Acoustic and mechanical responses of coal subjected to dry–wet cycling: a petrophysical perspective

Abstract Underground coal mine reservoirs experience repeated impoundment and drainage, exposing coal pillars to dry–wet cycling and causing cumulative deterioration. This study investigated the acoustic and mechanical responses of Wudong coal specimens subjected to 0, 1, 3, and 5 dry–wet cycles through mass measurements, P-wave velocity (Vp) tests, and uniaxial compression. The results show that dry mass, saturated water content, and Vp evolve nonlinearly with increasing cycle number, suggesting staged pore–crack rearrangement and water–coal interaction. A pronounced acoustic response occurs around the third to fourth cycle, while saturated-state Vp is more sensitive than dry-state Vp because of water filling, mineral swelling, and changes in local acoustic impedance. The uniaxial compressive strength and elastic modulus decrease by up to 49.7% and 28.4%, respectively, after three cycles. Meanwhile, the failure mode changes from shear-dominated failure to tensile–shear and tensile–compressive hybrid failure. The deterioration is mainly attributed to water adsorption, clay-mineral swelling, dissolution and migration of fine particles, and pore redistribution. These findings indicate that Vp, combined with mass and mechanical parameters, can provide a useful basis for evaluating moisture-induced damage in coal pillars of underground coal mine reservoirs.