The conventional Hoek–Brown (H–B) failure criterion does not explicitly account for the combined effects of anisotropy and high confining pressure on rock strength. In bedded rocks, anisotropic mechanical behavior arises from the orientation-dependent weakening induced by bedding planes, which breaks the stress symmetry relative to the loading direction. To address this limitation, this study proposes a modified H–B criterion that incorporates a dip-angle-dependent anisotropy parameter and a high-confinement correction term to describe the brittle–ductile transition of bedded rocks. The proposed criterion was implemented in the discrete element software 3DEC through secondary development of the built-in H–B module using the FISH language. Triaxial compression tests were conducted on bedded marble and sandstone specimens with different bedding dip angles under uniaxial compression and confining pressures of 40, 80, and 100 MPa to examine their stress–strain responses and failure characteristics. Comparisons between experimental results and predictions from existing failure criteria show that the proposed criterion provides improved prediction accuracy and effectively captures the variation in strength with bedding dip angle and confining pressure. Numerical triaxial tests were further performed on Martinsburg Slate specimens with bedding dip angles of 0°, 45°, and 90° under confining pressures of 50, 100, and 200 MPa. The results indicate a clear transition from elastoplastic softening to ductile deformation with increasing confining pressure. The compressive resistance follows the order α = 0° > α = 45° > α = 90°, highlighting the role of bedding-induced strength asymmetry in the brittle–ductile transition of bedded rocks.

The static fragmentation of naturally anisotropic rocks is poorly understood despite extensive research on dynamic fragmentation. This study systematically investigates the static fragmentation behavior of jointed sandstone under uniaxial and triaxial compression, focusing on joint inclination angles from 0° to 90°. The results reveal a U-shaped relationship between joint inclination angle and compressive strength, with both the minimum compressive strength and axial strain occurring at a joint inclination angle of 60°. The elastic modulus exhibits a nonmonotonic variation, reaching its maximum within the range of 65°–75° because of the interaction between the weak planes and the loading direction. Three distinct failure-mode transitions are identified: (1) axial splitting predominates at low inclination angles (0°–30°); (2) shear failure dominates at intermediate inclination angles (45°–60°); (3) axial splitting becomes dominant again at 90°. In addition, the fragment size distribution curves and average fragment sizes are determined, and the relationship between joint inclination angle and average fragment size is established. When the joint inclination angle is not greater than 45°, the specimens undergo severe failure and exhibit a relatively high degree of fragmentation. Fractal analysis demonstrates that the fragment size distributions of these sufficiently fragmented specimens follow a fractal law. These findings provide a useful basis for predicting rock fragmentation in tunneling and mining operations.