Mechanical properties and failure mechanism of heterogeneous combined rock mass under triaxial compression

The mechanical behaviour of heterogeneous combined rock masses with jointed surfaces is crucial for the safe extraction of deep underground resources. This study examines the effects of joint inclination and confining pressure on the strength and failure of heterogeneous combined rock masses by means of triaxial compression tests and PFC2D simulations. Rock-like samples composed of a strong and a weak matrix separated by a rough joint (joint roughness coefficient = 10) were tested under multiple confining pressures and a wide range of joint angles. The peak strength exhibits a pronounced U-shaped dependence on joint angle: for a given confining pressure, the strength at θ = 60° is up to about 25 ∼ 30% lower than at θ = 0°, whereas increasing confining pressure substantially enhances the strength at small joint angles. Numerical results show that microcracks mainly develop in the weak-side matrix at small joint angles, whereas up to 88% of microcracks coalesce along the joint plane at θ = 60°, indicating a transition from matrix-dominated to joint-dominated failure. Increasing confining pressure suppresses tensile opening and promotes shear microcracking. A modified Rafiai-type strength criterion provides accurate predictions for samples with varying joint angles and confining pressures.