Study on the evolution of surrounding rock stress and optimization of efficient gas extraction in protective layer mining

Aiming at the problem of efficient gas extraction from the protected layer in the process of double-protected layer mining of coal seam group, numerical simulation is used to reveal the distribution characteristics of the surrounding rock stress field and its dynamic evolution mechanism under the composite mining conditions, and to propose the optimization of the extraction holes layout scheme. The study shows that the implementation of double protective layer mining can extend the range of pressure relief area of the protected layer and reduce its stress level. After mining the lower protective layer, the pressure relief boundary angles in the inclined direction of the protected layer were 85.5° (upper edge) and 70.5° (lower edge), and the strike relief angle was 61.5°; the upper protective layer mining adjusted the upper and lower boundaries of the protected layer inclined pressure relief angles to 74.5° and 77.5°, respectively. The volume fraction of gas in the extraction holes increased significantly from 25%～35% in the original state to more than 85%, and the gas flow rate of a single hole increased from 0.019～0.069 m³/min to 1.099～1.839 m³/min, with a large increase in the pure amount of methane. The extraction parameters showed a rapid decay trend when the digging face advanced more than 150 m, indicating that the unpressurized gas extraction process has significant spatial heterogeneity and temporal dynamics.