Influence of Microstructure Orientation on Polyaxial Strength of Rock

Although the commonly used rock failure criteria such as the Mohr-Coulomb and the Hoek-Brown neglect the influence of the intermediate principal stress (2σ) on the strength of rock, experimental evidence appearing in many publications on the polyaxial strengths of rock strongly suggests that the 2σ effect exists for many rocks. Along with the accumulation of the polyaxial test data, a number of rock failure functions taking into consideration of the 2σ effect have been proposed. Most of the suggested polyaxial strength criteria, however, focus on the phenomenological description of rock strength observed in the true triaxial tests, so that they fail to provide the theoretical explanation for the cause of the strengthening effect of 2σ. In this study, a series of numerical polyaxial tests on the transversely isotropic rock samples are conducted to verify that the presence of microstructures and their alignment to a prevalent orientation could be one of the plausible causes of the 2σ effect. In order to incorporate the strength anisotropy into a failure condition, the Mohr-Coulomb criterion is extended to the anisotropic version by defining the internal friction angle and the cohesion as respective scalar functions of orientation vector. These two distribution functions employ the 2nd order symmetric traceless tensor which describes the directional bias of the strength parameter from the orientation average. Critical Plane Approach is applied to find the 3-D stress conditions at failure and the orientation of the corresponding failure plane. The simulation results hint that both the inclusion of the weak planes in the rock samples and their alignment to the preferred direction with respect to the loading direction are closely related to the 2σ dependency of rock strength.