Mechanical-seepage responses and damage constitutive model of coal under cyclic stress-pore pressure coupling

During coal seam mining, pore gas pressure and in-situ stress exhibit dynamic coupling characteristics involving multi-cycle synchronous loading and unloading. Coal deformation and gas migration are jointly driven by these two factors, triggering coal-rock gas dynamic disasters in mines. To further reveal the mechanical-seepage response characteristics of coal and their dominant controlling mechanisms under multi-cycle coupling of in-situ stress and gas pressure, triaxial seepage tests are conducted under three cyclic loading-unloading paths: stress-only (OS) cycling with fixed pore pressure, pore pressure-only (OPP) cycling with fixed stress, and stress-pore pressure coupled (SPPC) cycling. A damage constitutive model for coal under SPPC conditions is developed based on statistical damage theory. The results indicate that stress and pore pressure both promote axial compression and radial expansion of coal, while exerting an opposite competitive relationship on volumetric strain. This leads to their differentiated effects on seepage channels: stress facilitates volumetric compression, thereby inhibiting gas seepage, whereas pore pressure induces volumetric expansion, thus enhancing gas seepage. With an increasing number of cycles, axial strain of coal under all three cyclic paths exhibits a fluctuating increase, whereas radial strain shows a fluctuating decrease. Under the coupled effect of stress and seepage fields, coal subjected to SPPC cycling demonstrates a more sensitive deformation response and greater susceptibility to damage accumulation. By the end of the tests, its cumulative residual strain is the largest (axial 0.203% and radial −0.059%). Additionally, permeability of coal under all three cyclic paths at the end of loading exhibits exponential decay with an increasing number of cycles, indicating that accumulated cyclic damage hinders the recovery of pore-fractures apertures to their initial state, resulting in diminished seepage capacity. Permeability of coal under OS and SPPC cycling at the end of unloading gradually decreases with more cycles, whereas the opposite is observed under OPP cycling. High-velocity gas repeatedly scours loose coal particles or clay minerals adhering to fracture surfaces, facilitating the expansion of fracture channels. Contribution rate analyses reveal that coal strain evolution is dominantly controlled by stress, with average contribution rates of 97.5%, 64.9%, and 79.7% to axial, radial, and volumetric strains, respectively. Permeability evolution is absolutely dominated by pore pressure, with a contribution rate exceeding 97.6%. Finally, based on the characteristic that coal element strength follows a Weibull distribution, and considering the strength degradation effect of pore pressure on coal, a coupled damage variable is proposed, and a damage constitutive model based on the D-P (Drucker-Prager) criterion under SPPC conditions is developed. This model effectively describes the deformation response characteristics of coal under SPPC during loading-unloading stages. The results provide theoretical support for further elucidating the mechanical mechanism of coal-rock gas dynamic disasters in mines.