Energy Evolution of Coal at Different Depths Under Unloading Conditions

To explore the differences in deformation failure and energy evolution and simulate the real excavation conditions of coal masses at different depths during the coal-mining process, an unloading failure experiment was conducted. The experiment included coal samples at different depths from the no. 15 coal seam of the Ji group in the Pingmei coal-mining area. The sample depths were 300, 600, 700, 850, and 1050 m. The in situ stress environment, physical rock properties, and excavation disturbance influence were considered in the analysis. The results show that as the depth increases, the peak strength and residual strength of coal samples increase nonlinearly and that the deformation capacity simultaneously increases. The elastic strain energy stored before the unloading of coal, the total energy input, the accumulated elastic energy, and the dissipated energy generated during failure all increase with increasing depth, and the ratios of these results for the 1050–300 m samples were 10.79, 3.75, 3.78, and 4.67, respectively. A prepeak stage energy evolution model was established for coal samples from different depths, and the energy transformation trends of the deformation and failure of coal were explored from the perspective of the energy evolution rate. The energy dissipation efficiency ( u ed ) was defined to represent the energy dissipation generated per unit of energy input. When the energy input is mainly transformed to dissipation energy, the failure of coal occurs ( u ed > 0.5), and the energy is predominantly associated with crack growth at this point, so the coal is quickly destroyed. This conversion occurs earlier as the depth increases. The AE energy release trend exhibited a good correspondence with the increases in the energy dissipation, dissipation rate, and dissipation efficiency with depth during coal unloading failure, which indicates a more intense failure of the coal body at deeper depths.