Study on the failure law of overlying rock under coal seam in close distance coal seam and Feasibility discrimination of upward mining

Aiming at the key problems of failure evolution law and feasibility study of overlying strata in ascending mining of close distance coal seam group, this paper takes the recovery of 4 ^− 3 and 4 ^− 4 coal seams in Hongliulin Coal Mine as the engineering background, and carries out systematic research through theoretical modeling, numerical simulation and field engineering measurement. Firstly, the stress field model of the roof rock mass of the coal mining face is established according to the elastic mechanics, and the maximum failure depth of the overlying strata on the working face is obtained by combining the Mohr-Coulomb criterion. The average maximum instability height of the 5 ^− 2 coal seam roof is 12.46 m, and the maximum value is 16 m, which is much smaller than 25 m-55 m between the 5 ^− 2 coal seam and the 4 ^− 4 coal seam. According to the study of the distribution of the caving zone, the 4 ^− 4 and 4 ^− 3 coal seams are located outside the caving zone. The minimum distance between the top of the caving zone and the 4 ^− 4 coal seam floor is 16 m, and the maximum distance is 40.9 m, which is consistent with the calculation results of the maximum failure depth of the overlying strata on the working face. According to the numerical simulation results, 4 ^− 4 and 4 ^− 3 coal are in a continuous state under the conditions of 5 ^− 2 and 4 − 2 coal mining. The maximum dip angle of the 4 ^− 4 coal seam is 3.7 °, and the maximum dip angle of the 4 ^− 3 coal seam is 3.5 °. According to the field measurement results, the height of the caving zone in 5 ^− 2 coal mining is determined to be 14.43 m, and the caving ratio is 4.53. According to the above research results, under the mining conditions of 5 ^− 2 and 4 ^− 2 coal seams in Hongliulin Mine, the maximum roof failure depth did not reach 4 ^− 4 and 4 ^− 3 coal seams, and the continuity/integrity of these two coal seams was good, which could be recovered after the stability of 4 ^− 3 and 4 ^− 4 coal seams in the later stage of mining.