Mechanical Behavior and Fracture Mechanism of Phyllite with Different Foliation Angles Under Uniaxial Compression

Foliation-induced anisotropy in phyllite leads to complex deformation and failure responses, making excavation stability analysis and support design particularly challenging for geo-energy and geo-resource applications. To elucidate the influence of foliation angle (β) on mechanical behavior and fracture mechanisms, uniaxial compression tests were conducted and Particle Flow Code numerical simulations were performed on phyllite specimens with seven different foliation angles. The anisotropic mechanical behavior of phyllite under uniaxial compression was systematically analyzed across varying foliation angles. Furthermore, the interaction mechanism between matrix and foliation-induced cracks was investigated based on relevant theoretical frameworks. The foliation angle β has a significant effect on the stress–strain curve of phyllite. The elastic modulus generally increases with increasing β, whereas the peak strength displays an overall U-shaped distribution. As β increases, the macroscopic failure mode shifts from matrix-dominated fracturing to foliation plane-controlled slip/opening, and then trends back toward matrix-dominated fracturing at high β. In the simulations, microcrack type, initiation sequence, and the internal mechanism of instability vary markedly across β. Combined with the energy release rate criterion of fracture mechanics and the compression bar stability theory, the mutual induction mechanism between matrix cracks and foliation cracks is further clarified. The penetration or deflection behavior of matrix cracks near weak planes is jointly controlled by the crack propagation direction and foliation angle. When the foliation plane is parallel to the loading direction, the buckling instability of thin rock flakes resulting from tensile cracking along weak planes constitutes the primary factor contributing to the reduction in the strength of specimens where the foliation plane is normal to the loading direction. This finding clarifies the intrinsic cause of the prediction deviation of the conventional Jaeger weak plane theory and can provide a more accurate theoretical reference for the stability evaluation of layered rock mass engineering.