Law of overburden breaking and energy release in the mining of difficult coal resources in steeply inclined extra-thick coal seam group

Aiming at the problem of safe mining of difficult coal resources in steeply inclined extra-thick coal seam group,using the methods of physical simulation experiment and theoretical analysis,the characteristics of overburden change,breaking height and microseismic energy produced by different mining technologies in steeply inclined extrathick coal seam group are analyzed and compared. The theoretical calculation formula of energy accumulated by hard roof breaking in steeply inclined extra-thick coal seam is derived,and the comparative analysis results of different mining technologies provide some ideas for the scheme design of deep mining in steeply inclined extra-thick coal seam group. The results show that the central rock pillar of horizontal sublevel fully mechanized top coal caving mining in steeply inclined extra-thick coal seam group is broken intensively to form the linkage effect between the rock pillar and the upper coal seam roof,which is easy to cause dynamic disasters such as rock burst. The strong supporting effect of section coal pillar in longwall mining makes the increase of overburden breaking height and range smaller in the deep mining process of steeply inclined extra-thick coal seam group. By constructing the mechanical model of hard roof breaking in steeply inclined extra-thick coal seam,the calculation formula of hard roof breaking accumulated energy in steeply inclined extra-thick coal seam is derived. The accumulated microseismic energy of steeply inclined extra-thick coal seam group with strike longwall is 24.28×10⁵ J,which is 15.67% less than that of horizontal sublevel mining. In the process of horizontal sublevel fully mechanized top coal caving mining,the average energy of single mining is lower than that of longwall mining,and the waste of coal resources due to coal pillar left is avoided. The energy of roof breakage is not fully released,which will easily lead to the peak effect of energy release in the next breakage. Moreover,the large-scale concentrated breakage of the middle rock pillar will easily bring some challenges to the safe production of the mine. Based on the idea of saving coal resources while avoiding a large-scale concentrated breakage of overlying strata,it is suggested that on the basis of the original horizontal sublevel fully mechanized top coal caving mining and roof weakening regulation,the weakening degree of roof and middle rock pillar should be increased to avoid the mine safe production problems caused by the large-scale concentrated breakage of rock pillars. The research results provide a reference for effective utilization and safe mining of coal resources in the steep coal seam with complex conditions.