Anisotropic rocks are commonly encountered in underground engineering, yet their mechanical behavior under complex stress states remains inadequately understood. This study explores the effects of stress direction and magnitude on the stress–strain response, strength, failure pattern, and failure angle of weakly anisotropic sandstone through true triaxial compression tests. The experiments were systematically designed to simulate realistic engineering geological conditions, with \(\sigma_{3 }\) ranging from 5 to 20 MPa and \(\sigma_{2 }\) from 7.5 to 50 MPa. The direction of \(\sigma_{1 }\) was varied at 0°, 30°, 45°, 60°, and 90°, while \(\sigma_{2 }\) was applied either parallel or perpendicular to the bedding plane. The results demonstrate that the strength of weakly anisotropic sandstone is predominantly governed by the direction of \(\sigma_{1 }\) and the magnitudes of \(\sigma_{2 }\) and \(\sigma_{3 }\) , with negligible sensitivity to the direction of \(\sigma_{2 }\) . A shoulder-shaped strength anisotropy is observed with respect to the \(\sigma_{1 }\) direction. These strength characteristics are well captured by the modified Mogi–Coulomb criterion incorporating direction-dependent cohesion and friction angle. The failure of weakly anisotropic sandstone is governed by the coupling of structural and stress-induced anisotropy, and this coupling is enhanced when the \(\sigma_{1 }\) direction corresponds to the transition between bedding-dominated and matrix-dominated shear failure. The discussion on the strength behavior of different anisotropic rocks reveals that the effects of \(\sigma_{2 }\) direction and magnitude depend on the degree of structural anisotropy. This study provides new insights into the mechanical behavior of anisotropic rocks under true triaxial stress states and facilitates their theoretical analysis and numerical modeling.
