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Both theoretical and field research have shown that protective layer mining with pressure-relief gas extraction in protected coal seams is an effective and economical way of controlling coal and gas outbursts in underground mines. Given the influences of coal seam angles, layer spacing, pressurerelief angles, coal pillars, and other factors in the protected areas of protected seams, the protection range is often smaller than the area of protective seam mining. This problem can be solved by using a gas drainage method based on coal seam geology and the conditions of coalbed methane (CBM) occurrence during protective layer mining. If a protective layer is prone to outbursts, crossing boreholes and bedding boreholes can be used to eliminate outburst hazards. The crossing boreholes should control the upper side of the roadway contour for at least 20 m and the lower side for at least 10 m. For other coal seams, the boreholes should control both sides of the roadway contour for at least 15 m. Crossing boreholes can not only extract CBM from the protective layer but can also extract from the protected coal seam, thus leading to a larger protection range in the protective layer than that predicted by the pressure-relief boundary, as well as expanding the protection range. The actual protection range of the protected layer was determined using an index of residual gas content. The results of the study indicate that the pressure-relief angle increased from 56° to 69° in the strike direction and from 75° to 90° in the inclination direction. The protection range was approximately equal to the mining area of the working face of the protective layer, thereby extending the protection range. One of the important reasons for protection range expansion was the influence of gas drainage via crossing boreholes. The protection range extension mitigated the outburst danger over a large area of the coal seam, thus enabling high efficiency and safe exploitation.

Extending the protection range in protective seam mining under the influence of gas drainage