Experimental study on the damage and permeability characteristics of similar natural gas-bearing coal–rock composites

Investigating the damage and permeability characteristics of gas-bearing coal–rock composites is essential for understanding the mechanisms driving dynamic disasters in coal and rock gas composites. This study focuses on the uniaxial compressive strength of natural coal–rock specimens, from which similar natural composites with transition interfaces were created using the pouring-core sampling method. We conducted mechanical-seepage experiments on gas-bearing coal–rock composites under various confining stresses, gas pressures, and stress paths. The results show a positive correlation between the specimens' compressive strength and confining stress; as confining stress increases, both total input energy and elastic strain energy of the specimens increase, while dissipated energy decreases. When specimens reach instability, they require more energy to maintain this state. At peak strength, the stored elastic strain energy in the specimens gradually increases. Furthermore, higher gas pressure reduces the specimens' bearing capacity. At peak strength, specimen strain decreases, resulting in diminished deformation capacity. Among different testing paths, specimens exhibit the highest strength under the loading axial stress path, while the lowest strength occurs under the compound loading axial and unloading confining path, exhibiting significant deformation and expansion. In contrast, under the unloading confining stress path, specimen strain is minimal, and the overall energy required for specimen instability—including elastic strain energy and dissipated energy—remains relatively low.