Mechanical Response and Seepage Evolution of Coal Under Triaxial Compression and the Permeability Model

The stress–seepage coupling characteristics of the coal pillar dams in underground reservoirs of coal mines are critical to evaluating their service safety. To this end, triaxial compression–seepage coupling experiments on dry and saturated coal samples were conducted under different confining pressures, the effects of water content and confining pressure on the mechanical response and seepage characteristics of coal were analyzed, and a theoretical model characterizing the full-process permeability evolution was established. The results show the following: (1) The crack compaction stage of the stress–strain curve for the coal under triaxial loading gradually shortens, while the elastic deformation stage tends to be the dominant deformation stage. The lateral confinement effect of confining pressure leads to an exponential increase in peak axial stress, peak deviatoric stress, and elastic modulus of the coal sample. (2) Water–coal interaction deteriorates the mechanical properties of coal samples. Compared with dry coal samples, the peak axial stress, peak deviatoric stress, cohesion, and internal friction angle of saturated coal samples decreased by 20.12%, 21.39%, 26.9%, and 2.76%, respectively. However, the ductile effect of water on the failure characteristics of coal samples is weakened by the presence of confining pressure. (3) The permeability of coal samples first decreases and then increases with increasing axial strain. Based on the experimental data, the response surface of “permeability—axial pressure—confining pressure” and the empirical formula of “fracture permeability–maximum normal stress on cross-section” of coal samples were established. These functions enable numerical representation of permeability evolution during both the elastic and damage stages; nonetheless, they lack a foundation rooted in physical principles. (4) Considering the relationship between permeability and effective stress as well as the stress-induced fracture damage effect, a piecewise damage-based permeability model was constructed by introducing a damage variable defined based on elastic modulus degradation and the theory of porous media. Fitting results of experimental data demonstrate that the model can well reproduce the full-process permeability evolution of coal samples (i.e., the trend of first decreasing and then increasing). This research provides theoretical and experimental support for the stability evaluation and fluid structure coupling analysis of coal pillar dams in underground reservoirs of coal mines.