Study on the law of failure and instability of overburden rock under multiple mining movements of extra-thick coal seam and the transformation mechanism of bearing structure

In order to further explore the motion instability and structural evolution of overburden rock in strata fully mechanized caving mining of super thick coal seam, taking the mining of super thick coal seam in Zhundong mining area of Xinjiang as the research background, the research method combining physical simulation, numerical simulation, theoretical analysis and engineering case verification is adopted. This paper studies the characteristics of overburden structure shape change, rock size distribution and force chain network distribution in the process of stratified mining, analyzes the influence of multiple mining actions on the bearing structure stability of key layers, and reveals the bearing structure transformation mechanism of overburden rock in stratified mining of huge thick coal seams. The results show that with the increase of slicing mining times, the near-field overlying rock gradually transforms from block to bulk, and the overlying rock presents a structural transformation process of "masonry beam-short masonry beam-extrusion balance arch". In the initial stage of stratified mining, the overlying rock is bounded by high and low key layers to form a multi-zone distribution feature of rock block and force chain network. Multiple mining disturbances make the low bearing area degenerate into caving area. In the lower stratified mining, the overlying rock distribution evolves from three zones to two zones. The load bearing characteristics of the force chain can be well characterized by defining the average contact force intensity ƒ ₀ and the anisotropy coefficient a _n . The strength of the force chain in the low bearing area gradually decreases with the increase of slicing mining times, while the strength of the force chain in the high bearing area in the lower slicing mining suddenly increases, which proves that the bearing capacity of the arch structure is greater than that of the beam structure. Under the influence of multiple mining movements, the length of rock blocks decreases, the load transfer of overlying rock increases, and the internal damage of rock mass worsens, all of which lead to the instability of the bearing structure of the key layer. The loss of bearing capacity of the lower key layer leads to the upward movement of the bearing structure of the overlying rock, and the bearing form changes from "beam type" to "arch type". The field measurement results of support pressure verify the accuracy of the above research, and the research results can provide theoretical basis and scientific guidance for the safety mining of thick coal seam.