Experimental investigation on the permeability, acoustic emission and energy dissipation of coal under tiered cyclic unloading

Because of periodic disturbance during coal seam excavation and support, the surrounding rock is subjected to cyclic unloading-reloading. To study the effect of pressure-relief on coal fields during gas drainage and mining, the permeability, acoustic emission (AE) and energy dissipation properties of coal under tiered cyclic unloading were experimentally investigated. Permeability enhancement rate (PER), AE signal change rate and a damage variable were defined to describe this process. Hysteresis, memory and Felicity effects are revealed. These results show that, the step-wise increase of strains and permeability display a hysteresis effect under tiered cyclic unloading. The axial residual strain increases exponentially, while the radial residual strain increases as a quadratic function, and the PER increases. The AE signals show Kaiser effect when the unloading capacity is small. Conversely, the Felicity effect is displayed when the unloading capacity is large. The cumulative AE signal and the axial strain exhibit good synchronisation and memory characteristics. The dissipated energy increases as a quadratic function, and the developed damage variable also increases, the critical damage variable of coal failure is estimated to be D_c ≈ 0.8.