Simulation on mechanical and failure characteristics of sandstone with elliptical hole under tension-shear effect

Under the influence of geological environment and engineering disturbance, elliptical hole-defects exist widely in engineering rock mass. Excavation unloading causes rebound tensile stress in rock mass. The tension-shear stress zone is formed under the hole defects, which induces the tension-shear failure of rock mass and greatly reduce the stability of engineering rock mass. In order to study the mechanical properties and failure behavior of the rock mass with an elliptical hole under tension and shear, the numerical model was built using discrete element numerical simulation based on the rock mechanics test results. Furthermore, the tension-shear numerical modelling tests of  rock mass with an elliptical hole of different hole inclination angleαand the ratio of long to short axiskwere carried out, and the meso-mechanism of crack evolution was revealed from the point of view of stress tensor. The results show that whenkis constant, with the increase ofα, the shear strength approximately shows a “W” shape under low normal tensile stress (1–3 MPa), and the minimum value is obtained whenαis 120° or 150°, and the maximum value is obtained whenαis 90°. Under high normal tensile stress (4–6 MPa), the shear strength increases at first and then decreases, and the minimum and maximum values are obtained whenαis 0° and 90°, respectively. Whenαis constant, for the rock mass with an elliptical hole ifαis not 90°, the shear strength decreases nonlinearly with the increase ofk. The sensitivity of stress concentration of the elliptical hole to normal tensile stress decreases at first and then increases with the increase ofα, and the sensitivity is the highest whenαis 0°. The sensitivity is the lowest whenαis 90°, and the sensitivity is higher whenαis 120°and 150° than that whenαis 30°and 60°. The strength of the rock mass with an elliptical hole is obviously worse than that of intact rock mass, and the degree of deterioration is positively related to the normal tensile stress. The level of crack initiation stress increases with the increase of normal tensile stress, and the crack initiation angle decreases with the increase of normal tensile stress. The failure type of the rock mass with an elliptical hole under tension and shear is the tensile failure caused by anti-wing crack penetration. Under the effect of tension and shear, the maximum tensile zone is formed by the coupling of tensile stress and compressive stress in the rock mass, and the boundary near the side of the shear loading surface is the crack propagation path. The crack starts from the plastic yield at the elliptical hole. After the crack initiation, the stress is released and redistributed by the particle contact fracture, and the crack propagates along the direction of the maximum principal stress after the redistribution, which shows the nonlinear propagation mode of the crack macroscopically.