Estimation of Intact Rock Strength Under True-Triaxial Stress State Using Conventional Triaxial Compression Test Data

The strength and deformation behavior of intact rock has a functional dependence on the in-situ state of stresses, pore-pressure, temperature, pore fluids, etc. Rock and rock masses are generally subjected to a more generalized form of stress. In three mutually perpendicular principal stress directions, rock experiences three different principal normal stresses (True-triaxial state). Experimentally assessment of rock strength under true-triaxial stress state has its challenges. Hence for ease, the effect of intermediate principal stress on strength is neglected or ignored. This results in a conservative design. In the current research, we have addressed the challenge by estimating the true-triaxial strength of intact rock using three-dimensional (3D) strength criteria. The criteria parameters are estimated using conventional triaxial test data, as it omits the true-triaxial test requirement. In the study, nine criteria are assessed over twelve rocks. Out of nine criteria, two are 2D-strength criteria Mohr-Coulomb and Hoek Brown, and one is Mogi criterion, which requires true-triaxial data to estimate its parameter. It is concluded from the study that Drucker-Prager criteria in its three forms, inscribe, circumscribe, and middle circumscribe, provides very poor estimations of intact rock strength under true-triaxial stress state. Modified Wiebols-Cook criterion best estimates rock strength under a generalized stress state.