Study on failure depth of floor under impact load disturbance in overlying coal seam mining of middle group coal

In order to study the development of floor failure under the disturbance of impact load in the middle and upper coal seam mining of close distance coal seam, theoretical calculation, numerical simulation and field observation are used to observe the range of floor failure under the disturbance of close distance coal seam mining. Based on the semi-infinite plane theory combined with Mohr-Coulomb failure criterion and yield condition, the stress state of the floor at any point of the working face under the dynamic load and static load of roof collapse is analyzed, and the depth of floor failure is calculated to be 17.32 m. The numerical simulation of close distance coal seam mining is carried out. After advancing 25 m in the strike direction, the development of in-situ plastic zone is basically stable. The plastic zone of the floor develops in the process of gradual mining, which is characterized by “vertical and horizontal diffusion” under the coal wall. The maximum failure depth of the floor is 16.12 m. The borehole TV method is used to observe the floor failure before and after the mining of the working face. The failure depth of the floor below the working face is near 16.85 m. It is concluded that the failure caused by the mining disturbance of 6₁ coal seam in the middle group coal seam causes damage to the stability of the roof of 6₂ coal seam and 6₃ coal seam in the lower coal seam, which verifies the reliability of the theoretical model of impact load. The roof support should be strengthened in the mining of 6₂ coal seam and 6₃ coal seam. The research results provide a reference for the influence range of floor failure in the lower coal seam under the disturbance of close distance coal seam mining.