STUDY OF FAILURE PATTERN OF SOFT ROCKS UPON SATURATION

Abstract

In nature, rock formations usually occur in triaxial state of stress, which is a long term time-dependent process. While carrying out the stability analysis of large dam etc. it is necessary to study the stress-strain behaviour of foundation rock under dry as well as saturated condition. Some times it is found that the dry strength of the rock is sufficient to withstand the heavy load of the dam body, while on saturation the strength decreases drastically. Hence prior to the construction of such a heavy structure, extensive experimentation programme should be carried out to study the behaviour of rock in dry as well as in saturated condition. The objective of the present work is to study the engineering behaviour of soft rock under saturated condition in triaxial testing at different confining pressures. The effect of different degree of saturation on deformational and strength parameter (viz. uniaxial compressive strength, triaxial strength, modulus of elasticity etc.), mode of deformation, type of failure etc. have been studied. In present case, the soft sandrock collected from lower Shiwalik range have been used for experimental work and about 40 specimens were tested. Based on these study, it is observed from the triaxial test result that as the degree of saturation increases failure behaviour in the post peak range, tends to be more ductile in nature. The uniaxial compressive strength is found to decrease drastically (i.e. one-ninth of a~c) when the degree of saturation increases from 20 percent to 100 percent. It is also observed that Mohr's rupture envelop is nonlinear at high confining pressure for low degree of saturation while it is linear at low confining pressure with high degree of saturation. It is also observed that the modulus of elasticity is more or less a linear function of confining pressure but its value decreases with increase in degree of saturation.