Shear Strength of Dam-Foundations Rock Interface - A …igs/ldh/conf/2010/articles/t143.pdfthe...
Transcript of Shear Strength of Dam-Foundations Rock Interface - A …igs/ldh/conf/2010/articles/t143.pdfthe...
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.