Deformation and Seepage Characteristics of Coal Under True Triaxial Loading–Unloading

True triaxial loading–unloading tests of coal samples were performed using a multifunctional true triaxial fluid–solid coupling experimental system. The deformation, seepage, damage, strength, and failure characteristics of the coal samples were analyzed. Based on this, the variation mechanism of the permeability and yield conditions under different gas pressures was investigated, and the true triaxial loading–unloading of the coal sample was verified using numerical simulation software. The results show that the volumetric strain of the samples exhibited a process of compaction, flatness, and re-compaction with an increase in time. The permeability was approximately negatively correlated with volumetric strain. With an increase in gas pressure, the threshold value of the damage factor increases gradually, and the initial damage point of the sample gradually moves backward. At a pressure of 2 MPa, the degree of damage to the sample was relatively large. After failure, the internal cracks in the specimen developed and expanded rapidly, resulting in an upward trend in permeability. At pressures of 2.5 and 3 MPa, although the yield failure generated new cracks, owing to a decrease in cementation strength between particles and particle separation, shear slip blocked some natural leakage channels, resulting in an unclear permeability variation range. In the post-peak stage, the increasing volumetric strain caused the sample to continue to densify, and the permeability of the sample further decreased. When gas pressure was applied, it was equivalent to the axial prestress of the sample. The greater the gas pressure, the smaller the shear stress of the sample when it entered the yield state. After the yield, the sample was continuously subjected to a larger three-way pressure, resulting in a longer plastic-stage duration.