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Engineering Geology 9

The effect of pH of water and mineralogical properties on the slakedurability (degradability) of different rocks from the

Lesser Himalaya, India

Vikram Gupta ⁎, Iqrar Ahmed

Geotechnical Laboratory, Wadia Institute of Himalayan Geology, 33 General Mahadeo Singh Road, Dehra Dun - 248 001, India

Received 2 April 2007; received in revised form 23 August 2007; accepted 17 September 2007Available online 4 October 2007

Abstract

Slake durability is an important geotechnical parameter and is a measure of degradability of rocks due to the process ofmechanical and chemical breakdown. It is closely related to the mineralogical composition and the texture of the rocks. In thispaper, mineralogical examination along with slake durability tests under variable pH conditions, both in acidic and alkalineenvironments, on the limestone, shale and siltsone were evaluated to understand the relationship between mineralogy and thedegradability of rocks. The study revealed that rocks rich in calcium carbonate and or magnesium carbonate are adversely affectedin 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 aremore 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

1. Introduction

Environmental Geotechnology evaluating the engi-neering behavior of different rockmass under differentset of environmental conditions is an emerging field(Manassero and Deangeli, 2002). The slake durability,especially for weak rocks, is an important engineeringparameter (Franklin and Chandra, 1972; Rodrigues,1991; Dick and Shakoor, 1995; Gokceoglu et al., 2000)and represents the degradability of rocks due to the

⁎ Corresponding author. Tel.: +91 135 2525403; fax: +91 1352625212.

E-mail addresses: vgupta@wihg.res.in, vgupta_wihg@yahoo.com(V. Gupta).

0013-7952/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.enggeo.2007.09.004

process of chemical and mechanical breakdown as inexfoliation, hydration, solution, oxidation, abrasion. It isclosely related to the mineralogical composition of rocksand their texture (Beavis, 1985). Furthermore the pH ofthe fluid influences the mechanical and chemicalbreakdown of rocks indicating the necessity for theassessment of effect of pH on slake durability.

Many studies have been carried to understand theeffect of different environmental conditions on the me-chanical properties of rocks like quartzite, limestone,granite, sandstone, rocksalt, dolomite (Colbach andWild,1965; Simpson and Fergus, 1968; Knipe and Rutter,1990). It was also shown that the mineral compositionalong with their texture like crystal interlocking, crystalshape and size, surface roughness, crystal area, crystal

Fig. 1. Location map of the study area. The sampling site are marked in the figure.

Table 1Index properties of different rocks of the Lesser Himalayan sequencecollected from around Mussoorie township

Rocktype

Sampleno.

Density(g/cm3)

Waterabsorption(wa)

Porosityn (%)

Specificgravity

Dry(γd)

Saturated(γsat)

Limestone Lst-1 2.73 2.74 0.36 2.52 2.64Lst-1 2.76 2.77 0.45 4.97 2.86Lst-1 2.53 2.57 1.39 11.9 2.63Lst-1 2.81 2.82 0.19 2.86 2.84Lst-1 2.70 2.73 1.12 9.60 2.85

Shale Sh -1 2.66 2.67 0.40 3.56 2.74Sh -2 2.71 2.72 0.56 3.18 2.75

Siltstone Slst-1 2.55 2.61 2.08 7.88 2.64Slst-2 2.65 2.67 0.84 2.60 2.69

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perimeter length and effective porosity are greatly relatedto slake durability of rocks (Moon, 1993; Papadopouloset al., 1994; Dhakal et al., 2002; Yilmaz and Karacan,2005; Kolay and Kayabali, 2006). Kamon et al. (1996)investigated the engineering properties of lime undervariable pH conditions. Singh et al. (1999) reported thatthe strength characteristics of the rocks decrease linearlyas the strength of the acid increases. Singh et al. (2006)also reported the strength characteristics of marble ishighest at pH 7 and further the strength reduction isgreater in the acidic conditions than in the alkalineenvironment. Singh et al. (2005) investigated the effect ofacidity of water on the slake durability index of shales inthe laboratory conditions and further used artificial neuralnetwork system and neuro-fuzzy system for the predictionof slake durability index of the shaly rocks. Sunil et al.(2006) investigated the effect of different pH solutions onthe engineering properties and the chemical character-istics of soil.

In the present paper, we discuss the effect of variablepH of water on the slake durability of different rockscollected from the Lesser Himalaya.

2. Material and methods

Rock samples used in this study were collected fromfresh exposures surroundingMussoorie township (Fig. 1).The sampling sites form a part of the Mussoorie Synclineand 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 KrolFormation (Sh-1) and Blaini Formation (Sh-2) and twosamples of siltstone from Tal Formation (Slst 1–2) wereanalyzed. The sampling locations are marked in Fig. 1.

About 30 cm×20 cm×15 cm block of each samplewas collected in the field. The selection of samples wasbased on the homogeneity in visible macroscopic featureslike texture and color.Mineralogical, textural and physicalproperties like the dry density (γd), the saturated density(γsat), the water absorption (wa), the porosity (n) and thespecific gravity (G) of each sample were determined priorto performing slake durability tests (Table 1). The mineral

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composition and textural features of the rocks werestudied by means of optical microscopy and X-raydiffractometry. All the physical properties cited abovewere determined as per the standard testing procedures.Slaking fluid of different pH was prepared by adding 1molar HCl for acidic solution and 1molar NaOH for basicsolution in distilled water.

3. Mineralogical properties

The mineral constituents along with its texture likecrystal interlocking, crystal shape and size, surfaceroughness, crystal area and crystal perimeter length greatlyinfluence the engineering properties of the rocks. Thesewere studied in thin sections using optical microscopy.

Fig. 2. Photomicrographs of the thin sections showing (a) Lst-1having finewhich are filled with recrystallized calcite (b) Lst-2 having coarse grains withand calcite veins (c) Lst-3 having large number of pores of variable size and sgrains arranged in an interlocking fashion (e) Lst-5 having fine to medium grafine and sub-rounded grains of quartz and calcite (g) Sh-2 having fine, sub-parallel fashion and supported by clay matrix (h) Slst-1 having fine and sub-rby carbonaceous matrix (i) Slst-2 having fine and sub-rounded grains of qua

Microscopic studies on the limestone reveal that Lst-1is very fine grained having sub-angular to sub-roundedgrains, arranged in an interlocking pattern. The size ofthese grains is less than 63 micron. These are traversedby number of randomly oriented micro cracks filled withrecrystallized calcite (Fig. 2a). Lst-2 and Lst-4 are coarsegrained. The average size of the grains varies between0.07 mm and 0.42 mm. The grains are sub-angular tosub-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 theLst-2 (Fig. 2b). Lst-3 is highly weathered, porous andvery fine grained limestone. Number of pores of variableshape and size are present. It has porosity of 11.90%.Some of the pores and cracks are filled with

grains traversed by large number of randomly orientated micro-crackssub-angular and sub-rounded grains traversed by coarse grained quartzhape (d) Lst-4 having coarse grains with sub-angular and sub-roundedins traversed by recrystallized calcite along the fracture (f) Sh-1 havingrounded and greenish color grains of quartz and calcite aligned in theounded grains, mainly of quartz, muscovite and feldspar and supportedrtz, muscovite and feldspar and supported by clay matrix.

Fig. 3. Typical results of the XRD analyses for the rock samples used in the present study.

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recrystallized calcite (Fig. 2c). Lst-5 is fractured, fine tomedium grained limestone. The average size of thegrains is 0.035 mm. The fractures are filled with

recrystallized calcite with average grain size of about0.75 mm giving the impression of well developed combshaped structure (Fig. 2e).

Table 2Two-cycle slake durability classification of rocks (Johnson andDeGraff (1988)

Cycle 2 (% Retained) Cycle 1 (% Retained) Durability

b30 b60 Very low30–60 60–85 Low60–85 85–95 Medium85–95 95–98 Medium high95–98 98–99 HighN98 N99 Very high

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In order to semi quantitatively analyze the chemicalcomposition of limestones, X-ray diffractometry onwholerock powder was carried out (Fig. 3). The analyses revealthat Lst-1 is a pure calcite with 97% CaCO3 and 3%dolomite [CaMg (CO3) 2],whereas, Lst-2 is 99%dolomiteand 1% calcite, Lst-3 is 80% dolomite, 8% calcite and12% 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 grainedhaving sub-rounded grains (Fig. 2f) and comprises 28%quartz and 60% calcite along with 12% illite andkaolinite whereas, Sh-2 from the Blaini Formation isgreen colored with well developed parallel alignment ofmatrix. It is mainly supported by clay matrix (Fig. 2g). Itcomprises 23% quartz, 24% chlorite, 20% feldspar and33% illite (Fig. 3f–g). Number of shear fractures havingdisplacement of the order of 5–7 mm is visible in thethin section.

Siltstones (Slst1-2) from the Tal Formation are finegrained 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 andsupported by carbonaceous matrix (Fig. 2h), whereas, inSlst-2, the grains are elongated and supported by claymatrix (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 specificgravity (G) were determined and presented in Table 1. Theevaluation of the basic engineering properties was carriedout on 25mmdiameter cores of all rock types. These coreswere washed thoroughly and oven dried for 24 h at110 °C. For the saturated density, the water absorption andthe porosity, oven dried cores were dipped in water andweighed constantly at fixed interval of time until aconstant weight was attained. Dry and saturated density iscalculated as the ratio of dry and saturated mass to theirvolume. Porosity was calculated by water saturationmethod whereas the specific gravity was calculated bywater displacement method (Brown, 1981).

5. Slake durability test

Slake durability test (Franklin and Chandra, 1972) isused to predict the potential deterioration of durability ofrocks, due to the influence of climate. This test wassuccessfully done on sulphate bearing rocks such asgypsum and anhydrite (Bell, 1994; Papadopoulos et al.,1994; Kayabali et al., 2006), the marble (Singh et al.,

2006) and the laterite (Sunil et al., 2006) under differentpH condition to understand their degradability underdifferent pH conditions.

In the present study, in order to understand the effectof the variation in pH of water on the deterioration of therocks from the Lesser Himalaya, the slake durabilitytests were performed on five samples of limestone andtwo samples each of shale and siltstone. In order toperform the tests, ten rock pieces of each rock type andeach weighing between 40 and 60 gm were taken. Theedges of these rock pieces were made blunt with chisel.These rock pieces were rotated in a drum half immersedin 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 theproduct of the slaking from the rock could pass throughthe sieve into the water. The rock pieces retained in thedrum are oven dried at 110 °C for 24 h, cooled andweighed. The test is repeated for all the rock samples atpH of the liquid 2, 4, 6, 8, 10 and 12. Five cycles of teston each rock types was carried out. The Slake DurabilityIndex (SDI) which is a measure of the degradability ofrocks, corresponding to each cycle was calculated as thepercentage ratio of final and initial dry weight of rocksin the drum. The lower value of SDI represents thehigher susceptibility of rocks to degrade under a givenset of condition. Johnson and DeGraff (1988) based onthe durability of rocks classified them into six classesranging from rocks with very low durability to rockswith very high durability (Table 2). The results arepresented in Fig. 4.

6. Evaluation of results and discussion

A consistent pattern exists between slake durabilityIndex (SDI) of different rocks and the pH of the testingsolution (Fig. 4). In general, it has been observed that forrocks containing calcite or dolomite, higher amount ofslaking (the susceptibility of rocks to degrade) occurs atlower pH (Fig. 4a–f). This is because the calcite anddolomite are mainly composed of CaCO3 which is

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 andsiltstone collected from the Lesser Himalaya near Mussoorie township (Square indicate the results of the first cycle; and the triangle indicate thesecond cycle).

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Fig. 5. Average slake durability index for second cycle (Id2) of various limestone when tested under different pH condition.

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highly susceptible to be attacked by acidic agents. Atlow pH value, the dissolution of calcium carbonate isfaster as the acid is more able to attack the free chargedparticle that binds the carbonate (CO3) (Singh et al.,2006). The speed of the deterioration varies with thenature of the acid. The anomalous value at pH 6 for Lst-5 (Fig. 4e) is probably related to material characteristicssuch as the existence of the fractures.

The slake durability index for the first (Id1) and thesecond cycle (I2) for limestones tested under differentpH solution appear to be greatly related to texture ofrock, rather than the ionic strength of the testingsolution. Lst-3 with lowest average SDI (98.81%) forsecond cycle (Id2) tested under different pH solution ishighly weathered and porous (Fig. 5), whereas Lst-4with 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

arranged in an interlocking fashion. The average SDIfor second cycle (Id2) tested under different pH solutionfor Lst-1, Lst-2, and Lst-5 is 99.08%, 99.04% and99.02% respectively representing the limestone toexhibit high to very high durability (Fig. 5 and Table 2).

The average SDI for second cycle (Id2) for all set ofpH conditions for coarse grained limestone (Lst-4) andfor fine grained limestone (Lst-1) has been plotted inFig. 6. It has been found that for all pH condition, thecourse grained limestone is less susceptible to degradeas compared to the fine grained limestone. This isprobably because for fine grained rocks, more surfacearea is exposed to be attacked by the outside agents andin the present case, it is the testing fluid.

Two samples of shale tested for slake durabilityexhibit 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.

Fig. 7. Average slake durability index for second cycle (Id2) ofsiltstones when tested under different pH condition.

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quartz (28%) exhibit a pattern similar to limestone,where it has been observed that higher amount ofslaking (degradability) occurs at lower pH (Fig. 4f). Theanomalous value at pH 12 may be related to the presenceof quartz and discontinuities. Sh-2 from the BlainiFormation mainly comprises quartz (23%), feldspar(20%), chlorite (24%) and illite (33%), shows that theamount of slaking is independent of the pH of the testingfluid (Fig. 4g). The anomalous value at pH 8 may berelated to the presence of clay bands and shear fractures.

Both the siltstones (Slst 1–2) from the Tal Formationmainly comprise fine grained quartz (13%), muscovite(61–70%) and feldspar (12–17%). The test resultindicates that the higher amount of slaking (degrad-ability) occurs at pH 8 (Fig. 4h–i). The comparison ofthe average SDI for Id2 for all set of pH condition forsiltstone reveals that the SDI for Slst-1 is lower thanSlst-2 (Fig. 7) implying that the degradability of Slst-1 ishigher than Slst-2. This may be related to the sub-rounded nature of the grains of Slst-1 which provide lessfriction as compared to the elongated and sub-angulargrains of Slst-2 providing higher frictional resistance.

7. Conclusions

The present study was carried out on limestone, shaleand siltstone samples collected from the Lesser Hima-layan sequence near the Mussoorie township, to assesstheir degradability under different sets of pH condition. Ithas been observed that the degradability of rocks isgreatly influenced by their mineral constituents andtexture. For rocks containing an appreciable amount ofcalcium 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

reduction in strength is mainly due to chemical reactionof the calcium carbonate (CaCO3) with acidic solutionwhich reduces the bond strength between the differentparticles. It has also been observed that fine grainedlimestone is more susceptible to degrade as compared tothe coarse grained. The knowledge of the degradabilitybehavior of the rocks of the Lesser Himalaya underdifferent set of pH condition will be of asset to thegeotechnical engineers as lot of development activities inthe form of hydroelectric power projects are eithercoming up or being planned in the area.

Acknowledgements

The authors thank the Director, Wadia Institute ofHimalayan Geology, Dehra Dun for providing all thenecessary facilities to carry out the work and grantingpermission to publish the paper. All the required testshave been carried out in the Geotechnical Laboratory ofthe Wadia Institute of Himalayan Geology, Dehra Dun.

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