Study on the Permeability Model and Efficient Gas Control Technology of a Coal Seam with a Thick Dirt Band

The presence of thick dirt bands within coal seams significantly increases the difficulty of gas extraction. In this study, a coal–rock–coal combination permeability model was developed to investigate the permeability evolution of the underlying protected coal seam with a dirt band during mining of the overlying protective layer. A comparative analysis was conducted to evaluate the effects of different borehole layouts on the pressure-relief gas extraction efficiency of the protected coal seam. The results indicated that the permeability of the underlying 11# coal seam initially decreased and then increased under the influence of mining-induced disturbances in the overlying 8# coal seam. The permeability of the 11# coal seam reached its peak value of 0.23 mD at approximately 50 m behind the working face, which is 10.8 times higher than the permeability before pressure relief. Moreover, borehole layout played a crucial role in the efficiency of gas extraction in coal seams with dirt bands. The bedding borehole extraction method was hindered by the dirt band, which obstructed gas flow between different stratified coal seams and subsequently reduced the gas extraction efficiency of the protected layer. In contrast, cross-layer boreholes, serving as artificial gas flow channels, effectively mitigated the obstructive influence of the dirt band on gas flow, thereby enhancing the gas extraction efficiency across different coal seams. Based on these findings, cross-layer boreholes were implemented in the 8# coal seam in panel III (lower section) of the Baode Coal Mine to extract gas from the 11# coal seam. Compared with conventional inseam boreholes, the cross-layer borehole method exhibited superior performance, confirming its feasibility. This study provides theoretical support and technical references for the efficient control of gas in coal seams with dirt bands and offers valuable insights into gas disaster prevention in coal mines.