IN-SITU SHEAR STRENGTH OF PHYLLITE ROCK MASSE.Rakesh Reddy., Veer Surendra Sai University of Technology, rakesh15794@gmail.comE.Saibaba Reddy., Veer Surendra Sai University of Technology,esreddy1101@gmail.co" esreddy1101@gmail.comB.Haritha Reddy., Veer Surendra Sai University of Technology, harithareddy1293@gmail.comABSTRACT: Hydroelectric power projects are pollution free and renewable sources of energy. These projects involve the construction of dams across the flowing stream of water. Construction of dams in rock regions requires stability analysis of the foundation and the supporting rocks at the banks of the reservoir area. Stability of the concrete foundation over the rocky strata and stability of the banks depend upon the shear strength of the concrete-rock and rock-rock interfaces, respectively. Therefore, evaluation of shear strength parameters (cohesion and friction angle) at rock- rock and concrete- rock interface is the most important primary step for stability analysis and design of dams. Determination of shear strength parameters involves the in-situ tests in drifts on the dam axis site and interpretation of test results. In view of the above, site-specific shear strength parameter interpretation is carried out for Amochu hydroelectric power project site, Bhutan. This project envisages construction of 175 m high concrete gravity dam across river Amochu. The existing rock type on both left and right banks of the site is observed to be weathered Phyllites. From the analysis of 20 field tests in 2 drifts, it has been found that for rock-rock interface shear strength values are approximately 50% higher at the right bank as compared to the left bank. On the other hand, for concrete-rock interface, this variation is around 10% only. Therefore, seeing the large variation of estimated shear strength in rock-rock interface, it is proposed to use individual bank stability analysis whereas, marginal variation in concrete-rock interface suggests the use of the combined analysis of the data from both the banks for foundation stability analysis. Further, rock specific (for Phyllites) shear strength equation is also proposed in the present paper based on the values of shear strength parameters (cohesion and friction-angle) from other projects in the Phyllite rocks.INTRODUCTIONHimalayan region is endowed with huge hydropower potential. Numerous rivers and streams traverse through these hilly regions. The design of foundations of dams in rocky strata includes determination of bearing capacity, settlement analysis and sliding/slope stability analysis. Bearing capacity and settlement analysis involve the ability of the rock foundation to support the imposed loads without shear failure and without excessive settlements. Sliding stability analysis involves the ability of the rock foundation or slope to resist the imposed loads without shearing or sliding. Both analyses must be coordinated and satisfied in the design. For performing theses analyses shear strength parameters viz. cohesion and friction-angle are used as input parameters. There are two major categories of analysis, a) Analysis at the concrete foundation and rock joint interface and b) Analysis at the rock over rock interface. Henceforth, determination of shear strength parameters is required for the concrete over rock (C/R) interface as well as rock over rock (R/R) interface on the project site. Former is for stability analysis of concrete dam resting over rocky strata while later is concerned with the ability of the surrounding rocks to hold the reservoir..GEOLOGY OF THE PROJECTThe Exploratory drift on the right bank of the river is located on the dam axis at an elevation 263.00 m, i.e. about 28.0 m above the river level. The drift has encountered fine grained dark gray quartzite-phyllite. The rocks in the drift are also traversed by 7 sets of joints as given in Table 1.The Exploratory drift on the left bank of the river is located on the dam axis at an elevation 245.00 m. The drift has encountered fine grained dark gray quartzite- phyllite. The rocks in the drift are also traversed by 9 sets of joints as given in Table 2.Page 1 of 9Table 1. Details of joint sets in right bank drift [3]Sl.JointOrientationDescriptionJ1Smooth1(foliationN 00 to N350/40-70joint)planar2J2N335 to N340/50-80Roughplanar3J3N90 to N100/65to 85Roughplanar4J4N290 toRoughN310/60to80planar5J5N290toN310/35to50Roughplanar6J6N10 to N30/20to 40Roughplanar7J7N30 to N60/40 to75RoughplanarTable 2. Details of joint sets in left bank drift [3]Sl.JointOrientationDescription1J1N 00 to N350/20 to 80Smooth planar(foliationjoint)2J2N40 to N60/40 to 80Rough planar3J3N185 to N200/17 toRough planar454J4N240 to N270/65 toRough planar805J5N240toN260/10 toRough planar206J6N160 to N180/80toRough planar857J7N330 to N335/40 toRough planar758J8N130 /70Rough planar9J9N155 to N165/35Rough planarto40TEST PROCEDUREIn-situ shear tests at rock-rock and concrete- rock interfaces are conducted at the dam axis site of proposed hydroelectric project at Amochu (Bhutan). The tests are conducted in two drifts named as DR-5 on right bank and DL-6 on left bank. Blocks of rock mass for R/R interface test and concrete blocks for C/R interface test are prepared for testing purpose as per provisions of IS 7746:1991 and ISRM: 1981 [1,2]. Fig.1 shows the schematic diagram of the test-setup as given in IS 7746:1991 [1]. Fig.1 Schematic diagram of in-situ shear test set-up [1].Following step by step testing procedure is carried out on the site:Diamond wheel chisel and hammer along with rock breakers are used to separate the rock mass of block size (70cm 70 cm 35 cm) from the parent rock to create R/R interface. Steel frame of 20 mm thickness MS plate is placed over the block and then it is filled with cement grout. For C/R interface, the rock surface is leveled as much as possible by removing all undulations with the help of a chisel and a hammer. The rock breaker is also used in case of hard rock. After leveling the rock surface, the concrete blocks (70 cm x 70 cm x 35 cm) are prepared by using steel mould. All the blocks are cured for 28 days before the test. 20 mm thick MS plates are used to prepare side and top reaction pads and strengthened by RCC. The care is taken to keep the top and side reaction pads concentric with the block. It should be noted that the horizontal reaction pad has to be prepared such that it does not break during the application of horizontal thrust. Vertical load is applied using a 200 tones capacity hydraulic jack. Hallow cylinders of aluminum alloy are used to fill up the gap between the top reaction pad and the Page 2 of 9In-situ shear strength of phyllite rock masshydraulic jack. Shear force is applied by another 200 tones capacity hydraulic jack from the side reaction padNormal loads of 20, 25, 30, 35 and 40 tones were applied in the five tests, respectively. The shear load is applied at an angle of 150 with the horizontal so that the sheared plane coincides with a plane of weakness in the rock. This is achieved by two wooden wedges of appropriate geometry placed across the jack. Five such blocks for R/R interface and C/R interface are prepared. Each block is tested for a particular normal stress which is kept constant during the test. The shear force and displacement of block are measured and recorded during the test. The vertical, horizontal, and lateral displacements of the block, produced during the test are measured by 9 dial-gauges, 4 for normal displacement, 3 for shear displacement, and 2 for lateral displacement. The observations are recorded till failure and continued even after the failure at which no further rise or fall in shear strength is observed with increasing shear displacement to get the information regarding residual frictional resistance. ASSUMPTIONInterpretation of the test results are based on the following assumptions:Effect of pore water pressure is implicitly taken as the tests are conducted in the saturated condition. Complete interface area is involved in the process of shearing. Shear strength is governed by Mohr-Coulomb criteria only. All the reaction pads are infinitely stiff and not prone to any deformation. The rock mass has been considered to be ideally intact i.e. without cracks and voids. FORMULATIONNormal stress (n) and shear stress () are obtained, respectively, from applied normal load (Pna) and applied shear load (Psa) during the test, using the Eq. 1 and 2.t =Ps=Psa cosa(1)AAs=Pn=Pna + Psa sin a(2)nAAWhere, Ps, Pn , A, and , are net shear force, net normal force, gross cross-sectional area of the interface, and inclination of the applied shear force, respectively. It is worth mentioning here that to keep the normal stress constant (as required in the specified test procedure), the applied normal force is reduced by an amount Psa sin after each increment in the applied shear force.TEST RESULTS AND DISCUSSIONThe test data is collected in terms of applied shear load and observed shear displacement at the applied constant vertical load. This exercise is then repeated for five vertical loads as shown in Table 3Table 3. Applied vertical load and corresponding test sequenceTest No.Vertical loadTest No. 120 tonTest No. 225 tonTest No. 330 tonTest No. 435 tonTest No. 540 tonThe collected data is used to derive the shear stress (using Eq. 1) and corresponding shear displacement. Further, the plots of shear stress versus shear displacement are prepared for R/R interface at right bank drift (after here DR-5), R/R interface at left bank drift (after here DL-6), C/R interface at DR-5, and C/R interface at DL-6 as shown in Figs. 2 to 5, respectively [3].Figs. 2 to 5 are clear depiction of the shear behavior of the interfaces. Initially, required shear stress for shear displacement increase and it reaches at a peak value that is known as peak shear strength. After this point, curves start showing an asymptotic behavior and shear stress reduces to aPage 3 of 9value known as residual shear stress. Table 4 shows the shear displacement achieved at the peak shear stress, values of peak shear stress and residual shear stress for specified normal load, corresponding to R/R interfaces.Fig. 2: Shear stress V/S shear displacement plot or R/R interface shear tests in drift DR-5Fig. 3: Shear stress V/S shear displacement plot for R/R interface shear tests in drift DL-6Fig. 4: Shear stress V/S shear displacement plot for C/R interface shear tests in drift DR- 5 Fig. 5: Shear stress V/S shear displacement plot for C/R interface shear tests in drift DL-6Table 4. Comparison of shear displacement curves for various normal loads at R/R interfaceAppliedShearPeakshearResidualdisplacement atnormalstressshearstresspeakshearload(kg/cm2)(kg/cm2)stress (mm)TonnesDL 6DR 5DLDR 5DLDR 566209.8012.865.946.794.676.362514.2117.205.917.885.136.70301.177.606.7011.045.9110.25357.5312.846.3111.435.919.854012.019.899.4613.017.6911.83Table 5. Comparison of shear displacement curves for various normal loads at C/R interface.AppliedShearPeak shearResidualdisplacement atnormalstressshear stresspeak shearload(kg/cm2)(kg/cm2)stress (mm)TonnesDL 6DR 5DLDRDLDR65652011.307.594.664.133.953.23255.1912.615.195.934.454.823014.2012.265.323.594.263.20358.6911.966.585.595.925.264013.8811.817.127.136.005.99Similarly, Table 5 above shows the shear displacement achieved at the peak shear stress, values of peak shear stress and residual shear stress for specified normal load, corresponding to C/R interfaces. The comparative study based on the Figs. 2 to 5 and Tables 4&5 leads to following important observations:There is no definite trend of observed shear displacement (at peak shear stress) with applied vertical load. The reason might be Page 4 of 9In-situ shear strength of phyllite rock massthe presence of discontinuity on the rock surface in the form of joints or cracks.There is an increasing trend (not necessarily monotonous for example See DR-5) in peak shear stress with the applied vertical stress. Similarly there is an increasing trend in residual shear stress with the applied vertical stress. Observation (b) and (c), suggest that the peak and residual shear strength of the rock mass increases with the applied normal stress. This conclusion supports the use of Coulumb criteria in the determination of shear strength parameters at the interface.SHEAR STRENGTH PARAMETERSIn selecting shear-strength parameters for rock, the analysis should not be limited to a single approach, rather should incorporate several approaches into the selection process. Comparison of values obtained from several alternative approaches, together with a thorough understanding of rock-mechanics principles and prerequisites will provide the necessary basis for selection of meaningful shear-strength parameters. However, as discussed above in the present paper, the Coulumb criterion of shear strength (f) is used as given in Eq. 3.t f= c +s ntanf(3)Where, c is cohesion, n is applied normal stress, and is angle of friction at the interface, is used as the governing behavior for the interpretation of the shear test results in the present study. The shear stresses at the failure and the residual shear stresses are plotted against the corresponding normal stresses. Then, this data set is used to fit the Equation 3 and to determine the cohesion (c) and friction angle (). Figs. 6 to 11, show the scatter plots of shear stress (f) (peak and residual) versus normal stress (n) and the best fit lines for C/R interface in DL-6, DR-5 and combined DL-6 & DR-5, R/R interface in DL-6, DR-5, and combined DL-6 & DR-5, respectively. The constant term of the best fit line represents the cohesion (c) and the slope is tangent of friction-angle (tan ). Fig. 6: Shear stress V/S normal stress plot for C/R interface in drift DL-6Fig. 7: Shear stress V/S normal stress versus plot for C/R interface in drift DR-5Fig. 8: Shear stress V/S normal stress plot for C/R interface in combined DR-5 & DL-6Fig. 9: Normal stress V/S shear stress plot for R/R interface in drift DL-6Page 5 of 9Fig. 10: Shear stress V/S normal stress plot for shear tests on R/R interface in drift DR-5 Table 6. Shear Strength Parameters on rock -rock interfaceLocation ofPeak Shear StrengthResidual ShearDriftParametersStrength ParametersDrift.RLCohesion,FrictionCohesion,FrictionNo.(m)cAngle,crAngle,(kg/cm2)(kg/cm2)r(degree)(degree)DR-263.000.6555.500.2153.505DL-6245.001.8339.101.2836.00Combined0.7350.700.2448.30DR-5 & DL-6Table 7. Shear Strength Parameters on concrete-rock interfaceLocation ofPeak Shear StrengthResidual ShearDriftParametersStrengthParametersDrift.RL (m)Cohesion,FrictionCohesion,FrictionNo.c (kg/cm2)Angle, cr (kg/cm2)Angle,(degree)r(degree)DR-263.001.6233.300.9432.705DL-6245.002.1334.801.6631.70Combined1.5736.101.0633.82DR-5 & DL-6Fig. 11: Shear stress V/S normal stress plot for R/R interface in combined DR-5 & DL-6Values of the peak and residual shear strength parameters are tabulated below in Tables 6&7 for R/R and C/R interfaces, respectively. The data shows large variations among the shear strength parameters i.e. cohesion and friction-angle for both R/R and C/R interfaces. There are also variations in peak and residual shear strength parameters. These variations show that there are abrupt changes in various geological and mechanical rock mass properties from one bank to other. However, there are cases where the shear strength parameters are almost similar in magnitude on the left bank and the right bank for both R/R and C/R interfaces [4,5]. Further, it is recommended that the average values for a particular dam site may be obtained by plotting data from both the banks together rather than taking an arithmetic average of cohesion and friction values from both the banks as it may give better representative values (See Figs. 8 &11). INTERPRETATION OF SHEAR STRENGTH PARAMETERSTwo major design issues viz. stability of the rock and stability of the dam over rock are of central importance during the dam construction. Thus, R/R interface values are required for stability analysis of the rocks while C/R interface values are required for the stability of the dam foundation.Being on the conservative side, designer may like to use the lowest observed values of shear strength parameters for the above two cases. However, this may not be the case as the rock mass is not homogeneous throughout the dam axis. In view of this, a combined analysis of the values obtained from left and right bank is proposed in the present study. For illustration purpose, Tables 8&9 show the shear strength of R/R and C/R interfaces, respectively, for 40 ton of vertical load (which is applied on 70 cm x 70 cm block). These values are determined using Eq. 3 and using the shear strength parameters as given in Tables 6&7.Page 6 of 9In-situ shear strength of phyllite rock massTable 8. Shear strength on rock to rock interface (for 40 ton of vertical load)Location ofPeak ShearResidualDriftStrengthShear(kg/cm2)Strength(kg/cm2)DR-512.5311.24DL-68.477.21Combined DR-5 & DL-610.709.40Table 9. Shear Strength on concrete to rock interface (for 40 ton of vertical loadLocation ofPeak ShearResidualDriftStrengthShear(kg/cm2)Strength(kg/cm2)DR-56.996.18DL-67.806.70Combined DR-5 & DL-67.526.53In case of R/R interface, the right bank values are 47.97% higher as compared to the left bank values. Similarly, values determined by residual shear strength parameters are 55.93% higher as compared to the left bank values. Therefore, it seems better to use individual bank stability analysis due to this large relative variation in the shear strength for R/R interface. For C/R case, the relative variation is quite small as compared with R/R case. Variation is of the order of 10 %. Hence, the combined values of shear strength parameters may be used instead of using separate values on separate banks for foundation design. Lesser variation in concrete over rock may be due to uniformity in the concrete surface as compared to the rock over rock case. Residual shear strength parameters may be utilized for long term stability analysis [6, 7].SHEAR STRENGTH PARAMETERS FOR PHYLLITES AT OTHER H.E. PROJECTSThe shear strength parameters obtained from different project sites in India and Bhutan have been discussed [5, 8, & 9] for rock to rock and concrete to rock interfaces based on in-situ test data. A list of shear strength parameter values for the Phyllite rock is shown in Table 10 from various sites in India and Bhutan which also includes the data from the present study.For R/R interface, internal friction angle values range from 360 to 700 while for C/R interface, this ranges from 310 to 660, which is of almost same order of magnitude. So, it seems that for Phyllites, values of internal friction are of the order of 30 to 70 degress. For R/R interface, cohesion values range from 0 to 2.70 kg/cm2 while for C/R interface, this ranges from 0 to 2.13 kg/cm2, which is also of almost same order of magnitude. So, it seems that for Phyllites, cohesion values are of the order of 0 to 2.5 kg/cm2.Further, if we assume a vertical load of 40 ton on 70 cm x 70 cm block then for the lowest and highest set of values of c and for Phyllites, respectively, shear strength is 4.71 and 24.93 kg/cm2 (using Eqn. 3). It is analyzed that the maximum contribution of cohesion value in shear strength is about 10% only. Therefore, it is concluded that the maximum shear strength gain in Phyllites is due to internal friction only. Due to low cohesion values for Phyllite rock mass, it is suggested that shear keys, rock-anchoring, or rock bolting may be provided to attain the desired factor of safety against sliding [10, 11]. Based on this observation, shear strength Equation 3 can be approximated as Equation 4 for Phyllites, which ignores the cohesion value.t @ sn tanf(4)Based on the limited available data in Table 10, tan varies from 0.58 to 2.75, if it is assumed that varies from 30 to 70 degrees and shear strength will vary by 4.8 times within this range of internal friction. Further, this huge variation in shear strength due to internal friction, strengthens our view about conducting separate stability analysis for both the banksPage 7 of 9Table 10. Shear strength parameters from different projects in India and Bhutan for PhyllitesShear Strength ParametersRock-Rock InterfaceConcrete-Rock InterfaceS.Name ofRockPeakResidualPeakResidualNo.the ProjectTypeValuesValuesValuesValuesC0Cr0C0Cr0MPafMPafMPafMPaf1.Chamera Dam ProjectPhyllites----0.1353.300.0049.50(India), [9].2.Greater Shillong DamPhyllites0.2770.000.0169.000.2566.000.0162.00(India), [9].3.Sankosh Main DamPhyllites0.1760.000.0057.00----(Bhutan), [9].4.Amochu (Right Bank),Phyllites0.0755.500.0253.500.1633.300.0932.70Bhutan, [3].5.Amochu ( Left Bank ),Phyllites0.1839.100.1336.000.2134.800.1731.70Bhutan, [3].CONCLUSIONBased on the present study, following important conclusions may be drawn for the test drift at Amochu hydroelectric project site, Bhutan.The drifts have been encountered for fine grained, dark gray quartzites and phyllites. Most of the joints observed to be rough planar. Values of cohesion coefficient vary between 0.21 kg/cm2 to 1.83 kg/cm2 for R/R interface with a combined value (right and left bank together) of 0.73 kg/cm2 and 0.24 kg/cm2 for peak and residual shear strengths, respectively. For C/R interface, it varies between 0.94 kg/cm2 to 2.13 kg/cm2 with a combined value of 1.57 kg/cm2 and 1.06 kg/cm2 for peak and residual shear strengths, respectively. Values of friction-angle vary between 36.00o to 55.50o for R/R interface with a combined value of 50.70o and 48.30o for peak and residual shear strengths, respectively. For C/R interface, it varies between 31.70o to 34.80o with combined values of 36.10o and 33.82o for peak and residual shear strengths, respectively. The use of individual bank stability analysis is recommended as in case of R/R interface, the right bank values of example shear strength are around 50% higher as compared to the left bank values. On the other hand, for C/R case, the relative variation is only around 10%. Hence, the combined values of shear strength parameters may be used for foundation design on both the banks.For Phyllites, the values of internal friction are of the order of 30o to 70o and cohesion values are of the order of 0 to 2.5 kg/cm2. It is also shown that the major contribution in shear strength gain in Phyllites is of internal friction while cohesion values contribute by 10% only. Therefore, the use of shear keys, rock-anchoring, or rock bolting is suggested in Phyllites. Based on the study of the available data from Bhutan and Indian sites, an empirical shear strength equation in case of Phyllites may be approximated as t @ sn tanf , where, tan varies from 0.58 to 2.75. However, validation of this equation is subjected to the availability of more data on Phyllites and is seen as future scope for the extension of this study. REFERENCESIS 7746 (1991, 1996): Indian Standard code of practice for in-situ shear test on rock. ISRM Part 1 (1981): Suggested methods for determining shear strength Page 8 of 9In-situ shear strength of phyllite rock massCSMRS report on In-Situ Shear Strength Parameters of Rock Mass in Right Bank Drifts at Dam Site of Amochu Project, Bhutan, 2011. Singh, Rajbal (2007). Field Shear Test, Chapter 11 of Engineering in Rocks for Slopes, Foundations and Tunnels, Ed. Prof. T. Ramamurthy, pp. 256264. Singh, Rajbal and Sharma, V.M. (1990). Determination of Foundation Deformability and Shear Strength Characteristics of a Concrete Dam, Indian Geotechnical Conference (IGC-90), Bombay, pp. 371373. Barton, N. and Choubey, V. (1977). The shear strength of rock joints in theory and practice. In: Journal, Rock Mechanics The International Society for Rock Mechanics, Vol 10, No. 1-2, pp 1-54. Bhandari K.K., Verma V.K., Singh Rajbal, Jeur S.D. and Thosar S.P. (2004), Stabilisation of Rock Mass with MAI anchors at Tala Hydroelectric Project in Bhutan, Journal of New Building Materials and Construction World, Issue-11, pp. 36-41. Singh, Rajbal, Dev, Hari and Dhawan, A.K. (2000). Characterisation of Foundation Rock for a Concrete Gravity Dam, Indian Geotechnical Conference IGC-2000, Mumbai, pp. 6768. Singh, Rajbal (2009). Measurement of In situ Shear Strength of Rock Mass, Journal of Rock Mechanics and Tunnelling Technology, ISRMTT, Vol. 15, No. 2, pp.131142. Design of Tunnel Support, Rock Mechanics Vol. 6, No. 4, pp 189-236. ISRM (1987) International Society for Rock Mechanics Suggested Methods for Rock bolt Testing. Page 9 of 9