Mechanisms of failure in coal samples from underground water reservoir

Using goaf as water storage space plays an important role in the ecological environment and economic development of arid mining areas, while water content affects the rock strength and the stability of coal pillars in underground water reservoirs by changing crack propagation. In this work, variable-angle shear tests (compression-shear tests) were conducted for coal samples with moisture contents of 0%, 7.10%, 15.68%, 22.90%, and 23.09% to develop new insights into the mechanisms of crack propagation and strength weakening, combined with acoustic emission (AE) technique and infrared thermal imaging technique. The results show that the main cracks in dry coal samples exist along the shear surface, while the fracture surface in the saturated samples deviates from the shear surface and many irregular shear cracks form. The shear strength, cohesion, and internal friction angle of coal samples decrease linearly or exponentially with water content. Given this result, we discuss the Mohr-Coulomb criterion for coal samples based on water content. Increasing the water content weakens the effect of increased heating at the center of the coal samples, and especially the saturated coal samples emitted less infrared radiation than the loading device in the destructive phase. The average infrared radiation temperature (AIRT), AE energy rate and shear stress distribution are all correlated, which reflect the characteristics of the destructive process within water-containing samples. After the formation of macro-fractures, the increased contact area of fracture surfaces with the air causes heat to suddenly reduce. These results indicate the failure mechanism involving the destabilized deformation of coal samples under compression shear, which gives a useful reference for designing waterproof coal pillars and underground reservoir dams.