Strength and deformation of coal-rock composites with pre-cracks under triaxial compression

Fractures in coal seams and adjacent strata affect mining stability. This study examines the effects of confining pressure (0, 5, and 10 MPa) and fracture inclination (30°, 45°, and 60°) on coal–rock composites containing a single initially open, unfilled prefabricated fracture. Conventional triaxial compression tests, acoustic emission (AE) monitoring, and PFC3D simulations were combined. Compressive strength increased markedly with confining pressure, while fracture inclination had a smaller influence. Crack development shifted from predominantly tensile cracking under low confinement to mixed tensile–shear failure at higher confinement. AE activity progressed through quiet, active, and intense stages, consistent with the stress–strain response. PFC3D displacement fields showed radial displacement from three-dimensional crack initiation sites, with limited deformation in the coal units. These results clarify failure mechanisms in fractured coal–rock composites and support stability assessments in deep mining.