Study on the Permeability Evolution Laws and Damage Characteristics of Gas-Bearing Coal Under Different Cyclic Loading–Unloading Conditions

This study examines the seepage and damage behavior of coal under cyclic loading and unloading, typical in multi-layer coal seam mining. Four stress paths were designed: isobaric, stepwise, incrementally increasing, and cross-cyclic, based on real-time stress monitoring in protected coal strata. Seepage tests on gas-bearing coal were conducted using a fluid–solid coupled triaxial apparatus. The results show that axial compression most significantly affects axial strain, followed by volumetric strain, with minimal impact on radial strain. Permeability variation closely follows the stress–strain curve. Under isobaric cyclic loading (below specimen failure strength), specimens with higher initial damage (0.6) exhibit a sharp permeability decrease (75.47%) after the first cycle, with gradual recovery in subsequent cycles. In contrast, samples with lower initial damage (0.05) show higher permeability during loading, which eventually reverses, with unloading permeability surpassing loading permeability. Across all paths, a significant increase in residual deformation and permeability recovery exceeding 100% indicate the onset of instability. Continued cyclic loading increases damage accumulation, with different evolution patterns based on initial damage levels. These findings provide valuable insights into the pressure-relief permeability enhancement mechanism in coal seam mining and inform optimal gas drainage borehole design.