Mechanical-Seepage Characteristics of Coal Under Variable Loading Paths and the Modified Damage Permeability Model

As the key load-bearing structure of a coal mine underground reservoir, the coal pillar dam body is subjected to the long-term coupling effect of overburden stress and mine water erosion. To investigate the coupled mechanical-seepage evolution mechanism of coal-rock under mining-induced stress, triaxial compression-seepage tests were conducted on coal samples taken from the coal pillar dam body of the underground reservoir in Chahasu Coal Mine. The effects of confining pressure and stress loading path on the mechanical deformation and permeability evolution of coal-rock were analyzed, a damage-permeability model incorporating fracture compaction, damage-induced cracking, and matrix deformation was established, and sensitivity analysis of key parameters was performed. Additionally, the model’s predictive accuracy was verified against experimental data, and its overall characterization capability was further assessed. The results show that: (1) The peak axial stress, peak deviatoric stress and elastic modulus of coal samples increase exponentially with increasing confining pressure. A distinct inflection point is observed in the stress–strain curve under the unloading confining pressure-axial loading (UCP-AL) and constant confining pressure-axial loading (CCP-AL) paths, with the stress increasing at a markedly higher rate before the inflection point than after it. (2) Under different stress loading paths, the permeability of the coal samples first decreases, then slowly increases, and then rapidly increases with increasing axial strain, and it shows higher sensitivity to deviatoric stress. The minimum permeability exhibits an exponentially decreasing trend with increasing confining pressure. (3) A damage-permeability model incorporating a Logistic-type damage variable was developed to continuously characterize the permeability evolution of coal specimens over the entire loading process from elastic deformation to macroscopic failure. Furthermore, the key model parameters (ω, ζ, and τ) are strongly correlated with confining pressure. (4) The single-stress-parameter model exhibits limitations in characterizing permeability evolution under triaxial stress conditions. To address this deficiency, a multi-parameter modified damage-permeability model was established, in which the effective total stress is used to describe the fracture compaction effect, while the effective deviatoric stress accounts for the damage-induced cracking effect. These laboratory-scale findings provide an experimental and theoretical basis for understanding the stress–damage–seepage evolution of coal. Further upscaling and calibration using field monitoring data (piezometric and drainage measurements from underground reservoir) is required before its application in seepage prediction and stability evaluation of in situ coal pillar dams.