Mechanical Properties of Pressure Water-Immersed Coal Under Different Cyclic Stress Paths
摘要
Pressurized waterlogged environments and cyclic stress perturbations are causative factors in the instability failure of coal pillar dams in underground coal mine reservoirs. In this study, we characterized the mechanical properties of pressure water-immersed coal samples by performing constant lower limit, variable lower limit, and equal amplitude cyclic loading and unloading tests using a self-developed coal pressure water immersion test device. The results of these tests quantitatively revealed the macroscopic and microscopic mechanisms of the deterioration of the mechanical properties of the coal samples. The results showed that the pressure water–coal interaction exacerbates the internal damage of coal samples, resulting in decreased sample strength; as the number of cycles was increased, the samples subjected to variable lower limit cyclic loading and unloading showed the lowest strength, formed multiple localized deformation zones under stress, generated the most active acoustic emission signals, and experienced hybrid tensile–shear failure, with a large area of penetrating cracks appearing on the surface of the fracture surface. Under constant lower limit cyclic conditions, the coal samples exhibited moderate fragmentation. In contrast, samples subjected to equal amplitude cyclic primarily displayed tensile failure characteristics, accompanied by relatively stable acoustic emission signals and minimal damage accumulation. Consequently, these samples demonstrated comparatively higher strength retention. The load/unload response ratio can reflect damage to coal samples under different cyclic stress paths: YB > YA > YC. These results provide an important theoretical basis for the early warning, prevention, and control of instability hazards for coal pillar dams in underground reservoirs.