INFLUENCE OF CONFINING PRESSURE ON MODULUS OF ELASTICITY OF SANDSTONE

Abstract

Rock formations usually occur in triaxial state of stress in nature, which is a long-term time - dependent process. The relationship between the stresses in rockmass and the strength of the rock may play an important role in carrying out the stability analysis of an underground excavation as well as foundation of high concrete gravity dams. The stability of deep excavations normally depends upon the response of the rockmass to the stress field induced around the excavations. In order to make use of the knowledge of stresses induced due to various reasons such as tectonic activity around underground excavations or in the foundation of a dam, it has, been felt very necessary to have available a criterion or a set of rules which may predict the response of a rockmass to a given sit of induced stresses. Consequently, a substantial amount of work has been done by past researchers from time to time in order to study the effect of confining pressures on the deformational and strength parameters of rock such as ultimate strength, modulus of elasticity, cohesion angle of internal friction etc. as normally used during the design. The objective of the present work is to examine the strength behaviour of fairly medium strength rock (In the present study, Micaceous sand stone collected from Naina Devi area of Shivalic ranges of Himachal Pradesh has been chosen having uniaxial compressive strength as 240 to 280 kg/cm2) under confining pressures (T3) ranging from 10 to 150 kg/cm2. Based on the experimental study, it is observed that the modulus of elasticity is more or less a linear function of the confining pressures as also observed by the earlier researchers. The increase in strength at failure is found to be significant (4 times the unconfined compressive strength) with the increase in confining pressures even when the C-3 is approximately half the uniaxial compressive strength of rock. The correlations between major principal stress, minor principal stress and uniaxial compressive strength of rock have been proposed, which more or less match with the correlations proposed by Hoek and Brown (1969,1980) and BieniaWski (1974). An attempt has also been made to predict the approximate value of confining pressure when the rock would tend to exhibit ductile behaviour.