Study on the breakage and energy evolution characteristics of overlying strata based on numerical calculation

The intensity of mining-induced earthquakes generally increases as coal mines dive deeper into Earth’s interior. Consequently, this poses a greater threat to the safety of underground miners and residents, and results in limited mine productivity. Given the frequent occurrence of mining-induced earthquakes under multiple layers of thick and hard roofs in deep coal mines in China, this study uses the Dongtan coal mine as an example and uses the methods of microseismic monitoring, theoretical analysis, and numerical simulation to elaborate on the coordinated migration and fracture patterns of multiple key layers of overlying rocks during deep coal mining. It is found that during the mining activities process in the Dongtan coal mine, the advancing working face is accompanied by high-energy (E ≥ 105 J) events, which are concentrated in the high-position thick and hard rock strata. The evolution of the strain energy accumulation–release before and after key stratum overhanging roof rupture shows that during coal mining, suspended and overhanging roofs appear above the coal seam. The fracture of the key strata is delayed in nature, facilitating the accumulation of strain energy in the rock strata. The fracture heights for complete fracture of the sub-key and main key strata are 55.48 m and 250.47 m, respectively. Meanwhile, strain energy accumulated in the middle and fixed ends of the key strata owing to the bending stress. The elastic strain energy of the rock strata in front of the coalface showed a trend of increasing and then decreasing after the fracture of the key strata. Compared to the elastic strain energy of the sub-key stratum after the suspended roof rupture, it increased by 14.3% following the overhanging roof rupture. With an increase in the overburden load, the ultimate overhanging roof length of the rock strata decreased, and the fracturing frequency of the rock strata increased. We expect that this study will enhance the understanding of the mechanism of mining-induced earthquakes to provide effective early warnings of earthquakes with large energies related to the fracture of the overlying strata.