Three-dimensional strength estimation of an anisotropic intact rock under high confining pressure through developing a Hoek-Brown criterion

Although a three-dimensional (3D) modified Hoek–Brown (H–B) criterion can be applied to evaluate the strength of homogeneous rocks in a stress state, there is still lack of a criterion with consideration of anisotropic characteristics under high confining pressure. In this paper, we aim to establish a 3D H–B criterion for accurately determining the strength of an anisotropic rock under high confining pressure. Based on a two-dimensional (2D) modified H–B criterion under high confining pressure (M–HB–HC), we examine the effect of an intermediate principal stress on strength by taking the transformation of rock behavior from brittle to ductile into account under confining pressure, and then, establish a new improved 3D M–HB–HC criterion. The William and Warnke's elliptical approximation ridge function is introduced to eliminate non-smoothness and non-convexity of the proposed function on the π plane. Furthermore, comparison of predicted strengths by several criteria is conducted with true triaxial test data of anisotropic rocks under different confining pressures. It is shown that the predicted strength by the 3D M–HB–HC criterion is the closest to test data and the proposed criterion works better than other criteria. Thus, it is expected that the 3D M–HB–HC criterion, with a clear physical meaning, is more suitable to the estimation of an anisotropic rock strength.