Mining-induced stress–gas permeability characteristics based on protective layer pressure-relief mining

Protective layer pressure-relief mining is an effective strategy for preventing coal and gas outbursts in deep seams, while simultaneously facilitating safe and efficient gas extraction from low-permeability coal. This study employed the self-developed ZST 1500 test system to perform permeation tests on the axial pressure-relief zone (APRZ) and radial pressure-relief zone (RPRZ) of the protected layer under various gas pressure conditions. The primary objective was to elucidate the gas permeability mechanism of the protected layer under combined mining-induced stress and gas pressure during pressure-relief mining. The results indicated that the application of axial stress and seepage pressure during the initial seepage stage led to axial compression and circumferential expansion of the sample, thereby resulting in the formation of equivalent stable seepage channels. Comparative analysis revealed that the RPRZ exhibited higher gas permeability than the APRZ, while the gas permeability gradually rose as the sample's circumferential stress decreased. Notably, the circumferential stress was identified as the predominant factor influencing the formation of equivalent stable seepage channels. The upper limit of gas permeability was determined by the number and extension of these equivalent stable seepage channels in the coal body. Furthermore, variations in seepage pressure significantly affected the degree to which the gas slippage effect enhanced sample gas permeability. A capillary-based permeability model for porous media effectively fitted the experimental permeability of the protected layer, thus validating the model's feasibility. The findings provide a theoretical basis for delineating gas accumulation zones, optimizing design of gas drainage boreholes, and enabling safe and efficient co-extraction of coal and gas in underground mining operations.