Disaster-inducing mechanism of the movement of high-position combination cantilever plate structures in non-uniform and extra-thick coal seam mining

The mining of non-uniform and extra-thick coal seams significantly affects the roof breaking structure in the mining field, which greatly impacts the mining pressure behavior in the working face. To reveal the mechanism behind the occurrence of large-energy events in the roof during the mining of extra-thick coal seams at a deep mine in Gansu Province, numerical simulation, physical simulation, borehole detection, and internal rock movement monitoring were used to investigate the roof breaking structure and disaster-inducing mechanism in the mining of non-uniform and extra-thick coal seams. The results showed that the fracture development height in the 40 m cumulative mining thickness area was much greater than that in the 20 m mining thickness area, leading to the formation of high-position combination cantilever plate structure above the 20 m mining thickness area. It was determined that the breaking movement of the structure was the main cause of roof disasters, and this was validated by the simulation results. Based on the flushing fluid leakage and drill falling during the drilling process of four ground boreholes, it was found that the fracture development height of the first mined 4 m thick oil shale liberated seam was only 75 m, located at the bottom boundary of subordinate key stratum 2. The fracture development heights in the 20 m and 40 m mining thickness areas of the second coal seam were 289 m and 504 m, respectively, approximately located at the bottom boundary of sub-key stratum 4 and the main key stratum, revealing the differences in roof fracture development in different mining thickness areas and further confirming the objective existence of the high-position combination cantilever plate structure. By correlating the changes in the internal rock movement optical fiber measurements with large-energy events in the ZY1 ground borehole, the disaster-inducing mechanism, whereby the movement of high-position combination cantilever plate structures triggered strong mining pressure manifestation, was clarified. The research results provide reference for safe and efficient production in working faces under similar conditions of non-uniform coal seam distribution or stratified mining of extra-thick coal seam.