Research on the mechanical behavior and energy evolution characteristics of composite coal-rock under true triaxial stress conditions

Deep coal mining is confronted with complex geological conditions and strong engineering disturbances. The environment characterized by high geostress, high gas pressure, high geothermal temperature, and intense mining disturbance frequently induces nonlinear large deformation and catastrophic instability of surrounding rock, which seriously restricts the safe and efficient exploitation of deep geological resources. To investigate the mechanical response and energy evolution characteristics of coal and rock under deep true triaxial stress conditions, true triaxial tests were conducted on raw coal, sandstone, and composite coal-rock specimens under different intermediate principal stresses with the aid of a multifunctional true triaxial fluid-solid coupling testing system. The results show that composite coal-rock exhibits stronger plastic deformation capacity and a smaller post-peak stress drop. The dissipated energy of coal increases significantly after the peak, whereas that of sandstone accelerates during the plastic stage. The energy evolution of composite coal-rock approaches sandstone at low stress and resembles coal at high stress. For coal, the fluctuation peaks of the energy release rate ( G_\mathrme ) and energy dissipation rate ( G_\mathrmd ) occur near the strength peak, and an increase in \sigma _2 reduces their amplitudes. For sandstone, the fluctuation peaks appear in the yield stage, but the influence of \sigma _2 is relatively weak. Composite coal-rock exhibits gentle but high-amplitude post-peak fluctuations, which are markedly suppressed by the increase in \sigma _2 . Post-peak elastic energy release has a greater effect on the brittleness index than pre-peak elastic energy storage. Coal failure is mainly controlled by a single dominant crack and is weakly affected by \sigma _2 , whereas sandstone exhibits a more complex fracture network at low \sigma _2 and a simpler pattern at medium and high stress levels. In composite coal-rock, secondary cracks in the sandstone layer propagate along the \sigma _1 direction, while those in the coal seam propagate along the \sigma _3 direction. These results provide theoretical support for surrounding rock control and dynamic disaster prevention in deep resource extraction.