Energy evolution and progressive damage characteristics of gas-bearing coal-rock combination under different loads

It is of significant importance to investigate the energy evolution characteristics and progressive damage behavior of gas-bearing coal-rock combinations under both conventional and graded cyclic loading conditions for the prevention and control of deep coal-rock-gas composite dynamic disasters. By laboratory testing and theoretical analysis, a series of coupled acoustic emission (AE)-seepage-damage triaxial tests were conducted under various confining pressures and stress paths. The mechanical response, progressive damage behavior, and energy evolution patterns of loaded gas-bearing coal-rock combinations were systematically analyzed. By combining macro-meso failure characteristics, the influence mechanisms of confining pressure and stress loading on the failure behavior of gas-bearing coal-rock combinations were revealed. The results show that the peak strength, elastic modulus and residual strength of the composite specimens increase linearly with confining pressure. Under the same confining pressure, the elastic modulus under graded cyclic loading is lower by 0.13−1.76 GPa, and the peak strength is lower by 8.91%−20.81% than those under the conventional triaxial loading. Increasing confining pressure significantly suppresses crack development, while graded cyclic loading exhibits the lower crack initiation stress and damage stress ratios, making specimens more prone to cracking at lower stress levels. Acoustic emission (AE) signals exhibit stage-wise variations corresponding to characteristic stresses during progressive damage. Peak acoustic emission counts increase with higher confining pressure. Elevated confining pressure restricts internal crack propagation and coalescence. Under graded cyclic loading, the Kaiser effect zone of acoustic emission counts extends, while the Felicity effect zone shortens. The total energy, elastic energy and dissipated energy under graded cyclic loading are all higher than those under conventional triaxial loading. Energy dissipation is more pronounced in intervals with the larger stress amplitudes. SEM observations reveal more complex and rougher failure surfaces under graded cyclic loading, with more evident cracks and pores, and fractal dimension values larger by 0.057−0.280 than that under conventional triaxial loading. High confining pressure induces greater damage effects than compressive restrictions, exacerbating interfacial failure in coal-rock combinations, with shear failure being the dominant failure mode. These findings provide a theoretical basis for preventing and controlling coal and gas composite dynamic disasters in high-stress and high-gas-pressure zones in deep coal mining.