Mechanical Failure Modes and Fractal Characteristics of Coal Samples under Repeated Drying–Saturation Conditions

The ecology of arid mining areas in western China is facing serious problems because underground coal mining destroys overlying aquifers, causing groundwater to shift along fracture zones and groundwater levels to drop. Therefore, it is important to construct strategically coal mine goafs as underground water reservoirs to store and utilize mining-affected water resources. During the long-term operation of underground water reservoirs, the stability of coal pillar dams is affected by the repeated rise and fall of mine water levels. To understand the repeated effects of water, nondestructive water immersion, ultrasonic velocity, uniaxial compression, and acoustic emission (AE) tests were performed to study the mechanical properties and damage mechanisms of coal samples under repeated drying–saturation conditions. The ultrasonic velocity test mainly achieved two functions: selecting consistent coal samples and providing evidence of changes after each drying and saturation process. Coal samples were grouped according to the number of drying–saturation processes (DSPs) to investigate and compare the mechanical and AE parameters. The post-peak phases of full stress–strain curves present perpendicular, L-shaped, and stepped forms, among which the latter two show obvious residual strength. The AE count, cumulative AE count, and stress can jointly determine the various stages and stress thresholds of the sample failure process (or crack development process), except the boundary between elastic stage and stable crack growth stage. Multiple immersions in water reduces the fractal dimension of broken blocks; that is, the degree of fragmentation decreases and the quantity and mass of large-size blocks increase.