NRC. Soil Stabilisation for Pavements. D and Varnes. USA. Investigation and Mitigation. G. 2008. Building Research Advisory Board. University of British Columbia.1520/C0033-03. USA. R. (2004). USA. National Cooperative Highway Research Program (2004). Quart Jnl Engng Geol. National Cooperative Highway Research Program. Washington DC. R. Vientiane. Development of Project 1-37A Design Guide Using Mechanistic Principles to Improve Pavement Design. PG. Southern Tasmania. G and Sherar. National Lime Association. Hencher. 2008. S.189. McGraw-Hill. Vol 1 and 2. Transportation Research Board. Page 6-2 Ethiopian Roads Authority .1 . Materials Engineering Branch. 2008.Disaster Risk Reduction. Slope stabilisation and stability of cuts and fills. Dispersive Soils . (1994).P. 4345-4353. Use of Geogrids in Pavement Construction. 12th International Conference of the International Association for Computer Methods and Advances in Geomechanics. M. USA. Colorado US DOI Bureau of Reclamation. Denver. Crown Copyright. Natural Resource Managament. Harmonised Test Methods for Laboratory Determination of Resilient Modulus for Flexible Pavement Design. In: Debris Flow Risk Assessment and Mitigation on the Scottish Trunk Road Network. New York. Best Management Practices. Dealing with road subgrade problems in Southern Africa. Blackwell/Springer. A revision of the graphical method for assessing the excavatibility of rock. Pettifer. Slope Engineering Branch. US ACE. Keller. R. US Army Corps of Engineers. Malaysia. US Dept Agriculture Forest Service.188. and Rollings. Handbook of Slope Stabilisation. (2008). 27. Scottish Executive. Field Guide. Department of the Army. Geotechnical Materials in Construction. P. P-91-09. IM. Guidelines for Slope Design. Laboratory testing. USA.1 . Paige-Green. (1991). Sassa. Martin. (2008). Rollings. (1998). (1996). Debris flow types and mechanisms. ETL ..Design for New and Rehabilitated Pavement Structures. Characteristics and Problems of Dispersive Clays. National Cooperative Highway Research Program (2001). FHWA/TRB/NRC. US ACE. Ch 9 in: MDT Geotechnical Manual. 1-28A. USA. Washington D. Transportation Research Board. MPWT (2008). Virginia. Arlington. Lime Stabilization Manual. Landslides . US DOI Bureau of Reclamation..High Risk of Tunnel Erosion. Berlin. US Department of the Interior.S. Nettleton. (2003). Engineering Geology Field Manual. (1997). Springer. J. Guide for Mechanistic and Empirical . ETL . GS and Fookes. Ortigao. (2012). National Lime Association (1972). JKR 21500-0011-10. (2002). Ministry of Public Works and Montana DOT. Montana Department of Transportation. Slope Maintenance Manual. 145-164. Use of Geogrids in Pavement Construction. P. (2005). A. Laos. Ch 11 in: Low Volume Roads Engineering. JAR and Sayao. K and Canuti. S and Moore. (2011).Appendix A Soil Stabilization Geotechnical Design Manual – 2013 JKR. Department of the Army. Transport. US Army Corps of Engineers.C. Geosynthetic Engineering: Geosynthetic Separators. 53:139. Central Federal Lands Highway Division. Denver. Dept of Transportation. Lexington. E. (2005). Washington DC. GWP Consultants. Rock excavation methods. Geotechnical Aspects of Pavements. Vancouver. Subgrade Stability Manual. Washington State DOT. Dept of Transportation. Federal Highways Administration. Ch 3 in: Context Sensitive Rock Slope Design Solutions. US DOT FHWA CFLHD. TS. LW. Statewide Urban Design and Specification (SUDAS). (2005). BRAB (1978). Earth and Ocean Sciences. Ch 10 in: Geotechnical Design Manual M46-03. GM. Foundation Engineering. (2013). Foundations on weak and/or compressible soils. State of Montana Dept of Transportation and US DOT FHWA. Ch 6D in: Geotechnical Design Manual. USA. Embankment Construction. University of Iowa. Publ No Federal Highways Authority. Indiana DOT. Ch 6 in: Geotechnical Manual. (2011). (2008). NHI 05-037. California Department of Transportation. Sharma. University of Iowa. WS Department of Transportation. (2003). Caltrans (2013). California Department of Transportation.08. Rock Slope Stability Analysis – Utilization of Advanced Numerical Techniques. USA. Ch 3 in: Bridge Design Specification. (2013). Abramson. pre-publication draft. Illinois DOT. Asphalt Paving Design Guide. Soil and Foundations. Caltrans. Federal Highways Administration. Engineering Analysis and Evaluations. 2013. Lexington-Fayette Urban County Government. Quarry Design Handbook.. Geotechnical Manual. Colorado. (2008).2013 Chapter 5 Geotechnical Report and Checklist US Department of the Interior. Canada. Ethiopian Roads Authority Page 6-3 . FHWA-CFL/TD-11002. CED Engineering. Indiana Dept of Transportation. US DOT FHWA (2006A). Revised Edition.Geotechnical Design Manual . Building Research Advisory Board. Asphalt Paving Association of Iowa. US DOT FHWA (2006B). Foundations of Structures. University of British Columbia. Colorado. (2004). State of the Art. Design Procedures for Soil Modification or Stabilisation. Bureau of Bridges and Structures. Illinois Department of Transportation. USA. Bibliography APAI (undated). Lee. Kentucky. Montana DOT (2009). FHWA NHI-06-088. DC. The Design and Construction of Residential Slabs-on-Ground. Eberhardt. Iowa State. Slope Design. Course No G04-005. J Wiley. Retaining Walls. MDT Research Programs. Soil cut design. Washington DC. S and Boyce. Washington. Measurement and Evaluation of Subgrade Parameters: Phase 1 Synthesis of Literature. (1998). CED Engineering. Reference Manual. (2001). Slope Stability and Stabilisation Methods. Iowa State. Appendix 4-4. US Army Corps of Engineers.189. US Army Corps of Engineers. (1998).1 . Pavement Interactive (2007). Ch 26 in: Geotechnical Design Manual. DC. Report No FHWA-RD-75-48. New York State. CED Engineering. Federal Highways Authority. www. Constructively Speaking. Denver. DOT. Federal Highways Authority. Ch 17 in: Geotechnical Design Manual. US DOT FHWA (2006A). Issue No 3. Use of Geogrids in Pavement Construction. Ch 8 in: Slope Stability. Use of Geogrids in Pavement Construction. US ACE. FHWA. New York State Department of Transportation. US DOT FHWA (2006B). Mechanically Stabilized Earth Walls and Reinforced Soil Slopes Design and Construction Guidelines. US ACE. US DOT FHWA (2001). Department of Transportation.(1994). Department of the Army. First published 1992. Cut slope stability. Colorado. USA. US ACE. Denver. (2003). Stabilisation with lime. Geotechnical Aspects of Pavements. US Army Corps of Engineers. (2003). Construction. US Army Corps of Engineers. US Army Engineer Waterways Experiment Station. A Review of Engineering Experiences with Expansive Soils in Highway Subgrades. US Department of the Interior. MCRP 3-17.1 . Reporting and Documentation. (2010). USA. Washington. Page 6-4 Ethiopian Roads Authority . (2010).pavementinteractive. ETL . Transportation Research Board. EM 1110-3-137. Guideline and Recommended Standard for Geofoam Applications in Highway Embankments. Triaxial Test. (1991). USA. Colorado. NCHRP Report 529. Department of the Army. USA. Washington DC. (1975). Department of the Army. (2004). US DOT. Dept of Transportation.Appendix A Soil Stabilization Geotechnical Design Manual – 2013 National Cooperative Highway Research Program. (2012). Reference Manual. (2013). NHI 05-037. FHWA NHI-06-088.org/article/triaxialtest South Carolina Dept of Transportation.64. Characteristics and Problems of Dispersive Clays. Mobilisation. ETL . Department of the Army. Ch 9 in: Military Soils Engineering. Federal Highway Authority Publication No. Checklist and guidelines for review of geotechnical reports and preliminary plans and specifications. DC. Federal Highways Authority Publ No ED-88-053. Engineering Geology Field Manual. NHI-00-043. Vol 1 and 2. Soil stabilisation for roads and airfields. FM 5-410. Department of Transportation. USA.188. (2002). Afghanistan Engineer District. Washington DC. US DOT FHWA. Collapsible Soils. Embankments. US ACE.. P-91-09. EM 1110-2-1902. Dept of Transportation.. Soil and Foundations. Washington D. US Army Corps of Engineers.7G/TM 3-34. US DOI Bureau of Reclamation. US ACE. Materials Engineering Branch. US Department of the Interior. US Army Corps of Engineers. Washington. US DOI Bureau of Reclamation. Ch 4 in: Soil Stabilisation for Pavements. (1984).C. US ACE. WS Department of Transportation. 53:139. Foundations of Structures. University of Iowa. Colorado. Ch 3 in: Context Sensitive Rock Slope Design Solutions. Foundation Engineering. Ch 10 in: Geotechnical Design Manual M46-03. Soil cut design. Publ No Federal Highways Authority. FHWA-CFL/TD-11002.Geotechnical Design Manual . (2011). Rock excavation methods.08. Foundations on weak and/or compressible soils. Washington State DOT. University of Iowa. Ethiopian Roads Authority Page 6-5 .2013 Chapter 5 Geotechnical Report and Checklist US DOT FHWA CFLHD. Central Federal Lands Highway Division. 2013. (2013). . the stabilized soil is usually not considered as a structural layer in the pavement design process. wetting agents) have all been attempted. and alteration in permeability and wetting properties (surface active agents.2. Blending gravel and. asphalts). The gravel acts as filler. wet subgrades. For this case. cation fixation in expanding clays (with potassium). the gravel blend can take on the same poorer support characteristics of the natural subgrade. To stabilize expansive subgrades and reduce volume change for example. more recently. Mechanical stabilization using thick gravel layers. The use of gravel layers has been discussed in Section 2. recycled pavement material with poorer quality soils can also provide a working platform. the drainage characteristics. constructability. Stabilization with admixtures. The thickness of the subgrade to be treated is based primarily on the project economics and the objective of stabilization. cement. Ethiopian Roads Authority Page A.1 . carbonates. creating a dryer condition and decreasing the influence of plasticity. Soil stabilization is usually performed for the following reasons: • • As a construction platform to dry very wet soils and facilitate compaction of the upper layer. unless a suitable replacement soil is not economically available. The process is often called soil modification when the purpose is to change the physical properties and thereby improve the quality of the subgrade soil. lime continues to be the most widely used and most effective additive for stabilization of subgrades and rapid gain of strength. The thickness. the importance of the transportation network. has been used outside Ethiopia to control the swelling and frost heave of soils and improve the strength characteristics of unsuitable soils. For this case. When only a thin zone or short roadway length is subject to improvement.4. and the purpose of stabilization. and asphalt. 200 sieve (75 µm) and the PI of the soil is shown in Figure A-1. removal and replacement can usually be the preferred alternative. However. Among these are the anticipated traffic loads. such as lime. deactivation of sulphates (with calcium chloride). depth or zone of the subgrade that may be selected for soil improvement depends upon a number of factors. the modified soil is usually given some structural value in the pavement design process.SOIL STABILIZATION Soil stabilization is a general term that involves the use of mechanical or chemical modifiers to enhance the strength of soils and reduce the change in moisture. the geometric design. Table A-1 presents a summary of the stabilization methods used in pavement design and constructions the types of soils for which they are most appropriate. To strengthen a weak soil and restrict the volume change potential of a highly plastic (expansive) or compressible soil. ion exchange (addition of divalent and trivalent salts). phosphoric acid). in conjunction with geotextiles or geogrids is an effective technique for improving roadway support over soft. and their intended effects on soil properties. if saturation conditions return.2013 Appendix A Soil Stabilization APPENDIX A . However. Literally hundreds of chemicals or additives have been tried in the past few decades. cementation (silicates. waterproofing (silicones.Geotechnical Design Manual . A flow chart for the determination of chemical treatment options for soil stabilization based on the percent of materials that pass the No. This chart should only be used as a broad guideline. rapid strength gain. reduced breakage Strength gain through minimum disturbance and consolidation Plastic ------ Coarse -----Rapid drying. slow long-term pozzolanic cementing Lime Admixtures Remarks Silts and clays Plastic Fast. cementing of grains Dense and strong. reduced plasticity.Appendix A Soil Stabilization Geotechnical Design Manual – 2013 Table A-1 Stabilization methods for pavements. slower than cement Chemicals Plastic Strength increase and volume stability Difficult to mix Asphalt Plastic and collapsible Reduce change in moisture Long-term moisture migration problem Geomembranes Plastic and collapsible Reduce change in moisture Long-term moisture migration problem Lime-fly ash Lime-cementfly ash Bitumen Water proofers Improvement Page A. From Rollings & Rollings (1996) reproduced in US DOT FHWA (2006A) Stabilization method and materials used Mechanical More gravel Blending Geosynthetics Portland cement Soil type None Reduce dynamic stress level Moderately plastic None Too difficult to mix Others Silts and clays Improved gradation. plus provides longterm separation Less pronounced hydration of cement Hydration of cement ------ Coarse with fines Same as plastic soils Dependent on quantity of plastic fines Non-plastic Same as lime None Same as lime No reactive material Covers broader range Same as lime Same as lime Covers broader range Coarse Strengthen/bind waterproof Some fines Fines Same as coarse None Asphalt cement or liquid asphalt Liquid asphalt Cannot mix Pozzolans and slags Silts and coarse grained material Acts as a filler.2 Ethiopian Roads Authority . reduce plasticity rapidly. coarsen texture rapidly. 3 . the use of an electric potential to increase the rate of lime migration has showed little success and is rarely used.Geotechnical Design Manual . This technique has been mainly used for rehabilitation projects and new construction. However.5 m by 1. on 1. reproduced in US DOT FHWA (2006A) A. From Austroads (1998). The use of drill-holes consists basically of drilling holes into the subgrade and backfilling with a lime slurry or lime slurry-sand mixture. In the case of expansive subgrades. Generally. lime treatment may be appropriate. the lime migrates or diffuses into the soil initiating the soil-lime reactions. Lime treatment of subgrade soils is intended to speed up construction.1 Lime modification Lime treatment or modification consists of the application of 1 – 3% hydrated lime to aid drying of the soil and permit compaction. and deep-plough techniques.8m grids. Lime may also be used to treat expansive soils. 300 mm diameter holes with depths ranging from 1. and no reduction in the required pavement thickness should be made. Once placed in the holes. this diffusion process can be quite slow and time-consuming before a substantial quantity of the soil is affected. Generally. are needed for better results. depending upon the extent of treatment desired. Lime modification may also be considered to condition a soil for follow-on stabilization with cement or asphalt.8 m to 6 m. drill-hole.2013 Appendix A Soil Stabilization Figure A-1 Guide for selection of admixture stabilization methods. Some Ethiopian Roads Authority Page A. The amount of lime to be added is the minimum amount that will reduce swell to acceptable limits. pressure injection. Lime will reduce swell in an expansive soil to greater or lesser degrees. efforts have been directed elsewhere towards modifying deeper layers using electrical. depending on the activity of the clay minerals present. This is in addition to the conventional mix in-place or batch mixing approaches. If it has been determined that a soil has potential for excessive swell. 5 kg of lime per gallon of water. A PI of about 10 is commonly taken as the lower limit for suitability of inorganic clays for lime stabilization. Lime is usually produced by calcining limestone or dolomite. typically of more variable and poorer quality. The technique consists of pumping lime slurry under pressures of up to 1400KPa. For new roads other more economic measures should be adopted such as excavation and replacement and drainage.2 Lime stabilization Lime stabilization is a technique for improving soft and weak subgrades beneath flexible pavements. Lime is applicable in clay soils (CH. as defined by AASHTO T99. the effect of lime content on subgrade properties and the thickness of the stabilized subgrade are the primary variables for stabilization design. The most common varieties of lime for soil stabilization are hydrated lime (Ca(OH)2). MH and ML and in granular soils containing clay binder (GC and SC. A. The lime-stabilized subgrade should be compacted to a minimum density of 95%. Often. depending upon soil conditions. or the slurry has fractured the surface and is flowing out. the technique of lime slurry pressure injection (LSPI) was developed. Plasticity is a rough indicator of reactivity. 1 kg to 1. use of the more caustic quicklime has also grown steadily over the past two decades. Refusal is reached when soil will not take additional slurry. and the slurry. CL. through hollow injection rods into the subgrade. 40 sieve. is also produced as a by-product of other chemical processes. near optimum lime content value. the lime content should be from 3 – 8% of the dry weight of the soil. In an attempt to obtain greater distribution of lime in swelling subgrades. For lime stabilization of clay (or highly plastic) soils. quicklime (CaO). a definite improvement in serviceability index was also noted for treated sections compared to companion untreated areas. The injection rods penetrate the soil in approximately 300 mm intervals. and the cured mass should have an unconfined compressive strength of at least 350 KPa within 28 days. with a PI greater than 10 and with at least 10% passing the No. While hydrated lime remains the most commonly used lime stabilization admixture in many countries.Appendix A Soil Stabilization Geotechnical Design Manual – 2013 experiences showed that lime migration was quite limited to the periphery of the hole. The lime slurry left on the surface immediately following injection is mixed into the top 100-150mm of soil and re-compacted. Consequently. However. and the dolomitic variations of these high-calcium limes (Ca(OH)2⋅MgO and CaO⋅MgO). Lime reduces the PI and renders a clay soil less sensitive to moisture changes. A wetting agent is often added to the slurry to assist in migration. The optimum lime content should be determined with the use of unconfined compressive strength and Atterberg limits measured for different lime-soil mixtures moulded at varying percentages of lime. hydrated lime is used in powder form. A primary benefit of lime stabilization is a greatly increased stiffness as a function of the lime content. Page A. although some lime. The pH values can be used to determine the initial. The pozzolanic strength gain in clay soils depends on the specific chemistry of the soil (if it has sufficient silica and alumina minerals to support the pozzolanic reactions). Such techniques are better suited for treating existing failed pavements on highly trafficked roads due to cost. or as slurry (a mix of water and lime). is injected to refusal. the slurry is running freely either around the pipe or out of previous injection holes.4 Ethiopian Roads Authority . careful selection. and texture effects all occur very rapidly (usually with one hour after addition of lime). which. can reverse the beneficial reduction in plasticity and swell potential. plastic clay soils become more like silt or sand in texture after the addition of lime. If the problem is expected. which. However. carbonation. plasticity reduction. and the damage it causes can be quite severe.2013 Appendix A Soil Stabilization During stabilization. and improved workability. testing of stabilized soils in the soaked condition is prudent. When soils are treated with lime. Consequently. Conventional soil-lime construction techniques are normally limited to maximum depths of 200 – 300 mm. the lime stabilized soil should be tested in the laboratory to see whether it will swell when mixed and exposed to moisture. lime. it has been observed that the lime-soil mixture may be subject to durability problems.5 . The deep plough lime stabilization method was created in light of this problem. If atmospheric carbon dioxide combines with lime to form calcium carbonate. there have been instances of poor strength retention when exposed to soaking. the cyclic freezing and thawing of the soil. leaching processes. Leaching of calcium can decrease the cation exchange in lime stabilized soils. if greater depths of stabilization are required in one lift. the calcium silicate and calcium aluminate hydrate cements may become unstable and revert back to their original silica and alumina forms. in turn. provided there is a thorough mixing of the lime and the soil. The changes in the soil-water chemistry also lead to agglomeration of particles and a coarsening of the soil gradation. The durability of lime stabilization on swell potential and strength may be adversely affected by environmental influences such as water. therefore. Sulphates present in the soil or groundwater can combine with the calcium from the lime and the alumina from the clay minerals to form ettringite (a hexacalcium aluminate trisulphate hydrate. if there is a suspicion of possible sulphate attack. The technique consists of: Ethiopian Roads Authority Page A.Geotechnical Design Manual . These drying. common in hydrated Portland cement). Although most lime stabilized soils retain 70% to 85% of their long-term strength gains when exposed to water. Therefore. The addition of lime also introduces calcium (Ca+2) and magnesium (Mg+2) cations that exchange with the more active sodium (Na+) and potassium (K+) cations in the natural soil water chemistry. The potential for these effects is greater when low lime contents are used. Freeze and thaw cycles can lead to strength deterioration. and environmental conditions that will trigger sulphate attack. and compaction of the stabilized material to minimize carbon dioxide penetration. diminished susceptibility to strength loss with moisture. hydration of the lime absorbs water from the soil and causes an immediate drying effect. then the most common design approach is to specify a sufficiently high initial strength gain to retain sufficient residual strength after freeze and thaw damage. Carbonation is another problem that is common after lime stabilization. Massive irreversible swelling can. as well as prompt placement after lime mixing. It is difficult to predict the exact combinations of sulphate. and good curing. occur in the subgrade. placement. Carbonation may be minimized by the use of ample lime content. clay mineralogy. and sulphate attacks. corresponds to a reduced swell and shrinkage potential. which has a volume that is more than 200% larger than that of its constituents. reversing the long-term strength increase resulting from the pozzolanic reactions. This cation exchange reduces the plasticity of the soil. but this is not a problem in many most parts of Ethiopia. these conventional techniques are inadequate. in most cases. Weigh the lime to the nearest 0. Record the pH for each of the lime-soil mixtures. spreading the lime required for stabilization of the layer. The following is the procedure to determine the optimum lime content using a pH test.40. Representative air-dried samples.30 and only one mix gave this value.4.1 g and poured into 150-ml (or larger) plastic bottles with screw on tops. then additional test bottles should be prepared with larger percentages of lime.600 mm) should reveal that adequate densities (greater than 95% at standard compaction). has a pH of about 12. additional tests are needed if the pH is anything below 12. the lowest lime content producing the highest pH of the soil-lime mixture can be taken as the initial design lime content. and 400 . equal to 20 g of oven-dried soil. are weighed to the nearest 0. A special three-toothed ripper attachment with a trapezoidal shaped shoe-plough bolted to the teeth is normally used for ripping operations. If the highest pH is 12. 6. compacting and test rolling using a 50-ton roller making six passes. Most lime. The optimum lime content should be determined corresponding to the maximum pH of the lime-soil mixture. Page A. Tests taken at various depths (0 – 200 mm. are obtained within the subgrade.4 or equal to the pH of lime itself.30 and two or more mixes gave the same readings. the mix with the lowest percent of lime is the amount required to stabilize the soil. After one hour.01 g and add it to the soil. then the mix with the lowest percent of lime will be chosen to stabilize the soil. when placed in a water solution. spraying water over the subgrade after initial dry mixing. 200 . In this technique.30 for various reasons. • • • • • • • • • • A sufficient amount of lime should be added to soils to produce a pH of 12. mixing the lime and soil with three passes of the plough to a depth of 600 mm. The reaction that takes place when lime is introduced to a soil generally causes a significant change in the plasticity (plastic and liquid limits) of the soil. This will insure that the percentage of lime required can be determined in one hour. final mixing using a deep ripper. compacting the 600 mm depth of stabilized material in one lift using either sheepsfoot or vibratory sheepsfoot rollers. The pH meter must be equipped with an electrode and standardized with a buffer solution having a pH of 12. If the pH does not go beyond 12. The pH is a good indicator of the desirable lime content of a soillime mixture. the preferred method is to prepare several mixtures at different lime treatment levels and determine the pH of each mixture after one hour. and finally. and 7 %. Shake for a minimum of 30 seconds. Besides. Shake the bottles for 30 seconds every 10 minutes. 4.00. If the pH readings go to 12. transfer part of the slurry to a plastic beaker and measure the pH. 5. Add 100 ml of CO2 free distilled water to the bottles.6 Ethiopian Roads Authority . In order to choose the preferred lime content in a subgrade soil.Appendix A Soil Stabilization • • • • • • • Geotechnical Design Manual – 2013 ploughing the subgrade to a depth of 300 mm prior to spreading the lime. Shake the soil-lime mixture and water until there is no evidence of dry material on the bottom.400 mm. Shake the bottle to mix the soil and dry lime. It is advisable to set up five bottles with lime content of 3. Specific values required to use this figure are the PI and the % of material passing the No. Mark the % of soil binder on the Y-axis of Figure A. At the intersection with the percent of soil binder mark.Geotechnical Design Manual .2. These properties are determined from the Atterberg limit and gradation tests on the untreated soil. 40 sieve. From National Lime Association USA (1972) Ethiopian Roads Authority Page A. Read the percent of lime represented at the intersection with the 100% soil binder line. Follow the curved line starting from the PI value down to the intersection point with the horizontal line (drawn from the percent of soil binder mark on the Y-axis).2 and the PI on the upper Xaxis. move vertically upward to the 100% soil binder line. • • • • • • • Determine the PI (wet method) and the percent of soil binder (% passing No. for a soil having a P1 of 39% and 55% soil binder. 40 sieve). the lime required is about 4.2.7 . Figure A-2 Chart to determine the initial lime content. The following procedures are needed in order to estimate the initial lime content from Figures A-2.2013 Appendix A Soil Stabilization An alternate method of determining initial design lime content is by the use of Figures A.25% as shown in Fig A. Enter the plot from the upper X-axis where the PI value is marked. For example. High cement contents will unavoidably induce high incidences of shrinkage cracking caused by moisture and temperature changes. Small amounts of Portland cements may reduce swell potential of subgrade soils. bituminous stabilization is performed in place with the bitumen being applied directly on the soil or soil-aggregate system with the mixing and compaction operations being conducted immediately thereafter. Soil-bitumen. For cement stabilization of granular and/or non-plastic soils. The lowest cement content that eliminates or reduces the swell potential to the minimum is the design cement content. Procedures for measuring swell characteristics of soils are found in ASTM D 4546. For soils to be stabilized with cement. Usually. Increasing the cement content increases the quality of the mixture.3 Cement stabilization Portland cement is used widely for stabilizing low-plasticity clays. the cement content should be 3 to 10% of the dry weight of the soil.8 Ethiopian Roads Authority . and may be considered too expensive for this sort of application. the product is termed a cement-modified soil. However. 40 sieve. The durability of bitumen-stabilized mixtures can be assessed by measurement of their water absorption characteristics. Page A. Sand-gravel bitumen. The determination of cement content to reduce the swell potential of fine-grained plastic soils can be accomplished by moulding several samples at various cement contents and soaking the specimens along with untreated specimens for four days. which increases strength in cohesionless soils. as defined by AASHTO T 134. and the cured material should have an unconfined compressive strength of at least 1050 KPa within seven days. and improves strength and stiffness. Bitumen increases the cohesion and load-bearing capacity of the soil and renders it resistant to the action of water. bitumen-stabilized soils have been used elsewhere in the world for sub-base construction for engineering and economic reasons. which stabilizes the moisture content of cohesive fine-grained soils. highly plastic clays that have been pre-treated with lime are sometimes suitable for subsequent treatment. depending on the soil. Only fine-grained soils can be treated effectively with lime for marginal strength improvement. At higher cement contents. However. The Portland cement should meet the minimum requirements of AASHTO M 85. and may be divided into three major groups: • • • Sand-bitumen. A. The cement content that accomplishes soil modification should be checked to see whether it provides an unconfined compressive strength great enough to qualify for a reduced pavement thickness design (Guyer 2011). such as clean sands. The cement-stabilized subgrade should be compacted to a minimum density of 95%. the end product is termed soil-cement.Appendix A Soil Stabilization Geotechnical Design Manual – 2013 A. which has improved properties of reduced plasticity or expansive characteristics. or acts as a binder or cementing agent.4 The application of bitumen Often. proper mixing requires that the soils have a PI of less than 20 and a minimum of 45% passing the No. which provides cohesive strength and waterproofing for pitrun gravelly soils with inherent frictional strength. At low cement contents. Use of bitumen as a stabilizing agent produces varied effects. Portland cement generally is not as effective as lime. and sandy and granular soils. but also potentially including polyester. the fibres may be used singly or spun into yarns by wrapping several fibres together. or nylon. Depending on the application. creating barriers to water flow in liners and cut-offs. Furthermore. A.9 .Geotechnical Design Manual . the selection of the size of the aperture is partially dependent on the gradation of the material into which it will be placed. provide protective linings for channels. geogrids. or created by a slit film process. Geocells are typically constructed of high-density polyethylene (HDPE). These materials are also known as engineering fabrics. Geocomposite materials are often created by combining two or more of the products described above to take advantage of multiple benefits. The function of the apertures is to allow the surrounding soil materials to interlock across the plane of the geogrid. polyethylene. most commonly polypropylene. and provide multi-layered earth-retaining structures. woven geogrids. Nonwoven geosynthetics are made by placing fibres in a bed. geocomposites may be formed by combining geosynthetics with more traditional geo-materials.2013 Appendix A Soil Stabilization A. which may have a roughened surface created to aid in the performance of the membrane by increasing friction with the adjacent soil layer. although generally only fairly simply weaving patterns are used. Both use a polymer fibre as raw material. and improving drainage.5. These polymers are then punched in a regular pattern and drawn in one or two directions. and geogrid composites. support heavy construction traffic on weak subgrade soils. They consist of polymeric materials manufactured in different forms. The geogrid is manufactured using high-density polymers of higher stiffness than are common for geotextiles. Existing commercial geogrid products include extruded geogrids. consist of a regular grid of plastic with large openings (called apertures) between the tensile elements. The most common applications to roads are for reinforcing embankments and foundation soils. Geomembranes are used to retard or prevent fluid from penetrating the soil and as such consist of continuous sheets of low permeability materials. either by raising the temperature.5 Geosynthetics Geosynthetics are a class of geo-materials that are used to improve soil conditions for a number of applications. A geotextile is a permeable geosynthetic comprised solely of textiles. applying an adhesive chemical. and geomembranes. stabilize steep slopes. Hence. Woven geosynthetics are manufactured by weaving fibres or yarns together in the same way as any form of textile. The cells in three dimensional panels are opened and filled with granular material. Geogrids. The fibres are then bonded together. which are usually created from polymers. a weaving process may be used in which the crossing fibres are left wide apart and the junctions between them are reinforced. the most Ethiopian Roads Authority Page A. as their name suggests. welded geogrids. or by mechanical means. Alternatively. including geotextiles. which adds weight to make the multi-layer system act as a gravity retaining wall. either in full-length or in short sections.1 Types of geosynthetics The generic term “geosynthetic” is often used to cover a wide range of different artificially manufactured materials. Geotextiles are usually classified as either woven or non-woven. Geocells are designed to protect slopes against erosion. These materials are made by forming the polymer into a flat sheet. or drainage facilities. Stabilization using these materials is achieved through a combination of separation. Some or all of these objectives are normally achieved through at least one of the four functions (separation. abutments. The filtration function is required because soils requiring stabilization are usually wet. Page A. which might occur during construction and later in-service due to pumping of the subgrade by traffic loads.Appendix A Soil Stabilization Geotechnical Design Manual – 2013 common example being the geosynthetic clay liner. Figure A-3 Main functions of geosynthetics in pavement systems Geotextile and geogrid materials are the most commonly used geosynthetics in pavement design.5. By acting as a filter. filtration (drainage). filtration. permitting the pore pressure to dissipate. The separation function of a geotextile prevents the subgrade and the sub-base from intermixing (Figure A-4). the geotextile retains the subgrade without clogging. and reduce the thickness of the structural cross section for a given design period. and reinforcement. A. improve or extend the estimated service life of the pavement. This is especially true when only the pavement itself is considered without fills and cut slopes. while allowing water from the subgrade to pass up into the sub-base.10 Ethiopian Roads Authority . reinforcement. and promoting strength gain due to consolidation. and containment) as shown in Figure A-3. A geosynthetic clay liner consists of a layer of bentonite sandwiched together with geomembrane or geotextile materials to create a very low permeability barrier.2 The use of geosynthetics in pavements The three primary uses of a geosynthetics in a pavement system are to serve as a construction aid over soft subgrades. higher shear strength surfaces. These are conditions where the subgrade is unlikely to support conventional pavement construction without substantial rutting. as indicated in Table A-2. A geogrid with good interlocking capabilities or a geotextile with high frictional capacities can provide tensile resistance to lateral aggregate movement. as with geotextiles.11 . From CED Engineering (1998) Undrained shear strength (KPa) of subgrade 60-90 CBR% of subgrade Geosynethetic function 2-3 Filtration. separation. Table A-2. reinforcement In general. Soft subgrade soils provide very little lateral restraint (containment). Hence. separation. Geotextiles serve best as separators. Geosynthetics also increase the system bearing capacity by forcing the potential bearing surface under the wheel load to develop along alternate. longer mobilization paths and. and the range of functions potentially served increases as the subgrade strength decreases.Appendix A Soil Stabilization Geotechnical Design Manual . while geogrids are better at reinforcing. ruts develop on the surface and also in the subgrade.2013 Figure A-4 Use of geosynthetic to separate a sub-base from the subgrade Geosynthetics also provide some level of reinforcement by laterally restraining the base or sub-base and improving the bearing capacity. some separation 30-60 1-2 Filtration. thus. Table A-3 lists subgrade conditions that are considered to be the most appropriate for the use of geosynthetics. either separately or as a geocomposite. filters and. Ethiopian Roads Authority Page A. some reinforcement <30 Below 1 Filtration. Geogrids. When geogrids are used. the specific function to be provided by a geosynthetic in pavements depends on soil properties. thus decreasing shear stresses on the subgrade. in the case of non-woven geotextiles. prevent the sub-base from penetrating the subgrade. drainage layers. Function of geosynthetic with respect to subgrade properties. either the sub-base has to be designed as a separator or a geotextile must be used in conjunction with the geogrid. when the granular material moves laterally. ML. The first step in designing an effective reinforced pavement system is to determine the properties of the subgrade including the grain-size distribution. or A7-6 soils Low strength Undrained shear strength < 13 KPa. Generally. as if the geosynthetic was not present. the wheel load will create excessive rutting in the underlying soil before the geosynthetic develops enough tension to provide the required restraint. high elastic modulus geosynthetics undergo less deformation. and in-situ shear strength or bearing capacity. to develop the tensile resistance required for subgrade restraint. or PT soils. CH. or MR< 30 MPa High water table Within zone of influence of surface soils High sensitivity High undisturbed strength compared to remoulded strength When used as reinforcement.3 Design considerations There are at least three design approaches which are in use for incorporating geosynthetics in pavements. CL. Often. which corresponds to the value at which 75% of the recorded soil strength readings of the top 450 mm of the subgrade are higher. the elastic modulus could be used as an important factor for selecting geosynthetic products of similar cost. Geosynthetics used to construct roads over very soft subgrade conditions typically serve to mechanically stabilize the subgrade. In addition. A. an assessment of the applicability of geosynthetics should be conducted using the following guidance and Table A-4.12 Ethiopian Roads Authority . If placed with wrinkles or in a loose condition on a weak subgrade. Page A. The pavement is designed according to standard procedures. Once the design subgrade conditions have been determined. The in-situ shear strength can be measured directly using vane shear devices or indirectly using bearing capacity correlations from CBR. and design by function. Modified from CED Engineering (1998) Condition Related Properties Poor soils USCS: SC. CBR < 3. A-7. Thus. and any design is determined by the subgrade strength. Reinforced flexible pavement subgrade design Different combinations of geosynthetics are recommended for use in flexible pavements based upon the subgrade soil conditions. The design subgrade shear strength is defined as the 25th percentile shear strength. and hence less soil rutting. design by specification. or AASHTO: A-5. the use of geosynthetics for stabilization is completed using the design-by-function approach. MH. it is very important to place a geosynthetic in a taut or stretched condition so that it can develop full tensile resistance. As the subgrade strength increases. A-6.Appendix A Soil Stabilization Geotechnical Design Manual – 2013 Table A-3 Appropriate subgrade conditions for geosynthetic use. Atterberg limits.5. A key feature of this approach is the assumption that the structural pavement design is not modified in the procedure. OH. AASHTO and/or USCS classifications and sensitivity are also needed for comparison with the information given in Table A-3. the application of the geosynthetics transforms from mechanical subgrade stabilization to base reinforcement. the location of the groundwater table. These are: design by cost. Design an unreinforced flexible pavement. A non-woven geotextile is recommended for separation of fine-grained subgrades. For design subgrade CBR strengths of 4.0.2013 Table A-4 Recommendations for geosynthetic use in flexible pavements. Perform a life cycle cost analysis.0 and 8. When the subgrade CBR is higher than 8. The subgrade soil CBR can be determined using Fig. From US ACE (2003) Design subgrade soil strength and geosynthetic applicability CBR < 0. Design an unreinforced flexible pavement. A-5 based upon shear strength (c). and the use of a geogrid for reinforcement should be considered. The shear strength (c) can be directly measured using vane shear devices. For subgrade CBR strengths greater than 4. base and sub-base must meet strength and gradation requirements. Geogrid reinforcement should be evaluated. Thus.0 or less. Perform a life cycle cost analysis.0. A life cycle cost analysis should be made to determine the cost effectiveness of geogrid reinforcement. the primary application is mechanical subgrade stabilization.5 < CBR < 4. Use the aggregatesurfaced procedure to design a working platform with geosynthetics. Ethiopian Roads Authority Page A. use this design procedure to determine the reinforced aggregate thickness reduction. • • • • 0. for this subgrade strength level both a geotextile and a geogrid may be necessary and the aggregate thickness can be reduced using empirical reinforced pavement thickness equivalency charts. the primary geogrid application is base reinforcement. it is recommended that a construction platform be designed to facilitate the construction of the flexible pavement.0.0 A geotextile is generally NOT recommended unless prior experience has indicated separation problems. Then. The construction platform will serve as the sub base for the flexible pavement system. the following procedure can be used to design the reinforced flexible pavement: 1. For design subgrade CBR strengths between 4. Do not use this design procedure. For design subgrade CBR strengths of 0. a geotextile separator is not recommended unless prior separation problems have been noted for specific construction materials. The primary application of a geogrid at high subgrade soil strengths is base reinforcement. a geotextile separator is not recommended unless there is an experience of separation problems with the construction materials. 4.5 Use a geotextile and a geogrid at the subgrade/base interface. Then use this design procedure to determine the appropriate aggregate thickness reduction. Then. Use geosynthetic reinforcement to solve site-specific construction problems.13 .0 The design subgrade strength exceeds the existing practice. CBR > 8. Perform a life cycle cost analysis. If the use of a geotextile and/or geogrid is warranted based upon the applicability assessment.Appendix A Soil Stabilization Geotechnical Design Manual .5 or less.0 < CBR < 8. Each aggregate layer. use local procedures for flexible pavement thickness design. Research has indicated substantial extensions in pavement service life and significant potential for base thickness reductions.0 A geotextile is recommended for finegrained subgrades with a design CBR of less than 4. At these soil strengths. both the mechanical subgrade stabilization and base reinforcement applications are appropriate. No aggregate thickness reduction is recommended. As the design subgrade strength increases. Figure A-5 Relationship between CBR and shear strength (c). The graph should contain various curves represent design indexes (DI).14 Ethiopian Roads Authority .Appendix A Soil Stabilization Geotechnical Design Manual – 2013 2. The next step is to determine the design traffic. the decision to use geosynthetics is made on the basis of the following procedure: • For design subgrade CBR strengths of 0. Entering the chart with the unreinforced flexible pavement thickness. Modified from US ACE (2003) Aggregate-surfaced reinforced pavement design Geosynthetics in aggregate-surfaced roads can be used to support two pavement applications: mechanical subgrade stabilization and aggregate base reinforcement. a line is drawn to the intersection of the equivalency curve to get the thickness of the equivalent reinforced thickness.5 or less. the primary application is mechanical subgrade stabilization. it is possible to empirically determine the required unreinforced pavement thickness above the subgrade from the graph. This can be done using a graph prepared earlier using local environmental conditions. The type of geosynthetics recommended for use in aggregate-surfaced roads is based upon the subgrade soil conditions. The design traffic should be determined according to local practices which results in a design index (DI). the use of a non-woven geotextile is recommended for separation. Using the CBR and the appropriate DI. and correlate the subgrade CBR strength with required pavement thickness. 3. and a biaxial geogrid is recommended Page A. At the end. The application is pre-determined by the subgrade soil strength. there is a need to design an unreinforced flexible pavement for a given subgrade conditions. The design index combines the effect of average vehicle axle loadings and expected traffic volume as expressed by the road classification systems used for design purposes. At these soil strengths. Geosynthetics used to construct pavements over very soft subgrade conditions typically serve to mechanically stabilize the subgrade. the primary application of the geosynthetic changes from mechanical subgrade stabilization to base reinforcement. 4. The reinforced aggregate thickness is determined by using again a locally prepared reinforced pavement thickness equivalency chart. Once the index and other relevant properties of the subgrade are determined as mentioned above. The reinforced bearing capacity factor for a geotextile alone is 5. geogrids can be used as a construction expedient to solve site-specific problems.Geotechnical Design Manual . 3. A non-woven geotextile is recommended for separation at subgrade strengths with a CBR of 2 or less. defined as either a single wheel load. Once it is known that geosynthetics are needed based upon the above applicability assessment. The shear strength (c) can also be directly measured using vane shear devices. For multiple-axle vehicles. 2. A nonwoven geotextile should also be used for separation when the designer has experienced separation problems with the construction materials during previous construction projects. the following procedure can be used to design the reinforced aggregatesurfaced pavement: 1. such as site mobility and localized soft soil deposits. The construction platform serves as a bridge over very soft material. At these low material strengths. for this subgrade strength level. For design subgrade CBR strengths between 2. The next step is to determine the design traffic. use one-half of the single.0 or less.0. the full depth of the aggregate fill should be used and no reduction in aggregate thickness is recommended. The use of a non-woven geotextile for separation is generally recommended for fine-grained subgrades with design CBR values of less than or equal to 4. Instead.0.15 .0 and 4. The use of a biaxial geogrid for reinforcement is also generally cost-effective in terms of aggregate savings. the cost effectiveness of using a geogrid at these subgrade strengths should be determined by performing a life-cycle cost analysis. The design traffic gear should be based upon the gear configuration of the heaviest vehicle expected in the traffic mix. If the use of a geotextile and/or geogrid is necessary.2013 • • • Appendix A Soil Stabilization for aggregate reinforcement. both the mechanical subgrade stabilization and base reinforcement applications can be carried out. and the aggregate thickness can be reduced using the appropriate reinforced bearing capacity factor as described in the following paragraphs. both a geotextile and geogrid are generally recommended. The non-woven geotextile is placed directly on the subgrade followed by the geogrid and then the aggregate fill. For example. a compaction aid for obtaining target densities. Selecting Nc based on allowable subgrade ruts is also Ethiopian Roads Authority Page A. Thus.7. and a construction expedient. The combined weight on the selected gear is used as the design vehicle weight. Next it is necessary to determine the reinforced bearing capacity factor (Nc).8. The bearing capacity factor for a geotextile separator and geogrid reinforcement is 6. The unreinforced bearing capacity factor (Nc) is usually 2. The primary geosynthetic application for subgrade CBR strength of greater than 4 is base reinforcement. use one-half of the total load on the heaviest two neighbouring axles.5 CBR or less. Both the unreinforced and reinforced bearing capacity factors should normally be determined using empirical data obtained from local test sections. the subgrade soil strength must be converted from CBR to shear strength (c) using Figure A-5. For design subgrade CBR strengths of 2. Thus. the primary geogrid application is base reinforcement.or dual-wheel axle load for singleaxle vehicles. Geogrid reinforcement is generally considered costprohibitive for these types of subgrades. or tandem-wheel gear load. the unreinforced aggregate thickness design should be used for subgrade strengths of 0. However. a dual-wheel load. Recommended bearing capacity factors are summarized in Table 2-10. These values reduce to Nc = 2. Nc. In order to find out the required aggregate thickness.Appendix A Soil Stabilization Geotechnical Design Manual – 2013 common: Nc = 5 for a low rutting (< 50 mm). the procedure requires the use of curves drawn by using the subgrade bearing capacity (cNc) and the expected single. double and tandem wheel loads.7 6. Use this design procedure for aggregate thickness reduction.3. 2. Choosing the curve for specific design wheel loads and using an appropriate cNc enables determining the required aggregate thickness. From US ACE (2002 and 2003) Design subgrade soil strength and geosynthetic applicability CBR < 0. 3.0 6. 2 Both a geotextile and a geogrid are recommended.8. Table A-5 Recommendations for geosynthetic use in aggregate-surfaced pavements. is often 2.0. Nc1 Nc Geotextile Geogrid Both2 Geotextile Geogrid CBR > 4.0 Perform a cost analysis. Perform a life cycle cost analysis.0 < CBR < 4.16 Ethiopian Roads Authority .5 < CBR < 2.7 6. No aggregate thickness reduction recommended. Both 5. 4.0 A geotextile is required for fine-grained subgrades.0 Both a geogrid and a geotextile are recommended. and 6 for large rutting (> 100 mm). A geogrid may also be costeffective.7 The unreinforced bearing capacity factor.5 for moderate rutting (50 – 100 mm). 5.5 Use a geotextile and a geogrid at the subgrade/base interface. 5. with the former serving primarily as a separation fabric.0 6. Determine the subgrade bearing capacity by multiplying the reinforced or unreinforced bearing capacity factor (Nc) by the shear strength of the subgrade (c). 0. respectively without a geotextile. 1 Page A. or 3.8.7 5.