Coal mining is gradually moving towards a deep area of more than 1000 m. At this depth, coal seams are under extremely high stress, leading to the formation of macroscopic fractures. The effects of cleats and macroscopic fractures on permeability evolution and comparative analysis based on established permeability models between intact and fractured coal are still limited. In this study, the permeability of intact and fractured coal specimens was tested by a multi-functional test system. The permeability data were fitted and analyzed based on the established permeability models. Our results show that the permeability curve of fractured coal has an L-shaped trend different from the S-shaped trend of intact coal permeability. The macroscopic fractures increased the permeability of coal samples by 1–3 orders of magnitude. The exponential model had a better fit for the permeability of intact and fractured coal specimens than the cubic model. The mean fitting degrees based on Chen’s and Yu’s models were 0.973 and 0.948, respectively. The slip of the fracture surface resulted in permeability fluctuations during the progressive deformation of fractured coal. The macroscopic fractures increased the slope of permeability in the post-peak stage and reduced coal compressibility and strength.

During deep underground coal mining, water-injection-related engineering methods are generally carried out to reduce the hazards of coal dynamic disasters. The energy evolution characteristics of coal can better describe the deformation and failure processes, as it is more consistent with the in situ behavior of underground mining-induced coal. In this study, experimental efforts have been paid to the energy evolution characteristics of water-saturated and dry anisotropic coal under true triaxial stresses. The effects of water saturation, intermediate stress, and anisotropic weak planes of coal on the true triaxial energy evolution were systematically evaluated. The results show that the overall energy is weakened due to the water adsorption for water-saturated coal samples. The water-weakening effect on the overall energy of water-saturated coal is more pronounced when perpendicular to the bedding plane direction than in the other two cleat directions. The accumulation elastic energy anisotropy index of dry and water-saturated coal samples is higher than 100.00%. Both accumulation and residual elastic energy of dry and water-saturated coal samples show an increasing-then-decreasing trend with intermediate stress increase. The results obtained in this study help understand the in situ behavior of coal during deep underground mining and control coal dynamic disasters.

The underground coal mining process is closely associated with frequent energy storage and consumption of coal mass with natural and induced fractures. Exploring the energy evolution characteristics of intact and fractured coal samples could be helpful for dynamic disaster control. In this study, laboratory true triaxial tests on the energy evolution characteristics of intact and fractured coal samples have been carried out and systematically discussed. The results show that the brittleness and peak strength are weakened due to the presence of macro-fractures in coal. The mean peak strength and brittleness for fractured coal are 29.00% and 74.59% lower than the intact coal samples. For both intact and fractured coal, the energy evolution curves are closely related to the deformation stages under true triaxial stresses. When subjected to the same intermediate stress, intact coal stores more elastic strain energy compared to fractured coal. Additionally, the rate of dissipative energy variation is two–three times lower in fractured coal samples compared to intact coal samples.