Information PaperShaft Friction Capacity of Mini-Pile: A Case-Study in Infrastructure Exhibition Gallery at City Hall Annex STRUCTURAL ENGINEERING BRANCH ARCHITECTURAL SERVICES DEPARTMENT September 2011 Structural Engineering Branch, ArchSD Information Paper - Shaft Friction Capacity of Mini-Piles Issue No./Revision No. : Draft 1 (September 2011) Page 1 of 14 File Code: Frictional Piles.doc CTW/MKL/CYK Issue/Revision Date :September 2011 piled foundation is widely used to support building structures if the structure is located on a site where surface sub-soil strata are not sufficiently strong enough to support the loads from the building structures. this contribution is important for floating piles where the bedrock is at great depth.6×SPT-N values for continuous flight auger piles to 0. Furthermore. Theory and Literature Review Figure 1 shows the forces acting upon an axially loaded pile.1. the shaft friction component is usually neglected. engineers in Hong Kong may rely on the Hiley Formula in predicting the load carrying capacity of piles.doc CTW/MKL/CYK Issue/Revision Date :September 2011 . 1.3 2. Hence. especially for cast-in-situ construction using augered or Odex methods which may have caused disturbance to the soil along the pile shaft. fmax. For percussive piles. or friction piles/floating piles relying on the shaft friction between the soil and the pile shaft./Revision No.1 Introduction In Hong Kong. and will present the findings for the shaft friction correlated against the SPT-N values of the sub-soil. or a combination of both. Engineers.7×SPT-N values for piles formed by boring with an auger and temporary casing. For non-percussion cast in-situ concrete pile. if the rock end bearing stratum is available at a reasonable depth. 2. ArchSD Information Paper . and hence shaft friction often contributes the bearing capacity in practical situations. and the design values have to be further verified by trial piles before construction. The ultimate shaft friction has also been estimated based on the results of the proof load test and with the help of the commercially available finite element program . however. This paper presents the field measurements carried out to measure the shaft friction in an instrumented frictional mini-pile in a project in a project of our Department at City Hall Annex for the development of a permanent planning and infrastructure exhibition gallery. Piles are either end-bearing on hard stratum. for end bearing piles (e. is a function of depth. and by integrating that resistance over the surface of pile can give the total shaft resistance Rs. In theory. rock-socketted steel H-piles and large diameter bored piles). 1. However. shaft friction is developed after small relative displacements between the soil and the pile shaft.Shaft Friction Capacity of Mini-Piles Issue No.1 Structural Engineering Branch. the proportion of shaft frictional capacity will be small when compared with the end bearing capacity. : Draft 1 (September 2011) Page 2 of 14 File Code: Frictional Piles.2 1. the design shaft friction (in kPa) varies from a maximum of 1.g. have uncertainties on the shaft friction between the soil and the pile shaft.Plaxis 3D Foundation. the unit shaft friction. The α-method is. applicable to cohesive soil with s=0. αmethod (Tomlinson 1971). and σ’h is the effective horizontal stress due to overburden. method of installation. The coefficient α depends on the nature and strength of the cohesive soil. and time effects. and is not applicable to the typical soil in Hong Kong. pile dimensions. Burland (effective stress) method (Burland 1973). Burland -method (1973).doc CTW/MKL/CYK Issue/Revision Date :September 2011 Structural Engineering Branch.Shaft Friction Capacity of Mini-Piles Issue No. the ultimate bearing capacity of a friction pile may develop up to 80 – 90% of its capacity through shaft friction. This method further assumes that the shaft resistance is independent of the effective overburden pressure./Revision No. where both cu and s are present. is therefore given by the following equation: fs = ca = αcu where α is an empirical adhesion factor to reduce the average undrained shear strength along the pile length. Nottingham and Schmertmann CPT method (1975. 1978). models the long-term drained shear strength Page 3 of 14 File Code: Frictional Piles. 2. Meyerhof method (1976)) have then been developed to compute the shaft resistance along a pile shaft. which is applicable to both cohesive and cohesionless soil.g. : Draft 1 (September 2011) . and the unit shaft resistance fs. ArchSD Information Paper . The unit shaft friction (fs) along the pile shaft is theoretically determined by the sum of pile to soil cohesion and friction components in the following equation: fs = ca + σ’h tans where ca and s are respectively the adhesion and friction parameters between the soil and the pile shaft. however. Nordlund method (1963). Numerous theoretical methods (e.Figure 1 Stresses and forces on an axially loaded pile (Source: O’Neill 2001) Because relatively large displacements are required in floating/friction piles to mobilize the end bearing capacity.2 The α (total stress) method suggests that the ultimate capacity of the pile is be determined from the undrained shear strength (cu) of the cohesive soil (Tomlinson 1971). 5 – 2. Meyerhof (1976) provides the correlation factor of the average mobilized shaft friction fmax (in kPa) with the SPT-N values to be 2 for driven pile and 1 for bored piles. Littlechild et al (2000). Similarly. Hong Kong. Ks = earth pressure coefficient. For shaft-grouted mini-piles. ArchSD Information Paper . based from a number of loading test results on the foundations in a KCRC project.6. and the level of confidence is not particularly where the scatter in SPT N values is large”. GEO (1996.5. Chan et al (2004) used a correlation factor of 2. GEO (1996. and found satisfactory performance in the subsequent loading test to twice the design working load.doc CTW/MKL/CYK Issue/Revision Date :September 2011 Structural Engineering Branch.85 for a frictional mini-pile with post-grouting construction method in a project for the former Kowloon-Canton Railway Corporation (the “KCRC”) in Tuen Mun. Based on the insitu measurements. GEO (2006) summarized that the correlation factor can range from 1. : Draft 1 (September 2011) ./Revision No. 2006) summarizes the work of various researchers and in-situ tests.8 – 1. The advantages of this method are that it is very easy to use and that SPT-N values data is typically available for a project.Shaft Friction Capacity of Mini-Piles Issue No.4 to 5.4N for design. and the unit shaft resistance fs is calculated using the following equation: fs = σ’v where (Bjerrum-Burland beta coefficient) = Kstan. Because of limited data in Hong Kong. GEO (2006) suggests that the average mobilized shaft friction fmax (in kPa) in Table 1 for different types of pile can be used. σ’v = average effective overburden pressure along the pile shaft. in-situ measurements have been carried out locally to correlate the shaft friction between the soil and the pile shaft.8 to 1.4 200 1. in-situ measurements on the shaft friction have not frequently been carried out for frictional mini-piles without shaft grouting.3 to 3. with latter being more commonly adopted to calculate the unit shaft friction and base resistance of friction piles.0 80 (Source: GEO 2006) 2. Meyerhof method (Meyerhof 1976) estimates pile capacity based on semi-empirical correlation between standard penetration test blowcount (SPT-N values) results and static pile load tests. and that the limiting value for SPT-N values can be taken to be Page 4 of 14 File Code: Frictional Piles. 2. Table 1 Suggested correlation between mobilized (maximum) shaft friction and SPT-N values for saprolites of Hong Kong Types of pile Bored pile Driven steel H-pile Suggested fmax /N Limiting N-values 0. this paper recommends that the average mobilized shaft friction fmax (in kPa) for mini-piles without shaft-grouting can be taken as 0.3 In Hong Kong. and provides the correlation factor of the average mobilized shaft friction fmax in (kPa) with the SPT-N values for different types of piles and methods of construction.4 GEO (2006) cautions that “[t]he design method involving correlations with SPT results is empirical in nature. Foundation Design and Construction (GEO 2006) published by the Geotechnical Engineering Office recommends the use of either Burland method or Meyerhof method. However.conditions of piles using the effective stress. reported that the correlation factors range from 1. and = friction angle between the soil and the pile shaft. 2006) further recommends the base resistance to be ignored in calculating the load carrying capacity of the pile. Proposed 5-storey Lift and Stairs Block Figure 2 Location of the proposed 5-storey block Photo 1 Architectural impression of the completed project Structural Engineering Branch. 3. An instrumented pile had been installed with strain gauges in a project in Central. : Draft 1 (September 2011) Page 5 of 14 File Code: Frictional Piles./Revision No. The location of the proposed new block is shown in Figure 2. Hong Kong in order to validate the recommended correlation factor between the shaft friction and the SPT-N values. In order to facilitate the public to access to the new gallery. Photo 1 shows the architectural impression of the completed project.doc CTW/MKL/CYK Issue/Revision Date :September 2011 . a new 5-storey block comprising a lift and staircase is added adjacent to the main entrance of the City Hall Annex.Shaft Friction Capacity of Mini-Piles Issue No. 3. ArchSD Information Paper .80.1 Project Details The project is to provide a new exhibition gallery at the existing City Hall Annex for the Planning Department. /Revision No.467N. The shaft friction of the pile section in the soil strata of fill and marine deposit have not been included in the design. limited site access. In the design. due to the early setting of the grout before extraction. The founding depth of the instrumented pile was at 45m below the ground level. Page 6 of 14 File Code: Frictional Piles.2 Ground investigation shows that the area is underlain by 15 metres deep of fill/marine deposit/alluvium overlaying 33 metres of completely decomposed granite. the design shaft friction fs (in kPa) was therefore taken as 0. the congested site also permits the use of small drilling equipment for mini-piles rather than the heavy machineries for large diameter bored or pre-bored rocksocketted steel H-piles. 4.doc CTW/MKL/CYK Issue/Revision Date :September 2011 Structural Engineering Branch.1 Design and Construction of Mini-Piles The frictional mini-pile for this project consists of a 152×152×37 kg/m UC installed in a prebored hole formed into soil with a temporary steel casing with minimum internal diameter of 305 mm and then grouted with cement grout followed by extraction of temporary steel casing before the setting of grout.5 mPD in accordance with the data obtained from the Hong Kong Observatory. Shear bars are provided to the steel sections. The ground water level is governed by the tidal effect of the nearby sea and the highest is at about +2. : Draft 1 (September 2011) . limited working space and the concern on noise and vibration problems affecting the nearby City Hall). Table 2 summarizes the soil profile from the ground investigation together with the range of SPT-N values. and this was considered not to be an economical option. Bedrock of Grade III granite can only be found at about 48 metres below ground. Typical details of the frictional mini-pile are shown in Figure 3. During the construction.3. ArchSD Information Paper .4N and the SPT-N value was limited to 80. or mini-piles) with pile length of approximately 53 metres will be required. Moreover. 4. The construction of the 14 nos. and the end-bearing capacity of the pile has also been ignored. The maximum theoretical safe loading capacity of the frictional mini-pile was 580kN. With a factor of safety of 3.Shaft Friction Capacity of Mini-Piles Issue No. Table 2 Summary of sub-soil strata Soil Type Depth interval (m) Fill/ Marine Deposit/ Alluvium 0 – 15m Completely Decomposed Granite 15 – 48m Grade III Bedrock > 48m SPT-N values 6 – 18 43 – 200 - Having considered various factors (including subsoil profile. the mini-piles will be installed at a depth of 7 or 8m more. As Grade III granitic bedrock can only be found at about 48 metres below ground. of frictional mini-piles commenced in May 2010 and was completed in July 2010. the piling option with 14 nos. especially that a lightweight structural steel superstructure scheme will be adopted. end-bearing piles (in the form of large diameter bored. the average mobilized shaft friction fmax (in kPa) for the mini-piles was taken as 1. or pre-bored rocksocketted steel H-piles. of frictional minipiles (whose loading capacity would be provided by the shaft friction between the soil and the pile shaft) was adopted. If the present design method is not followed. the casing of the upper portion of the instrumented mini-pile was accidentally left in to a depth of 27m below the ground level. /Revision No. ArchSD Information Paper .doc CTW/MKL/CYK Issue/Revision Date :September 2011 . of Geokon vibrating wire strain gauges were installed at ten levels along the pile length with 2 numbers of strain gauges at each level. of frictional mini-piles was designated as the instrumented pile in order to verify the actual loading behaviour and to validate the shaft friction values adopted in the design. Figure 4 Diagrammatical representation of the instrumented pile Structural Engineering Branch. : Draft 1 (September 2011) Page 7 of 14 File Code: Frictional Piles.Shaft Friction Capacity of Mini-Piles Issue No.2 Instrumentation One of the 14 nos. A total of 20 nos. The set-up of the instrumented pile is shown diagrammatically in Figure 4.Figure 3 Typical details of pile 4. Shaft Friction Capacity of Mini-Piles Issue No.4. Structural Engineering Branch.065mm and 6.3 Field Measurements and Analysis The shaft friction along the instrumented pile was calculated by measuring the pile head movement and strains along the pile during a static loading test to twice the theoretical load carrying capacity (i. The maximum pile head settlement at a load of 1160 kN was found to be 11. : Draft 1 (September 2011) Page 8 of 14 File Code: Frictional Piles. and shows that the strain along the pile decreases with the depth of pile in general.542mm respectively in the contract. 1160 kN). Figure 5 Load-Settlement Curve of the Instrumented Pile Figure 6 plots the variation of the strains along the pile length.132mm and the residual pile head settlement was 2.doc CTW/MKL/CYK Issue/Revision Date :September 2011 .515mm which are far below the limiting values of 28. This observation is in line with the expectation as the pile load was transmitted from the steel sections to the cement grout and then to the surrounding soil by shaft friction along the pile shaft. The load settlement curve of the pile is shown in Figure 5./Revision No. the axial load distribution along the pile length can be deduced from the elastic modulus of the pile section. From the distribution of strain distribution.e. ArchSD Information Paper . The load applied to the pile head through four loading cycles. and readings of the strain gauges and pile head movements were taken at each load increment throughout the loading test. 44 to 0.doc CTW/MKL/CYK Issue/Revision Date :September 2011 . : Draft 1 (September 2011) Page 9 of 14 File Code: Frictional Piles. and either a shorter pile length or a higher capacity for the same pile length is feasible. and also shows the SPT-N values along the pile shaft. The ratio of the shaft friction to the SPT-N values for the pile section in the CDG layer of the soil stratum is found to range from 0. Structural Engineering Branch. the static loading test revealed that there could be much more reserve capacity for this pile type due to the relatively small maximum and residual pile head settlement obtained.Shaft Friction Capacity of Mini-Piles Issue No. The relationship between the shaft friction and the SPT-N values can then be deduced accordingly. ArchSD Information Paper . However. Table 3 shows the calculation of the axial load distribution and the shaft friction to the SPT-N values along the pile length.Figure 6 Distribution of Average Strain Distribution along Pile Length Figure 7 shows the axial load distribution along pile length. The test results well proved the construction method and performance of the frictional mini-piles in this project./Revision No.50 for the load tested. Structural Engineering Branch. ArchSD Information Paper .Shaft Friction Capacity of Mini-Piles Issue No.doc CTW/MKL/CYK Issue/Revision Date :September 2011 Figure 7 Distribution of axial load distribution along pile length ./Revision No. : Draft 1 (September 2011) Page 10 of 14 File Code: Frictional Piles. /Revision No. : Draft 1 (September 2011) Page 11 of 14 File Code: Frictional Piles.Shaft Friction Capacity of Mini-Piles Issue No. ArchSD Information Paper .Structural Engineering Branch.doc CTW/MKL/CYK Issue/Revision Date :September 2011 Table 3 Calculation of Shaft Friction along Instrumented Pile . Shaft Friction Capacity of Mini-Piles Issue No. the load-settlement curve from the computer analysis matches with that of the loading tests. Figure 8 Validation of the Finite Element Model With the soil model/parameters established.e.4. it can be found that the ultimate load carrying capacity of the pile is about 2600 kN. By plotting the commonly adopted Davidson’s off-set criterion for loading test (i. the finite element model can then be used to simulate the load-settlement curve of the pile for an applied load beyond the maximum test load of the loading test until its failure. ArchSD Information Paper . Figure 8 compares the load settlement curve for the pile from Plaxis 3D Foundation and that from the static loading test. PL/AE + D/120 + 4mm). and the mobilized shaft friction fmax to the Structural Engineering Branch. : Draft 1 (September 2011) Page 12 of 14 File Code: Frictional Piles. the stress distribution along the pile shaft was further analysed by the commercially available finite element program Plaxis 3D Foundation. Figure 9 Load-Settlement Curve of the Pile by Finite Element Analysis The relationship between the maximum mobilized shaft friction and the SPT-N values can therefore be deduced. The loadsettlement curve of the pile in the finite element model is shown in Figure 9. By adjusting the soil parameters in the model in the program./Revision No.4 Finite Element Analysis With the data of the instrumented pile obtained from the static loading test.doc CTW/MKL/CYK Issue/Revision Date :September 2011 . G G (1976). 6(3). Journal of the Soil Mechanics and Foundations. and Tam. Hill. 1/96: Pile Design and Construction (Hong Kong: GEO). G D.4 for the bored piles without shaft-grouting in Hong Kong. S J. New Technological and Design Developments in Deep Foundations (Houston: University of Houston). Meyerhof.44 to 0. : Draft 1 (September 2011) Page 13 of 14 File Code: Frictional Piles. Ground Engineering.50 for the load tested. pp. C K. ArchSD Information Paper . The Structural Engineer. 89(SM3). “Shaft Friction of Piles in Clay”. ASCE. pp. pp. Nottingham. J Y C (2004). pp. 102(3). R L (1963). References Burland. pp. The relationship between the design shaft friction fs to SPTN values is obtained from the analysis of the results from the instrumented pile.1. A H K. 5. ASCE. “Prebored Friction Mini-pile Foundation for Light Rail Grade Separation”.Shaft Friction Capacity of Mini-Piles Issue No. Journal of the Geotechnical Engineering Division. GEO Publication No. “Bearing Capacity and Settlement of Pile Foundations”. J B (1973).doc CTW/MKL/CYK Issue/Revision Date :September 2011 . GEO (1996). Chow. It also demonstrates the benefit and option of using this pile type against the conventional pile types which have to be founded on or socketted into the rock where the rockhead level may be very deep. The result is in line with the in-situ measurements summarized in GEO (2006). The ratio of shaft friction fs to the SPT-N values is found to range from 0. Nordlund. The test results in this project further well proved the construction method and performance of frictional mini-piles. and the result shows that the correlation factor between the maximum mobilized shaft friction fmax to the SPT-N values is about 1. GEO Publication No. 30-42 Chan.1. Littlechild. 1/2006: Foundation Design and Construction (Hong Kong: GEO).33-45. L C (1975). 24-7. and Lee. 195-228. S C (2000). Structural Engineering Branch. This paper presents the use of the frictional mini-piles founding on soil in a project of our Department at City Hall Annex for the development of a permanent planning and infrastructure exhibition gallery. Tsang. Finite element analysis with the use of the data from the pile load test is used to estimate the maximum mobilized shaft friction of the pile. which is in line with GEO (2006) that the ratio ranges from 0. B D. R N. where the average mobilized shaft friction fmax to the SPT-N values ratio ranges from 0.4 for the bored piles without shaftgrouting in Hong Kong./Revision No.8 to 1. 82(20). Plumbridge. “Shaft Grouting of Deep Foundations in Hong Kong”. in N D Dennis. Use of Quasi-Static Penetrometer Data to Predict Load Capacity of Piles (Gainesville: University of Florida). “Bearing Capacity of Piles in Cohesionless Soils”. GEO (2006). Jr et al (eds). Concluding Remark Shaft friction along a pile is hard to be estimated accurately especially for castin-situ construction using augered or Odex methods which may have caused disturbance to soil along the pile shaft.8 to 1. 1–35.SPT-N values for the pile section in the CDG layer of the soil stratum ranges is found to be about 1. Journal of Geotechnical and Geoenvironmental Engineering. ArchSD Information Paper . J H (1978). DC: Departments of Transportation).O’Neill. pp. Pile Design and Construction Practice (London: E & FN Spon. 127(1). “Some Effects of Pile Driving on Skin Friction”. Schmertmann. Proceedings of Conference on Behavior of Piles. “Side Resistance in Piles and Drilled Shafts”. ICE. Poulos. FHWA-TS-78-209 Report: Guidelines for Cone Penetration Test. Elastic Solutions for Soil and Rock Mechanics (New York: John Wiley & Sons). Tomlinson. 3–16. M J (1971). H G and Davis./Revision No. Performance and Deign (Washington. 4th ed).Shaft Friction Capacity of Mini-Piles Issue No.doc CTW/MKL/CYK Issue/Revision Date :September 2011 . Tomlinson. Structural Engineering Branch. M W (2001). M J (1994). London. pp 107–14. : Draft 1 (September 2011) Page 14 of 14 File Code: Frictional Piles. E H (1974).