Gas permeability characteristics of the hanging wall and footwall in normal faults based on true triaxial experimental system

In deep mining environments, normal fault structures significantly influence coal seam permeability, which plays a crucial role in coal and gas outbursts. Utilizing a true triaxial multifunctional physical simulation system, this study conducted gas seepage and hydraulic fracturing experiments on normal fault-associated coal seams at depths of - 400 m, - 600 m, and - 800 m. The research systematically investigates the gas permeability evolution in hanging wall and footwall coal seams of normal faults and proposes enhanced control measures tailored to the Guodishan normal fault. The results showed: Permeability exhibited a negative exponential relationship with confining pressure. As confining pressure increases, the permeability disparity between the hanging wall and footwall diminishes. Under true triaxial conditions, the permeability of the coal in the hanging wall of the normal fault is generally lower than that in the footwall. This difference gradually increases with burial depth, with the permeability ratio between the two sides increasing from 2.96 to 10.2 times. Moreover, the permeability reduction in the hanging wall coal is greater than that in the footwall coal. The crack initiation pressure for fracturing is higher in the hanging wall. Post-fracturing, permeability increases significantly in both walls, with the footwall demonstrating superior enhancement. However, the permeability amplification factor decreases progressively with depth. Building on the evolutionary characteristics of the Guodishan normal fault and its permeability patterns, targeted gas control strategies were developed in the Pingdingshan mining area. This work provides theoretical and practical guidance for gas extraction and outburst prevention in fault-affected coal seams.