LAUSR

Abstract Because of coal seam excavation and periodic disturbance of back-and-forth coal cutting, the surrounding rock is subjected to tiered cyclic unloading and loading. Based on this, a seepage experiment under constant axial stress and tiered cyclic unloading-loading confining pressure was carried out, and two new damage variables were proposed. These results showed that, the accumulative plastic deformations and corresponding permeability increased step-wise with unloading levels, permeability decreased with cycle number in the same unloading level when the unloading level was small, it was the opposite when the unloading level was large. The Kaiser effect and the Felicity effect actually coexisted during the experiment. As the unloading level increased, the relationship between dissipated energy and cycle number converted from logarithm to linear relationship. Two damage variables showed that the critical value of the damage variables for coal failure Dc≈0.9.