Shear Strength of Dam-Foundations Rock Interface - A …igs/ldh/conf/2010/articles/t143.pdfthe values specified in IS 6512-1984 for different loading conditions and results are incorporated

Shear Strength of Dam-Foundations Rock Interface - A Case 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 gravity dams. Field studies have been conducted to determine

the shear strength parameters for the concrete – rock interface for 26.2m high and 700m long composite type

Upper 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 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 have been presented.

Indian Geotechnical Conference – 2010, GEOtrendz

December 16–18, 2010

IGS Mumbai Chapter & IIT Bombay

1. INTRODUCTION

For gravity dams on rock foundations, beside normal load

from the self weight of the structure, many 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 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.

Fig. 1: Forces Acting on a Solid Gravity Dam

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 of the cohesion (c) inherent in the

materials and at their contact and angle of internal 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)

(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. In-

situ direct shear tests are carried out to determine values of

c and f from the peak and residual direct shear strength.

The factor of safety is then determined and compared with

the values specified in IS 6512-1984 for different loading

conditions and results are incorporated for ensuring the

stability of dam against sliding. One of such in-situ direct

shear test is presented based on CWPRS Technical Report

No.4125(2004).

2. TEST LOCATION

A 16.7 m high Anicut has been constructed across river

Tunga and under operation since 1956.In the recent past,

1040 A.K. Ghosh

the Anicut has developed problems arising out of operation

of gates for the scour sluices. In order to tackle the problem

as well as to increase the storage capacity of the Tunga

reservoir, a 26.2m high and 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. The 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.

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 reach of 2 to 3 m depth is around 43 to 60%.

The RQD is fair to good ranging from 51 to 91% and mostly

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. 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 measure 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 and 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

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 period 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 the gap between casted test

block and spillway body wall is more, R.C.C. reaction 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.

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

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 and 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

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

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

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 failure

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 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

5. RESULTS AND DISCUSSIONS

A sketch showing the application of normal and 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.

(2)

(3)

W h e r e P

s = total shear force, P

n = total normal force,

P

sa=applied shear force ,P

na=applied normal force, P

sa cosα

= tangential component of applied shear force, Psasin

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.

Fig. 8: Shear StressVs Displacement Plots

1042 A.K. Ghosh

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 rock 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.

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

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

The study carried out lead to following conclusions:

1. The shear strength parameters c and φ are influenced

by 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, φ are

expected 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.

ACKNOWLEDGEMENTS

The 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.

REFERENCES

CWPRS Technical report 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.