Unconventional staggered distance simultaneous mining theory in extremely close and thin coal seams and its application

The exploitation of extremely close thin coal seams is featured of great interaction and it is hard to realize the simultaneous mining of upper and lower coal seams. This paper defines the extremely close coal seam and studies the unconventional staggered distance simultaneous mining in pressure-relief region formed by locating the lower coal mining face in the upper face. The mechanical model of unconventional staggered distance simultaneous mining is established and its theoretical formula is given. The numerical simulation analyzes the influence of staggered distance on the front abutment pressure distribution: when staggered distance increases, the degree of upper coal mining face front abutment stress concentration decreases and the degree of lower coal mining face front abutment stress concentration decreases first and increases later; when the staggered distance is larger than a certain value, the front abutment pressure in lower coal mining face is in “double peak” distribution. With theoretical analysis, the staggered distance of the left six simultaneous mining face in Zhengyang Mine is conformed to be 6 m. This research conclusion has been verified in the field and significant socio-economic benefits have been achieved, which can provide a reference for safe and efficient exploitation of extremely close coal seams.