Anisotropic evolution of mechanical and seepage properties in coal under mining-induced stress paths: experiments and modeling for enhanced gas extraction

As mining depth increases, the in-situ stress in coal bodies rises significantly, leading to an increased risk of dynamic disasters such as rock burst, coal and gas outburst. Deeply understanding the influence of stress paths on the anisotropic evolution of coal mechanics and seepage characteristics during mining is crucial for improving gas drainage efficiency and disaster prevention. In this study, self‑developed true‑triaxial compression‑seepage test system was adopted, and two groups of typical stress‑path tests were carried out on anthracite specimens, including triaxial proportional loading (Path 1) and simulated excavation unloading (Path 2). Test results show that the peak strength in the perpendicular-to-bedding direction is 3.48%‑6.94% higher than that in the parallel-to-bedding, and the failure mode is transformed from X‑type conjugate shear failure to combined shear‑splitting failure along weak structural planes. Permeability exhibits a four‑stage evolutionary characteristic under both stress paths, whereas the post‑peak permeability amplification in the parallel-to-bedding direction (16.5‑52.3 times) is considerably larger than that in the perpendicular-to-bedding direction (14.7‑22.7 times). Theoretically, an anisotropic damage‑permeability coupling model is established in this work. The relationship between the differential development of fracture networks and the amplification effect of the cubic law for permeability anisotropy is investigated, and the intrinsic mechanism accounting for the above‑mentioned direction‑dependent behaviours is interpreted. From an engineering perspective, it is demonstrated that gas drainage boreholes should be approximately arranged along the bedding direction to connect highly conductive shear‑tensile fracture channels. Meanwhile, targeted support is utilised to maintain confining pressure, by which the outburst risk induced by the sharp permeability increment during unloading can be mitigated. This research provides a theoretical foundation and implementable engineering countermeasures for safe and efficient gas drainage in deep coal mines.