Coal Damage and Permeability Characteristics Under Accelerated Unloading Confining Pressure

Based on the actual mining conditions and the stress state of coal on-site, the damage–permeability–acoustic emission (AE) experiments of gas-bearing raw coals were carried out under the conditions of conventional triaxial unloading (UCP) and accelerated unloading confining pressure (AUCP) with different gas pressures and confining pressures. The effects of unloading speeds increasing during the process of unloading confining pressure on the damage and permeability of coals were analyzed. Meanwhile, the intrinsic mechanism of the coal damage and permeability evolution in front of the working face is revealed. The results show that under the two unloading paths, the failure mode is both the tension–shear failure. The AE signals correspond well with the stress–strain and permeability evolutions of the coal samples. At the initial stage, the AE signal is relatively less, and it rises sharply when the coal approaches instability and failure, presenting the characteristics of rapid damage as a whole. Compared with UCP, AUCP has a greater impact on the damage and permeability characteristics of raw coal. The acute damage of coal is more severe and the time required for reaching instability and failure is significantly shortened under AUCP. The permeability increment under AUCP is obviously higher than that under UCP. When the sample is damaged, the unloading ratio of confining pressure is smaller under AUCP, and the sample shows more broken and more impact destructive effect. With the increase of initial confining pressure or the decrease of gas pressure, the time required for instability and failure increases, and the influence of accelerated unloading path on coal is strengthened. Therefore, the accelerated advancement of working face is more likely to induce coal gas dynamic disasters.