Characteristics of fracture development and gas extraction of a lower protected seam

To ensure the mining safety of working face in the protective seam and meanwhile extract pressure-relief gas of the lower protected seam and eliminate its outburst risk, the present study researched into fracture development of floor coal-rock mass of the protective seam and migration rule of pressure-relief gas from a protected seam so as to obtain an effective pressure-relief gas extraction method. The results show that after the upper protective seam was mined, mining-induced fracturing floor coal-rock mass fell into two zones: a floor heave fracture zone and a floor heave deformation zone. Lower protected seams located at different fracture subzones were different in the degree and characteristics of fracture development, and penetration borehole drilling from the floor roadway effectively extracted pressure-relief gas. The engineering practice of the Hongling coal mine in Shenyang indicated that for a lower protected seam in the floor heave fracture zone, taking the one time seam spacing (16 m) as the spacing between penetration boreholes ensured the mining safety of the No.321 working face in the protective seam and completely eliminated the outburst risk of the lower protected seam, the No. 12 coal seam.