Research and Application of Mining-Induced Fracture Evolution and the Mechanism of Pressure Relief Gas Migration and Storage in Overlying Strata during Soft Rock Protective Layer Mining

During deep coal seam development using traditional protective layer mining methods, resource extraction is often constrained by low permeability, high gas content, and high ground stress. It is necessary to develop a new protective layer mining approach to improve mining safety and efficiency. Taking the III 11 soft rock protective layer working face of Luling Mine in the Huaibei mining area as the engineering background, this study adopts a comprehensive research framework integrating physical similarity simulation experiments, fractal geometry theory, numerical simulation, and engineering practice. The results indicate that the mining-induced fracture network exhibits distinct fractal characteristics, and pressure-relief gas preferentially migrates along fractures toward low-pressure zones. Based on a fracture network with a development height of 130 m, a numerical model was established to simulate gas migration behavior, demonstrating that gas transport is dominated by pressure gradients and guided by fracture connectivity. A mining design methodology was proposed, together with a three-dimensional gas extraction technology specifically tailored for soft rock protective layer mining. Field application of the pressure-relief gas drainage technology was conducted at the III 11 soft rock working face. The results show that the residual gas pressure of the protected coal seam decreased to 0.2–0.25 MPa, while the residual gas content was reduced to 3.85–5.32 m3/t, effectively eliminating the risk of coal and gas outbursts. After gas extraction, the residual gas pressure and gas content of the overlying protected coal seam were also reduced to 0.2–0.25 MPa and 3.85–5.32 m3/t, respectively, indicating a significant reduction in outburst risk. Overall, this finding provides a technical pathway for deep coal and gas comining and further promotes the application of protective layer mining under deep and complex geological conditions.