Permeability and Energy Evolution of Coal under Unloading Confining Pressure

To understand the permeability and energy evolution of coals under the mining-induced stress, constant and unloading confining pressure tests are conducted. The permeability and energy evolution of coals under unloading confining pressure for simulating the excavation process of coal seam are compared with a constant confining pressure condition. The results show that the peak strength and elastic modulus under unloading confining pressure are lower than that under constant confining pressure. Besides, the unloading confining pressure results in a lower normalized permeability decrement and a larger normalized permeability increment than constant confining pressure condition. Moreover, permeability is well related to elastic and dissipative energy density. The energy evolution could well characterize the damage and permeability evolution. The damage is initiated earlier under constant confining pressure tests than under unloading confining pressure.