Influence of cyclic wetting-drying on the mechanical strength characteristics of coal samples: A laboratory-scale study

Water-coal interactions have gained much recent attention, although few studies focus on the strength of coal under wetting-drying cycles. This study investigates changes in the microstructure and mechanical strength of coal that are induced by water-coal interactions, at the macroscale and microscale. Fourteen specimens after continuous wetting and fourteen specimens after wetting-drying cycles were tested, indicating that peak stress and elastic modulus decrease with increasing continuous wetting time and number of wetting-drying cycles, while peak strain increases. We analyzed changes in the coal microstructure using a scanning electron microscope (SEM) and the Image Pro Plus 6.0 (IPP 6.0) software. The regression model for cyclic wetting-drying reveals that porosity and pore circularity are the main correlation indicators associated with uniaxial compressive strength. Therefore, different numbers of wetting-drying cycles could induce different degrees of damage to coal, which increases progressively through gradual changes to its microstructure. These findings indicate that wetting-drying cycles have a more significant impact on the stability of coal mass than continuous wetting. Our results are of use in determining the size of coal pillars, and expanding the knowledge base related to the mechanical properties of coal mass, water bursting, water recharge channels, and the stress-crack-permeability evolution law in mines.