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SHEAR STRENGTH OF ROCK JOINTS AND ROCK MASSES (part-II)Mahendra SinghProfessor, Department of Civil EngineeringIIT Roorkee, Roorkee-247 667
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Fig.1. Hill side failure along planar discontinuities
Fig. 2a. Portable field shear box to test rock joints (Ross-Brown and Walton, 1975)
Shear displacement
She
ar s
tres
s
Peak strength
Residual strength
Fig. 2b. Typical shear stress-shear displacement plot from direct shear test on rough rock joints
Normal stress
Sh
ea
r st
ren
gth
Residual strength
Peak strength
c
f
fr
Fig. 2c. Failure envelopes of shear strength for rough rock joint
Normal stress
She
ar s
tren
gth
(fb+i)
Failure of intact rock
Shearing on saw-tooth surfaces
Fig. 3. Patton’s simulation of asperities and bi-linear shear strength model for rock joints
Fig. 4. Barton’s shear strength model for rock joints
Fig. 5. Roughness profiles to estimate JRC (Barton and Chaubey, 1977)
Normal stress
She
ar s
tren
gth
finst
Tangent
cinst
Fig. 6. Instantaneous cohesion and friction angle for a non-linear failure criterion (Hoek, 2000)
Fig. 7. Estimation of friction angle of rock mass from RMR (Mehrotra, 1993)
Fig. 8. Estimation of Geological Strength Index (redrawn from Marinos et al., 2005)
s3
1 -3
s3=scrt
A'(s3)2
Mohr-Coulomb criterion
Modified Mohr-Coulomb criterion
sci
Fig. 9. Modified Mohr-Coulomb strength criterion
Fig. 10. Field modulus of elasticity (Mehrotra, 1992).