ine of al ⁎, gy, 33 Received 2 April 2007; received in revised form 23 August 2007; accepted 17 September 2007 granite, sandstone, rocksalt, dolomite (Colbach andWild, 1965; Simpson and Fergus, 1968; Knipe and Rutter, 1990). It was also shown that the mineral composition Available online at www.sciencedirect.com Engineering Geology 95 (20 ⁎ Corresponding author. Tel.: +91 135 2525403; fax: +91 135 1. Introduction Environmental Geotechnology evaluating the engi- neering behavior of different rockmass under different set of environmental conditions is an emerging field (Manassero and Deangeli, 2002). The slake durability, especially for weak rocks, is an important engineering parameter (Franklin and Chandra, 1972; Rodrigues, 1991; Dick and Shakoor, 1995; Gokceoglu et al., 2000) and represents the degradability of rocks due to the process of chemical and mechanical breakdown as in exfoliation, hydration, solution, oxidation, abrasion. It is closely related to the mineralogical composition of rocks and their texture (Beavis, 1985). Furthermore the pH of the fluid influences the mechanical and chemical breakdown of rocks indicating the necessity for the assessment of effect of pH on slake durability. Many studies have been carried to understand the effect of different environmental conditions on the me- chanical properties of rocks like quartzite, limestone, Slake durability is an important geotechnical parameter and is a measure of degradability of rocks due to the process of mechanical and chemical breakdown. It is closely related to the mineralogical composition and the texture of the rocks. In this paper, mineralogical examination along with slake durability tests under variable pH conditions, both in acidic and alkaline environments, on the limestone, shale and siltsone were evaluated to understand the relationship between mineralogy and the degradability of rocks. The study revealed that rocks rich in calcium carbonate and or magnesium carbonate are adversely affected in the acidic environment, whereas, the rocks rich in quartz, feldspar and muscovite are independent of the pH of the slaking fluid, which in turn, is more influenced by the texture of the constituent minerals. It has also been observed that fine grained rocks are more susceptible to degrade in comparison to the coarse grained rocks. © 2007 Elsevier B.V. All rights reserved. Keywords: Slake durability index; pH; Mineralogical properties; Lesser Himalaya Abstract Available online 4 October 2007 The effect of pH of water and m durability (degradability) Lesser Him Vikram Gupta Geotechnical Laboratory, Wadia Institute of Himalayan Geolo 2625212. E-mail addresses:
[email protected],
[email protected] (V. Gupta). 0013-7952/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2007.09.004 ralogical properties on the slake different rocks from the aya, India Iqrar Ahmed General Mahadeo Singh Road, Dehra Dun - 248 001, India 07) 79–87 www.elsevier.com/locate/enggeo along with their texture like crystal interlocking, crystal shape and size, surface roughness, crystal area, crystal The eering Geology 95 (2007) 79–87 perimeter length and effective porosity are greatly related to slake durability of rocks (Moon, 1993; Papadopoulos et al., 1994; Dhakal et al., 2002; Yilmaz and Karacan, 2005; Kolay and Kayabali, 2006). Kamon et al. (1996) investigated the engineering properties of lime under variable pH conditions. Singh et al. (1999) reported that the strength characteristics of the rocks decrease linearly Fig. 1. Location map of the study area. 80 V. Gupta, I. Ahmed / Engin as the strength of the acid increases. Singh et al. (2006) also reported the strength characteristics of marble is highest at pH 7 and further the strength reduction is greater in the acidic conditions than in the alkaline environment. Singh et al. (2005) investigated the effect of acidity of water on the slake durability index of shales in the laboratory conditions and further used artificial neural network system and neuro-fuzzy system for the prediction of slake durability index of the shaly rocks. Sunil et al. (2006) investigated the effect of different pH solutions on the engineering properties and the chemical character- istics of soil. In the present paper, we discuss the effect of variable pH of water on the slake durability of different rocks collected from the Lesser Himalaya. 2. Material and methods Rock samples used in this study were collected from fresh exposures surroundingMussoorie township (Fig. 1). The sampling sites form a part of the Mussoorie Syncline and the rocks exposed in the area belongs to unmetamor- phosed sedimentary sequence of the Blaini–Krol–Tal Succession of the Mussoorie Group of Krol Belt (Auden, 1934). Five samples of limestone from Krol Formation (Lst1-5), two samples of shale, one each from Krol Formation (Sh-1) and Blaini Formation (Sh-2) and two samples of siltstone from Tal Formation (Slst 1–2) were analyzed. The sampling locations are marked in Fig. 1. About 30 cm×20 cm×15 cm block of each sample sampling site are marked in the figure. was collected in the field. The selection of samples was based on the homogeneity in visible macroscopic features like texture and color.Mineralogical, textural and physical properties like the dry density (γd), the saturated density (γsat), the water absorption (wa), the porosity (n) and the specific gravity (G) of each sample were determined prior to performing slake durability tests (Table 1). The mineral Table 1 Index properties of different rocks of the Lesser Himalayan sequence collected from around Mussoorie township Rock type Sample no. Density (g/cm3) Water absorption (wa) Porosity n (%) Specific gravity Dry (γd) Saturated (γsat) Limestone Lst-1 2.73 2.74 0.36 2.52 2.64 Lst-1 2.76 2.77 0.45 4.97 2.86 Lst-1 2.53 2.57 1.39 11.9 2.63 Lst-1 2.81 2.82 0.19 2.86 2.84 Lst-1 2.70 2.73 1.12 9.60 2.85 Shale Sh -1 2.66 2.67 0.40 3.56 2.74 Sh -2 2.71 2.72 0.56 3.18 2.75 Siltstone Slst-1 2.55 2.61 2.08 7.88 2.64 Slst-2 2.65 2.67 0.84 2.60 2.69 composition and textural features of the rocks were studied by means of optical microscopy and X-ray diffractometry. All the physical properties cited above were determined as per the standard testing procedures. Slaking fluid of different pH was prepared by adding 1 molar HCl for acidic solution and 1molar NaOH for basic solution in distilled water. 3. Mineralogical properties The mineral constituents along with its texture like crystal interlocking, crystal shape and size, surface roughness, crystal area and crystal perimeter length greatly influence the engineering properties of the rocks. These were studied in thin sections using optical microscopy. Microscopic studies on the limestone reveal that Lst-1 is very fine grained having sub-angular to sub-rounded grains, arranged in an interlocking pattern. The size of these grains is less than 63 micron. These are traversed by number of randomly oriented micro cracks filled with recrystallized calcite (Fig. 2a). Lst-2 and Lst-4 are coarse grained. The average size of the grains varies between 0.07 mm and 0.42 mm. The grains are sub-angular to sub-rounded and are arranged in an interlocking fashion (Fig. 2b and d). A prominent vein of about 100 μmwide, filled with sub-rounded quartz and calcite cut across the Lst-2 (Fig. 2b). Lst-3 is highly weathered, porous and very fine grained limestone. Number of pores of variable shape and size are present. It has porosity of 11.90%. Some of the pores and cracks are filled with fine with and s m gra , sub- 81V. Gupta, I. Ahmed / Engineering Geology 95 (2007) 79–87 Fig. 2. Photomicrographs of the thin sections showing (a) Lst-1having which are filled with recrystallized calcite (b) Lst-2 having coarse grains and calcite veins (c) Lst-3 having large number of pores of variable size grains arranged in an interlocking fashion (e) Lst-5 having fine to mediu fine and sub-rounded grains of quartz and calcite (g) Sh-2 having fine parallel fashion and supported by clay matrix (h) Slst-1 having fine and sub-r by carbonaceous matrix (i) Slst-2 having fine and sub-rounded grains of qua grains traversed by large number of randomly orientated micro-cracks sub-angular and sub-rounded grains traversed by coarse grained quartz hape (d) Lst-4 having coarse grains with sub-angular and sub-rounded ins traversed by recrystallized calcite along the fracture (f) Sh-1 having rounded and greenish color grains of quartz and calcite aligned in the ounded grains, mainly of quartz, muscovite and feldspar and supported rtz, muscovite and feldspar and supported by clay matrix. eering 82 V. Gupta, I. Ahmed / Engin recrystallized calcite (Fig. 2c). Lst-5 is fractured, fine to medium grained limestone. The average size of the grains is 0.035 mm. The fractures are filled with Fig. 3. Typical results of the XRD analyses for t Geology 95 (2007) 79–87 recrystallized calcite with average grain size of about 0.75 mm giving the impression of well developed comb shaped structure (Fig. 2e). he rock samples used in the present study. 2006) and the laterite (Sunil et al., 2006) under different pH condition to understand their degradability under different pH conditions. In the present study, in order to understand the effect of the variation in pH of water on the deterioration of the rocks from the Lesser Himalaya, the slake durability tests were performed on five samples of limestone and two samples each of shale and siltstone. In order to perform the tests, ten rock pieces of each rock type and each weighing between 40 and 60 gm were taken. The 83eering Geology 95 (2007) 79–87 In order to semi quantitatively analyze the chemical composition of limestones, X-ray diffractometry onwhole rock powder was carried out (Fig. 3). The analyses reveal that Lst-1 is a pure calcite with 97% CaCO3 and 3% dolomite [CaMg (CO3) 2],whereas, Lst-2 is 99%dolomite and 1% calcite, Lst-3 is 80% dolomite, 8% calcite and 12% illite, Lst-4 is 95% dolomite, 3%calcite and 2% illite, and Lst-5 is 99% dolomite and 1% calcite (Fig. 3a–e). Shale (Sh-1) from the Krol Formation is fine grained having sub-rounded grains (Fig. 2f) and comprises 28% quartz and 60% calcite along with 12% illite and kaolinite whereas, Sh-2 from the Blaini Formation is green colored with well developed parallel alignment of matrix. It is mainly supported by clay matrix (Fig. 2g). It comprises 23% quartz, 24% chlorite, 20% feldspar and 33% illite (Fig. 3f–g). Number of shear fractures having displacement of the order of 5–7 mm is visible in the thin section. Siltstones (Slst1-2) from the Tal Formation are fine grained and mainly comprise quartz (about 13%), mus- covite (61–70%) and feldspar (12–17%) (Fig. 3h–i). Most of the grains in Slst-1 are sub-rounded and supported by carbonaceous matrix (Fig. 2h), whereas, in Slst-2, the grains are elongated and supported by clay matrix (Fig. 2i). 4. Index properties of rocks Following the standard procedure outlined in ISRM (1981), the dry density (γd), the saturated density (γsat), the water absorption (wa), the porosity (n) and the specific gravity (G) were determined and presented in Table 1. The evaluation of the basic engineering properties was carried out on 25mmdiameter cores of all rock types. These cores were washed thoroughly and oven dried for 24 h at 110 °C. For the saturated density, the water absorption and the porosity, oven dried cores were dipped in water and weighed constantly at fixed interval of time until a constant weight was attained. Dry and saturated density is calculated as the ratio of dry and saturated mass to their volume. Porosity was calculated by water saturation method whereas the specific gravity was calculated by water displacement method (Brown, 1981). 5. Slake durability test Slake durability test (Franklin and Chandra, 1972) is used to predict the potential deterioration of durability of rocks, due to the influence of climate. This test was successfully done on sulphate bearing rocks such as gypsum and anhydrite (Bell, 1994; Papadopoulos et al., V. Gupta, I. Ahmed / Engin 1994; Kayabali et al., 2006), the marble (Singh et al., edges of these rock pieces were made blunt with chisel. These rock pieces were rotated in a drum half immersed in a slaking fluid at about 20 °C for 10 min at 20 rpm. The drum was made of sieve mesh of 2 mm so that the product of the slaking from the rock could pass through the sieve into the water. The rock pieces retained in the drum are oven dried at 110 °C for 24 h, cooled and weighed. The test is repeated for all the rock samples at pH of the liquid 2, 4, 6, 8, 10 and 12. Five cycles of test on each rock types was carried out. The Slake Durability Index (SDI) which is a measure of the degradability of rocks, corresponding to each cycle was calculated as the percentage ratio of final and initial dry weight of rocks in the drum. The lower value of SDI represents the higher susceptibility of rocks to degrade under a given set of condition. Johnson and DeGraff (1988) based on the durability of rocks classified them into six classes ranging from rocks with very low durability to rocks with very high durability (Table 2). The results are presented in Fig. 4. 6. Evaluation of results and discussion A consistent pattern exists between slake durability Index (SDI) of different rocks and the pH of the testing solution (Fig. 4). In general, it has been observed that for rocks containing calcite or dolomite, higher amount of slaking (the susceptibility of rocks to degrade) occurs at lower pH (Fig. 4a–f). This is because the calcite and dolomite are mainly composed of CaCO3 which is Table 2 Two-cycle slake durability classification of rocks (Johnson and DeGraff (1988) Cycle 2 (% Retained) Cycle 1 (% Retained) Durability b30 b60 Very low 30–60 60–85 Low 60–85 85–95 Medium 85–95 95–98 Medium high 95–98 98–99 High N98 N99 Very high Fig. 4. The results of the first and second cycle slake durability tests conducted on five samples (Lst 1–5) of limestone, and two each of shale and siltstone collected from the Lesser Himalaya near Mussoorie township (Square indicate the results of the first cycle; and the triangle indicate the second cycle). 84 V. Gupta, I. Ahmed / Engineering Geology 95 (2007) 79–87 highly susceptible to be attacked by acidic agents. At low pH value, the dissolution of calcium carbonate is faster as the acid is more able to attack the free charged particle that binds the carbonate (CO3) (Singh et al., 2006). The speed of the deterioration varies with the nature of the acid. The anomalous value at pH 6 for Lst- 5 (Fig. 4e) is probably related to material characteristics arranged in an interlocking fashion. The average SDI for second cycle (Id2) tested under different pH solution for Lst-1, Lst-2, and Lst-5 is 99.08%, 99.04% and 99.02% respectively representing the limestone to exhibit high to very high durability (Fig. 5 and Table 2). The average SDI for second cycle (Id2) for all set of pH conditions for coarse grained limestone (Lst-4) and Fig. 5. Average slake durability index for second cycle (Id2) of various limestone when tested under different pH condition. 85V. Gupta, I. Ahmed / Engineering Geology 95 (2007) 79–87 such as the existence of the fractures. The slake durability index for the first (Id1) and the second cycle (I2) for limestones tested under different pH solution appear to be greatly related to texture of rock, rather than the ionic strength of the testing solution. Lst-3 with lowest average SDI (98.81%) for second cycle (Id2) tested under different pH solution is highly weathered and porous (Fig. 5), whereas Lst-4 with highest average SDI (99.28%) for second cycle (Id2) is coarse grained having sub-angular grains Fig. 6. Slake durability index for second cycle plotted for coarse grained (Lst for fine grained limestone (Lst-1) has been plotted in Fig. 6. It has been found that for all pH condition, the course grained limestone is less susceptible to degrade as compared to the fine grained limestone. This is probably because for fine grained rocks, more surface area is exposed to be attacked by the outside agents and in the present case, it is the testing fluid. Two samples of shale tested for slake durability exhibit entirely different pattern. Sh-1 from Krol For- mation, which mainly comprises calcite (60%) and -4) and fine grained (Lst-1) limestone for various set of pH conditions. ISRM suggested methods, 211. Pergamon Press, Oxford. eering quartz (28%) exhibit a pattern similar to limestone, where it has been observed that higher amount of slaking (degradability) occurs at lower pH (Fig. 4f). The anomalous value at pH 12 may be related to the presence of quartz and discontinuities. Sh-2 from the Blaini Formation mainly comprises quartz (23%), feldspar (20%), chlorite (24%) and illite (33%), shows that the amount of slaking is independent of the pH of the testing fluid (Fig. 4g). The anomalous value at pH 8 may be related to the presence of clay bands and shear fractures. Both the siltstones (Slst 1–2) from the Tal Formation mainly comprise fine grained quartz (13%), muscovite (61–70%) and feldspar (12–17%). The test result indicates that the higher amount of slaking (degrad- ability) occurs at pH 8 (Fig. 4h–i). The comparison of the average SDI for Id2 for all set of pH condition for siltstone reveals that the SDI for Slst-1 is lower than Fig. 7. Average slake durability index for second cycle (Id2) of siltstones when tested under different pH condition. 86 V. Gupta, I. Ahmed / Engin Slst-2 (Fig. 7) implying that the degradability of Slst-1 is higher than Slst-2. This may be related to the sub- rounded nature of the grains of Slst-1 which provide less friction as compared to the elongated and sub-angular grains of Slst-2 providing higher frictional resistance. 7. Conclusions The present study was carried out on limestone, shale and siltstone samples collected from the Lesser Hima- layan sequence near the Mussoorie township, to assess their degradability under different sets of pH condition. It has been observed that the degradability of rocks is greatly influenced by their mineral constituents and texture. For rocks containing an appreciable amount of calcium carbonate (about 60%), like limestone (Lst 1–5) and calc shale (Sh-1) in the present case, the degrad- ability is higher in the acidic solution (Fig. 4a–f). The Colbach, P.S.B., Wild, B.I., 1965. The influence of moisture content on compressive strength of rocks. 3rd Canadian rock mechanics symposium, Toronto, pp. 63–65. Dhakal, G., Yoneda, T., Kato, M., Kaneko, K., 2002. Slake durability and mineralogical properties of some pyroclastic and sedimentary rocks. Engineering Geology 65, 31–45. Dick, J.A., Shakoor, A., 1995. Characterizing durability of mud rocks for slope stability purposes. Geological Society of America, Reviews in Engineering Geology 10, 121–130. Franklin, J.A., Chandra, A., 1972. The slake durability test. International Journal of Rock Mechanics and Mining Sciences 9, 325–341. Gokceoglu, C., Ulusay, R., Sonmez, H., 2000. Factor affecting the durability of selected weak and clay bearing rocks from Turkey, with particular emphasis on the influence of the number of drying and wetting cycles. Engineering Geology 57, 215–237. ISRM, 1981. In: Brown, E.T. (Ed.), Suggested method: Rock Char- acterization, Testing and Monitoring. Pergamon Press, Oxford. 211 pp. Johnson, R.B., DeGraff, J.V., 1988. Principles of Engineering Geology. Wiley, New York. 497 pp. Kamon, M., Ying, C., Katsumi, T., 1996. Effect of acid rain on lime and cement stabilised soils. Japan Geotechnical Society 36 (4), 91–96. Knipe, J.R., Rutter, E.H., 1990. Deformation mechanism, rheology reduction in strength is mainly due to chemical reaction of the calcium carbonate (CaCO3) with acidic solution which reduces the bond strength between the different particles. It has also been observed that fine grained limestone is more susceptible to degrade as compared to the coarse grained. The knowledge of the degradability behavior of the rocks of the Lesser Himalaya under different set of pH condition will be of asset to the geotechnical engineers as lot of development activities in the form of hydroelectric power projects are either coming up or being planned in the area. Acknowledgements The authors thank the Director, Wadia Institute of Himalayan Geology, Dehra Dun for providing all the necessary facilities to carry out the work and granting permission to publish the paper. All the required tests have been carried out in the Geotechnical Laboratory of the Wadia Institute of Himalayan Geology, Dehra Dun. References Auden, J.B., 1934. The Geology of Krol belt. Records Geological Survey of India I 67 (4), 357–364. Beavis, F.C., 1985. EngineeringGeology. Blackwell,Melbourne. 231 pp. Bell, F.G., 1994. A survey of the engineering properties of some anhydrite and gypsum from the North and Midlands of England. Engineering Geology 38, 1–23. Brown, E.T., 1981. 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Slaking durability and its effect on the doline formation in the gypsum. Environmental Geology 47, 1010–1016. 87V. Gupta, I. Ahmed / Engineering Geology 95 (2007) 79–87 The effect of pH of water and mineralogical properties on the slake durability (degradability) ..... Introduction Material and methods Mineralogical properties Index properties of rocks Slake durability test Evaluation of results and discussion Conclusions Acknowledgements References