Indian Geotechnical Conference – 2010, GEOtrendz December 16–18, 2010 IGS Mumbai Chapter & IIT Bombay
Shear Strength of Dam-Foundations Rock Interface Interf ace - A Case Study Study Ghosh, A.K. Chief Research Officer e-mail:
[email protected]
Central Water & Power Research Station, Pune
ABSTRACT Shear strength parameters such as cohesion and angle of internal friction for dam-foundation interface play an important role in determining the stability aspects of grav ity dams. Field studies have been conducted conduc ted to determine the shear strength parameters for the concrete – rock rock interface for 26.2m high and 700m long composite type Upper Tunga Tunga dam, across river Tunga at Shimoga,, Karnatak a. The foundation rockmass, exposed as outcrop, has been found to be fresh and hard rock rock of Schistose variety. varie ty. A total of six locations at the spillway spi llway zone have been tested and the estimated values of cohesion (c) and friction angle (φ ) have been found to be 10 kg/cm2 and 59° respectively. A brief review of site including predominant geological features, testing procedures as well as findings hav e been bee n presented. 1. INTR INTRODU ODUCTI CTION ON
For gravity dams on rock foundations, beside normal load from the self weight of the structure, m any of the loads on the dam are horizontal or have horizontal components. These are resisted by frictional or shearing forces along horizontal or nearly horizontal planes in the body of the dam, on the foundation foundation or on horizontal or nearly horizontal weak planes in the foundation. Thus for a realistic assessment of the stability of the structure against sliding, estimation of the shear resistance of rock mass along any desired plane of shear or along the weakest discontinuity is essential. Since laboratory tests on small specimens do not reflect the influence of seams, fissures and local alterations on behaviour of in-situ rock, large scale in-situ shear tests are conducted under anticipated stress range.
One of the primary design requirements in case of concrete or masonry gravity dam built on rock foundation is to ensure adequate factor of safety for shear and sliding failure at the dam-foundation interface. The resistance to sliding is a function function of the cohesion (c) inherent inher ent in the materials and at their contact and angle of internal friction friction (φ) of the material at the surface of sliding.(Fig.1). In its simplest form, the friction factor criterion used for evaluating the factor of safety against sliding (FS) is as follows: (1) where N=downward vertical force, U=uplift force, H=horizontal forces, φ =friction angle for plane XX2 , c=cohesion on plane XX2 , L=base width of the dam. Insitu direct shear t ests are carried out to determine values of c and f from the peak and residual direct shear strength. The factor of safety is then determi ned and compar ed with the values specified specified in IS 6512-1984 for different different loading conditions conditions and results are incorporated incorporated for ensuring ensuring the stability of dam against sliding. One of such in-situ direct shear test is presented based on CWPRS CWPRS Technical Technical Report No.4125(2004). 2. TE TEST ST LOCA LOCATIO TION N
Fig. 1: Forces Acting on a Solid Gravity Dam
A 16.7 m high Anicut has been constructed across river Tunga and under operation since 1956.In the recent recent past,
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the Anicut has developed problems arising out of operation of gates for the scour sluices. In order to tackle th e problem as well as to increase the storage capacity of the Tunga reservoir, a 26.2m high an d 770m long composite dam has been under construction at the time of studies across river Tunga at about 100m downstream of the existing Anicut at Gajanur village of Shimoga district, Karnataka. Th e dam has non-overflow sections of length 18.50m on the left flank and 126m on the right flank and 321.50m long weir type concrete Spillway at the center portion comprising of 22 numbers of radial crest gates of size 11.75m×4.74m to discharge design flood of 2,60,000 cusecs. In order to determine the design value of factor of safety against shear and sliding, field studies have been conducted at the downstream of spillway blocks to determine the shear strength parameters at dam-foundation rock interface.
A.K. Ghosh
leveling of the surface by chiseling and keeping a gap of about 600mm from the body of the spillway. The blocks have been tested after allowing for a curing per iod of about 3 weeks. The spillway body wall itself has been used in most cases as reaction wall for application of shear load. However, in some cases where t he gap between casted t est block and spillway body wall is more, R.C.C. reacti on pad of size 1m ´ 1m has been constructed to facilitate the application of shear load. One of such RCC reaction pad along with concrete test blocks is shown in Fig.3.
3. GEOLOGY
The rock mass met with at the Upper Tunga Dam site is in general good quality Granites with occasional schistose zones. The core recoveries has been found to be good and mostly above 80% after 2-3 m depth whereas the recovery in the initial r each of 2 to 3 m depth is ar ound 43 to 60%. The RQD is fair to good ranging from 51 to 91% and m ostly above 70%. In the spillway portion, fresh and hard Hornblend Schist rock occurs much above the proposed foundation level i.e. right from river bed level. The schistose rock mass as outcrop with its roughness profile is shown in Fig.2.
Fig. 3: RCC Reaction Pad with Shear Test Blocks
For each test location, anchorage and girder arrangements have been specially built to facilitate the application of normal load. A view of the complete test set up at one of the locations is shown in Fig.4
Fig. 4: Complete Test Setup at One of the Locations Fig. 2: Hornblend Schist Rock Mass as Outcrop on Which Concrete Test Blocks Have Been Prepared
4. IN SITU SHEAR TEST
The test is carried out to measur e Peak and Residual direct shear strength as a function of the stress normal to the plane to be sheared- which in the present case is the interface between concrete an d foundation rock. Peak direct shear strength corresponds to the maximum shear stress in the shear stress vs. displacement curve whereas the Residual shear strength is the shear stress at which no further rise or fall in the shear strength is observed with increasing shear displacement. A total of 6 concrete blocks of sizes 700mm× 600mm × 600mm at the downstream of Spillway blocks has been casted on the foundation rock mass after
The testing procedure has been consisted of applying a predetermined normal load on the concrete test block and while maintaining this load constant, the shear load has been applied in small increments till the block failed. Two 200T capacity hydraulic jacks an d one 100T capacity hydraulic jack have been used for application of shear and normal load respectively after applying correction using calibration charts of the pressure gauges used. Roller has been introduced below the normal load to facilitate smooth movement of the test block during application of shear load.Based on the dimension of the casted concrete block ,wooden wedges are specially prepared and with the help of prepared wedges and 10mm thick MS plate with ball seating at the centre, shear load has been applied at an angle of about 15° so that the resultant of the normal and
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Shear Strength of Dam-Foundation Rock Interface...
shear forces passes within the middle third of the base of the test block. Detail view of loading arrangement for application of normal and shear load is separately shown vide Fig.5.
For Normal load
For Shear load
5. RESULTS AND DISCUSSIONS
A sketch showing the application of normal an d shear forces on the test block including the prepared wooden wedge is shown in Fig.7.
Fig. 7: Sketch Showing Application of Forces As per IS 7746:1991,both normal and shear stresses can be computed as follows.
Fig. 5: Detail View of Loading Arrangement During Test
Horizontal displacement corresponding to each increment of shear load has been recorded using two dial gauges of sensitivity 0.01 mm. After reaching peak fail ure stresses, each of the test blocks has been tested under several normal stresses to obtain corresponding residual shear stresses. After completion of each test, the test block is upturned and the failure surface has been examined to assess the mode of failure. Upturned views of blocks are shown vide Fig.6(a) and 6(b) respectively.
Test block-1
Test block-2
(2) (3) = total shear force, P n = total normal force, s =applied shear force ,Pna=applied normal force, Psa cosα sa = tangential component of applied shear force, P sasin α=normal component of applied shear force, α= inclination of applied shear force to the shear plane, A= area of shear surface.Based on equations (2) and (3), both normal and shear stress values for peak shear (at failure) and residual shear(after failure)have been computed. Values of shear stress and corresponding shear displacements are obtained after averaging the displacement readings of two dial gauges and a combined plot for all blocks is shown vide Fig.8. W
h
e r
e
P
P
Test block-3
Fig. 6: (a) Upturned Views of the Test Blocks
Test block-4
Test block-5
Test block-6
Fig. 6: (b) Upturned Views of the Test Blocks
Fig. 8: Shear StressVs Displacement Plots
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From the shear stress vs displacement plot it can be observed that most of the curves exhibit a distinct peak shear strength and a sudden fall in shear strength at failure as expected for tight joints like interface between concrete and good quality rock (IS 7746:1991). For most of the blocks, initiation of yielding has started without drop in value of shear load little earlier followed by gradual increase of the displacement over a comparatively small increase of the shear load. This can be explained by the shear resistance offered by the unevenness of the r ock surface at the contact plane after initiation of yielding till final failure when the shear load has suddenly dropped. Examination of failure surfaces of test blocks reveals that for block nos 1,2and 5 some rock intrusion has been sheared during failure. However for block nos 3,4and 6, failure has been at the concrete-rock interface and accordingly for computing residual strength, normal and shear stress values corresponding to these blocks have been utilized. Graphs of peak and residual shear strength vs normal stress is shown in Fig.9 A and B respectively from which the estimated values of cohesion(c) and angle of internal friction(φ) has been computed as 10kg/cm2 and 59° respectively.
A.K. Ghosh
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 been found to be 2.79gm/cc,7.53×105 kg/cm2,467 kg/cm2 and 26 respectively. Though compressive strength is at lower side due to failure of samples through foliation, from the RQD values and laboratory test results, rock mass can be designated as good quality schist. 6. CONCLUSIONS ��� ����� ������� ��� ���� �� ��������� ������������ 1. ��� ����� �������� ���������� � ��� φ ��� ���������� �� ��� ��������� �� ��� ���� ������� ��� ��� ��������. 2.
3.
�.
.���� ����������� �� ������� �������� �� ���� ���������, �� ��� �� ��������� ����, ������� �� �������� ���� ��������� �������� �� ��� ������� ��������� ������� �� ��� ���� ������� ,�� ��� ���� �����, �� ��� ���� �� ������� , ����� �� �� ������� �� ������ ������ ������� �� ������������� ���������. ��� ��������� �� ��� ������ ���� ���� �� ��� ���� �������� �� ��� ���� �����������. �� ����� ������ �� �, φ ��� �������� ��� ���� ������, ���� ������ �� ����� �� ��������� �� ���� ����� �� ��� ������ ���� ����� ���� ����� ���� ����� ��� ������������� ��������� ��� ��������� �� ����� �������� ���������� ��� ������. ���� �� ���� �� ���������� ���������� ����� ����� �������� �� ���� ������ ��� ������� ������ ������� ��� ��������� �� ��� ���, �� �� ��������� �� ������ ���� ��� �� ���� ������� ����� ��� ������� ������� �� ��� ������� ���� ��� ����������.
ACKNOWLEDGEMENTS ��� ������ �� �������� �� ��. �.�.�����, ��������, ����� ��� ���� �.�.�������, ����� ��������, ����� ��� ����� ������������� ��� ��������. ��� ���������� ��� � ������ �� ������� ��������� �� ����� ����� ������� ��� �� ����.�.�.������� ��� ����.�.�.�������, ���������� ���������� �� �����, ������ ����� �������������� ��� ������������ ��������� ���� ������.
REFERENCES
Fig. 9: Normal Stress Vs Shear Stress for Peak(A) and Residual (B) Conditions
High value of c and φ can be attributed to the increase of surface roughness caused by the saw tooth type of unevenness of the rock surface (Fig.2) on which the test blocks have been prepared (Gole C.V.et.al.1972). From
CWPRS Technical r eport no.4125(2004). Rock Mechanics Studies to Determine Shear Strength Parameters of Foundation Rock Mass for Upper Tung Project, Karnataka , pp 1-12. Gole C.V. et.al.(1972).Some Studies on evaluating Shea rand 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 Test on Rock( First revision ), pp 5-7. IS 6512:1984 –Indian Standard Code on Criteria for Design of Solid Gravity Dams, pp 14-15.
