Thermodynamic energy degradation and progressive fracture of deep coal-rock composites regulated by interface dip angles and lateral confinement

Coal-rock composites containing weak interfaces are subjected to elevated stresses in deep mining, making their mechanical response and progressive failure relevant to underground stability. This study uses the two-dimensional Particle Flow Code (PFC2D) to investigate coal-rock composites with interface inclinations of 0°, 30°, and 60° under lateral confining pressures of 0, 10, and 30 MPa. Biaxial compression simulations were conducted to examine macroscopic mechanical response, microcrack evolution, force-chain anisotropy, and energy allocation. The results show that interface inclination and lateral confinement jointly influence peak strength and failure mode. Increasing inclination promotes interface-controlled sliding under low confinement, whereas higher confinement restrains relative sliding and promotes cracking within the matrix. Force-chain fabric tensor analysis reveals inclination-dependent changes in internal load-transfer directions and anisotropy. At a lateral confining pressure of 10 MPa, increasing interface inclination is associated with a lower elastic-energy storage ratio at peak stress, indicating a change in energy allocation. For the specimen with a 60° interface inclination, increasing lateral confinement from 10 to 30 MPa was associated with a higher peak elastic-energy storage ratio and delayed interface sliding; the intermittent stress drops were accompanied by increases in dissipated energy. These findings provide a specimen-scale numerical interpretation of damage and energy redistribution in coal-rock composites within the investigated two-dimensional models and loading conditions.