Impact of Lower Protective Coal Seam Mining on Overlying Strata: Deformation, Pressure Relief, and Permeability Enhancement

This study focuses on the issue of gas overflow encountered during the mining of coal seams at the Anshun coal mine. Initially, a mechanical model is constructed to analyze the displacement and vertical stress distribution in the upper neighboring coal seam following the mining of the protective seam in the field. Subsequently, similar indoor simulation experiments, utilizing the digital image correlation method, are conducted to examine the evolution characteristics of the overlying rock strain during the back mining process of the lower protective seam. Numerical simulation is then employed to analyze the plastic zone, stress distribution, and evolution characteristics of the overlying coal seam during the recovery of the lower protective layer. The study finally investigates the unloading effect of the top plate and the permeability characteristics of the protected layer. The results demonstrate that the maximum main strain concentration area in the overlying rock increases as the working face progresses, transferring asymmetrically from the middle of the mining area to the cuthole side and the working face side, with the concentration on the cuthole side being notably larger. As the lower protective layer advances, the unloading range, unloading height, stress concentration area, peak value, and unloading coefficient are observed to increase gradually before stabilizing. The numerical simulation reveals a predominance of shear damage around the mining area, where high permeability facilitates gas flow. Additionally, the collapsed gangue in the mining area's center is recompacted, leading to reduced porosity and permeability. Stress concentration is noted on both sides of the coal seam, resulting in high stress at both ends and low stress in the middle of the unloading area. Furthermore, the permeability coefficient growth rate diminishes with the advancement of the lower protection layer, exhibiting negative growth in some areas, while the permeability coefficient in the unloading area remains high at the late stage of workface recovery, thereby constituting an efficient gas extraction zone. This study provides a theoretical foundation for the safe and efficient mining of protruding coal seams in southwest China.