Parameter optimization of gas extraction holes in layered mining of steep thick coal seam

This study focuses on optimizing the gas extraction borehole design for layered mining of steeply dipping ultra-thick coal seams. A systematic investigation was conducted on the influence of various borehole parameters on extraction efficiency by integrating numerical simulation with field data. Taking the steeply dipping coal seam at Dongxia Coal Mine as the engineering context, a numerical model measuring 100 m × 250 m was developed using the RFPA^2D software to simulate the fracture evolution process within the "three vertical zones" (caving zone, fractured zone, and bending subsidence zone) in overlying strata above the goaf. Based on the simulated gas distribution range in the goaf and the spatial overlap patterns of borehole extraction zones, the optimal combination of borehole parameters was determined. The results indicate that a borehole spacing of 90 m effectively covers the extraction blind zones between adjacent boreholes, while a borehole depth ranging from 13 to 40 m ensures coverage of the lower portion of the fractured zone and the upper part of the caving zone. An inclination angle of 4° to 6°, compatible with the coal seam dip, enhances the permeability of the fractured zone. Field application results verify that the effective utilization length of the optimized boreholes reached 110 m.