Mechanical Behavior and Fracture Evolution Mechanism of Composite Rock Under Triaxial Compression: Insights from Three-Dimensional DEM Modeling

Existing studies on transversely isotropic rock formations, a special geology, have mainly focused on its mechanical characteristics; whereas, investigations on its fracture process and damage microscopic mechanisms are relatively limited. To remedy this deficiency, in this study, a three-dimensional numerical model is established using discrete elements (PFC^3D), focusing on the effects of confining pressure (0, 5, 10, 15, and 20 MPa) and laminar inclination angle (θ_0°, θ_15°, θ_30°, θ_45°, θ_60°, θ_75°, and θ_90°) on the failure behavior of the composite rock. To demonstrate the accuracy of the simulations, the stress–strain curves and ultimate failure modes obtained from the numerical simulations were compared with the previous laboratory mechanical test results and X-ray CT images. Numerical models using the smooth-joint contact model were shown to simulate the laboratory results reasonably well. Numerical simulation results indicate that the confining pressure and laminar angle significantly influence the internal crack evolution patterns of the specimen. The internal cracks change from a concentrated to a discrete distribution as the confining pressure increases. The internal cracks of specimens with θ_0° and θ_90° laminar inclination emerges from the soft rock and eventually extends to the hard rock, while the inclined specimens crack from the laminar face and finally spread to the rock matrix, which can be explained by the graph of the increasing number of cracks. In addition, the internal principal stress and tangential stress in soft and hard rocks were monitored by arranging measurement circles, and it was found that the tangential stresses are the essential cause of the difference between the mechanical behavior of the two rock types.