Experimental investigation on the stress-dependent permeability of post-failure sandstone under loading–unloading conditions

The stress-dependent permeability (SDP) of post-failure rock is pivotal for mine goaf reuse and environmental protection. In this study, we employed a triaxial servo system to investigate the SDP of post-failure sandstone subjected to a mining-induced stress path. Initially, intact sandstone specimens underwent axial loading and confining unloading under various initial hydrostatic pressure, seepage pressures, and loading–unloading rates. Subsequently, the SDP and crack porosity were quantified by incrementally increasing the effective confining pressure to characterize how these properties evolve under different stress conditions. Notably, the induced crack patterns resulting from the loading–unloading stress path (LUSP) encompassed tensile dominated cracks, shear dominated cracks, and tensile-shear composite cracks, which exhibited dependence on the specific test conditions. Interestingly, the measured permeability and crack porosity of the anisotropic complex crack network exhibited less sensitivity to changes in effective confining pressure compared to that of single cracks and pore medium. This is likely due to the directional correlation between confining pressure and crack surfaces. Our experiments suggest that a power law provides a more accurate description of sandstone behavior post-failure under low uniform stress conditions than the exponential model. Gas permeability was found to be higher than water permeability, but within one order of magnitude. The sensitivity exponent of crack porosity ranged from 1.13 to 6.33, reflecting variations in crack morphology and complexity under different test conditions.