Experimental study on seepage properties, AE characteristics and energy dissipation of coal under tiered cyclic loading

In order to study the influence of cyclic mining stress on gas-containing coal seams during mining, the seepage properties, acoustic emission (AE) characteristics and energy dissipation of coal under tiered cyclic loading were experimentally investigated. Permeability recovery rate, damping ratio, AE energy rate and ring count rate were defined to describe this process. A new equation was established for the damage variable of coal with dissipation energy. These results indicate that, with tiered cyclic loading and unloading, the following occurred. 1) The absolute recovery of permeability first decreased gradually, and then increased while the relative recovery rate of permeability gradually increased. 2) AE ring count rate and AE energy rate all showed a volatility change in the initial stage, with significant increases in the yielding stage, and rapid increases in the failure stage. When the stress level was low, the AE signal disappeared after unloading to the last peak stress. When the stress level was high, the AE signal appeared in the whole unloading process. 3) The cumulative dissipation energy increased with an exponential function, and the damping ratio of coal first decreased and then increased. The damage variable also increased and reached its critical damage variable value of Dc ≈ 0.9 at coal failure