Research on permeability evolution law and gas outburst mechanism of coal near concealed fault

Hidden faults significantly influence the development of coal fractures and variations in permeability. Differences in fracture formation and permeability contribute to localized gas accumulation, heightening the risk of gas outbursts. To investigate the evolution of coal permeability and the mechanisms driving gas outbursts caused by concealed faults, this study focuses on the 9# coal seam of the Longfeng Coal Mine in the Northern Guizhou Coalfield. A comprehensive analysis was conducted using scanning electron microscopy, micro-CT scanning, and triaxial seepage experiments. The findings indicate that the fissure distribution differs significantly between primary and tectonic coals. In primary coal, fissures are predominantly located at the edges, with a fissure rate of 0.02 %, whereas tectonic coals exhibit a well-developed internal fissure network, with a fissure rate of 0.27 %. The strength of coal samples decreases as their distance from the fault decreases, with notable weakening observed in samples located 20 m and 50 m away. The tectonic influence has caused substantial structural damage to coal bodies situated closer to the fault. The results demonstrate that under various loading conditions, the permeability of coal located 20 m and 50 m from the fault is significantly higher than that of coal located 100 m away and primary coal. Specifically, at the initial stage, the minimum permeability stage, and the failure stage, the permeability of coal near the fault is approximately 3.2 times, 8.3 times, and 2 times greater than that of primary coal, respectively. This indicates that prolonged tectonic activity has severely fragmented the internal structure of the coal, resulting in dense fissure networks and fracture zones. Gas accumulates along these fissures near the concealed faults, forming localized enriched zones. Following mining activities, gas in these zones can rapidly spread to lower-pressure areas due to sudden pressure changes, leading to rapid gas release to the working face, which triggers gas outburst incidents. The study reveals the impact of concealed faults on the mechanical seepage characteristics of coal and the mechanisms underlying gas outbursts. These findings provide valuable insights for the comprehensive prevention and control of gas in fault-affected structural zones, contributing to safer and more efficient coal mine operations.