Shear Strength of Dam-Foundations Rock Interface - A Case Study

Ghosh, A.K.Chief Research Officer

e-mail: emarald249@yahoo.co.in

Central Water & Power Research Station, Pune

ABSTRACTShear strength parameters such as cohesion and angle of internal friction for dam-foundation interface play animportant role in determining the stability aspects of gravity dams. Field studies have been conducted to determinethe shear strength parameters for the concrete rock interface for 26.2m high and 700m long composite typeUpper Tunga dam, across river Tunga at Shimoga,, Karnataka. The foundation rockmass, exposed as outcrop,has been found to be fresh and hard rock of Schistose variety. A total of six locations at the spillway zone havebeen tested and the estimated values of cohesion (c) and friction angle () have been found to be 10 kg/cm2 and59 respectively. A brief review of site including predominant geological features, testing procedures as well asfindings have been presented.

Indian Geotechnical Conference 2010, GEOtrendzDecember 1618, 2010

IGS Mumbai Chapter & IIT Bombay

1. INTRODUCTIONFor gravity dams on rock foundations, beside normal loadfrom the self weight of the structure, many of the loads onthe dam are horizontal or have horizontal components.These are resisted by frictional or shearing forces alonghorizontal or nearly horizontal planes in the body of thedam, on the foundation or on horizontal or nearly horizontalweak planes in the foundation. Thus for a realisticassessment of the stability of the structure against sliding,estimation of the shear resistance of rock mass along anydesired plane of shear or along the weakest discontinuityis essential. Since laboratory tests on small specimens donot reflect the influence of seams, fissures and localalterations on behaviour of in-situ rock, large scale in-situshear tests are conducted under anticipated stress range.

Fig. 1: Forces Acting on a Solid Gravity Dam

One of the primary design requirements in case ofconcrete or masonry gravity dam built on rock foundationis to ensure adequate factor of safety for shear and slidingfailure at the dam-foundation interface. The resistance tosliding is a function of the cohesion (c) inherent in thematerials and at their contact and angle of internal friction() of the material at the surface of sliding.(Fig.1). In itssimplest form, the friction factor criterion used forevaluating the factor of safety against sliding (FS) is asfollows:

(1)

(1)

where N=downward vertical force, U=uplift force,H=horizontal forces, =friction angle for plane XX2 ,c=cohesion on plane XX2 , L=base width of the dam. In-situ direct shear tests are carried out to determine values ofc and f from the peak and residual direct shear strength.The factor of safety is then determined and compared withthe values specified in IS 6512-1984 for different loadingconditions and results are incorporated for ensuring thestability of dam against sliding. One of such in-situ directshear test is presented based on CWPRS Technical ReportNo.4125(2004).2. TEST LOCATIONA 16.7 m high Anicut has been constructed across riverTunga and under operation since 1956.In the recent past,

1040 A.K. Ghosh

the Anicut has developed problems arising out of operationof gates for the scour sluices. In order to tackle the problemas well as to increase the storage capacity of the Tungareservoir, a 26.2m high and 770m long composite dam hasbeen under construction at the time of studies across riverTunga at about 100m downstream of the existing Anicutat Gajanur village of Shimoga district, Karnataka. The damhas non-overflow sections of length 18.50m on the left flankand 126m on the right flank and 321.50m long weir typeconcrete Spillway at the center portion comprising of 22numbers of radial crest gates of size 11.75m4.74m todischarge design flood of 2,60,000 cusecs. In order todetermine the design value of factor of safety against shearand sliding, field studies have been conducted at thedownstream of spillway blocks to determine the shearstrength parameters at dam-foundation rock interface.3. GEOLOGYThe rock mass met with at the Upper Tunga Dam site is ingeneral good quality Granites with occasional schistosezones. The core recoveries has been found to be good andmostly above 80% after 2-3 m depth whereas the recoveryin the initial reach of 2 to 3 m depth is around 43 to 60%.The RQD is fair to good ranging from 51 to 91% and mostlyabove 70%. In the spillway portion, fresh and hardHornblend Schist rock occurs much above the proposedfoundation level i.e. right from river bed level. The schistoserock mass as outcrop with its roughness profile is shownin Fig.2.

Fig. 2: Hornblend Schist Rock Mass as Outcrop on WhichConcrete Test Blocks Have Been Prepared

4. IN SITU SHEAR TESTThe test is carried out to measure Peak and Residual directshear strength as a function of the stress normal to theplane to be sheared- which in the present case is theinterface between concrete and foundation rock. Peak directshear strength corresponds to the maximum shear stress inthe shear stress vs. displacement curve whereas the Residualshear strength is the shear stress at which no further riseor fall in the shear strength is observed with increasingshear displacement. A total of 6 concrete blocks of sizes700mm 600mm 600mm at the downstream of Spillwayblocks has been casted on the foundation rock mass after

leveling of the surface by chiseling and keeping a gap ofabout 600mm from the body of the spillway. The blockshave been tested after allowing for a curing period of about3 weeks. The spillway body wall itself has been used inmost cases as reaction wall for application of shear load.However, in some cases where the gap between casted testblock and spillway body wall is more, R.C.C. reaction padof size 1m 1m has been constructed to facilitate theapplication of shear load. One of such RCC reaction padalong with concrete test blocks is shown in Fig.3.

Fig. 3: RCC Reaction Pad with Shear Test BlocksFor each test location, anchorage and girder

arrangements have been specially built to facilitate theapplication of normal load. A view of the complete test setup at one of the locations is shown in Fig.4

Fig. 4: Complete Test Setup at One of the LocationsThe testing procedure has been consisted of applying

a predetermined normal load on the concrete test blockand while maintaining this load constant, the shear loadhas been applied in small increments till the block failed.Two 200T capacity hydraulic jacks and one 100T capacityhydraulic jack have been used for application of shear andnormal load respectively after applying correction usingcalibration charts of the pressure gauges used. Roller hasbeen introduced below the normal load to facilitate smoothmovement of the test block during application of shearload.Based on the dimension of the casted concrete block,wooden wedges are specially prepared and with the helpof prepared wedges and 10mm thick MS plate with ballseating at the centre, shear load has been applied at anangle of about 15 so that the resultant of the normal and

Shear Strength of Dam-Foundation Rock Interface... 1041

shear forces passes within the middle third of the base ofthe test block. Detail view of loading arrangement forapplication of normal and shear load is separately shownvide Fig.5.

For Normal load For Shear loadFig. 5: Detail View of Loading Arrangement During Test

Horizontal displacement corresponding to eachincrement of shear load has been recorded using two dialgauges of sensitivity 0.01 mm. After reaching peak failurestresses, each of the test blocks has been tested under severalnormal stresses to obtain corresponding residual shearstresses. After completion of each test, the test block isupturned and the failure surface has been examined to assessthe mode of failure. Upturned views of blocks are shownvide Fig.6(a) and 6(b) respectively.

Test block-1 Test block-2 Test block-3 Fig. 6: (a) Upturned Views of the Test Blocks

Test block-4 Test block-5 Test block-6Fig. 6: (b) Upturned Views of the Test Blocks

5. RESULTS AND DISCUSSIONSA sketch showing the application of normal and shear forceson the test block including the prepared wooden wedge isshown in Fig.7.

Fig. 7: Sketch Showing Application of ForcesAs per IS 7746:1991,both normal and shear stresses

can be computed as follows.

(2)

(3)W h e r e P

s = total shear force, P

n = total normal force,

Psa=applied shear force ,P

na=applied normal force, P

sa cos

= tangential component of applied shear force, Psasin

=normal component of applied shear force, =inclination of applied shear force to the shear plane, A=area of shear surface.Based on equations (2) and (3), bothnormal and shear stress values for peak shear (at failure)and residual shear(after failure)have been computed.Values of shear stress and corresponding sheardisplacements are obtained after averaging thedisplacement readings of two dial gauges and a combinedplot for all blocks is shown vide Fig.8.

Fig. 8: Shear StressVs Displacement Plots

1042 A.K. Ghosh

From the shear stress vs displacement plot it can beobserved that most of the curves exhibit a distinct peakshear strength and a sudden fall in shear strength at failureas expected for tight joints like interface between concreteand good quality rock (IS 7746:1991). For most of theblocks, initiation of yielding has started without drop invalue of shear load little earlier followed by gradual increaseof the displacement over a comparatively small increase ofthe shear load. This can be explained by the shear resistanceoffered by the unevenness of the rock surface at the contactplane after initiation of yielding till final failure when theshear load has suddenly dropped. Examination of failuresurfaces of test blocks reveals that for block nos 1,2and 5some rock intrusion has been sheared during failure.However for block nos 3,4and 6, failure has been at theconcrete-rock interface and accordingly for computingresidual strength, normal and shear stress valuescorresponding to these blocks have been utilized. Graphsof peak and residual shear strength vs normal stress isshown in Fig.9 A and B respectively from which theestimated values of cohesion(c) and angle of internalfriction() has been computed as 10kg/cm2 and 59respectively.

Fig. 9: Normal Stress Vs Shear Stress for Peak(A) andResidual (B) Conditions

High value of c and can be attributed to the increase ofsurface roughness caused by the saw tooth type ofunevenness of the rock surface (Fig.2) on which the testblocks have been prepared (Gole C.V.et.al.1972). From

the laboratory test of the rock cores of schist rock mass,average values of Density, Static modulus of Elasticity,Unconfined Compressive strength and Hardness have beenfound to be 2.79gm/cc,7.53105 kg/cm2,467 kg/cm2 and26 respectively. Though compressive strength is at lowerside due to failure of samples through foliation, from theRQD values and laboratory test results, rock mass can bedesignated as good quality schist.

6. CONCLUSIONSThe study carried out lead to following conclusions:

1. The shear strength parameters c and are influencedby the roughness of the rock surface and its strength.

2. .From observation of failure surfaces at test locations,

it can be concluded that, chances of distinct rock

intrusion compared to the natural roughness profile of

the rock surface ,in the test block, at the time of casting

, needs to be avoided to ensure proper failure at

concrete-rock interface.

3. The foliation in the schist rock mass at the test location

is not very conspicuous. As lower values of c, areexpected for such planes, more number of tests is

advisable in such cases as the result from shear test

along such plane can significantly influence the

selection of shear strength parameters for design.

4. Even in case of stratified foundation where shear

strength of soft layers and bedding planes control the

stability of the dam, it is necessary to ensure that dam

is safe against shear and sliding failure at its contact

with the foundation.

ACKNOWLEDGEMENTSThe author is grateful to Dr. I.D.Gupta, Director, CWPRS

and Shri R.S.Ramteke, Joint Director, CWPRS for their

encouragement and guidance. The assistance and support

of project engineers of Upper Tunga Project and of

Shri.H.R.Bhujbal and Shri.J.M.Deodhar, Laboratory

Assistants of CWPRS, during field investigations are

acknowledged sincerely with thanks.

REFERENCESCWPRS Technical report no.4125(2004). Rock Mechanics

Studies to Determine Shear Strength Parameters ofFoundation Rock Mass for Upper Tung Project,Karnataka , pp 1-12.

Gole C.V. et.al.(1972).Some Studies on evaluating Shearand Sliding Friction Factors for Rock Foundations,Proc. 42nd CBIP Annual Research Session, Vol. III,Madras, Tamil Nadu, India, pp 114.

IS 7746:1991 Indian Standard Code on In Situ Shear Teston Rock( First revision ), pp 5-7.

IS 6512:1984 Indian Standard Code on Criteria for Designof Solid Gravity Dams, pp 14-15.