Mechanical Properties and Acoustic Emission Characteristics of Water-Bearing Coal Specimens under a Coupled Compression-Shear Load

The construction of an underground coal mine reservoir plays an essential role in the ecological environment of arid areas. The moisture content and loading angle inclination significantly impact the strength and stability of the safety coal pillar of underground reservoirs. Water-bearing coal was investigated under a coupled compression-shear load using inclined uniaxial compression tests on coal samples with varying water contents (omega = 0%, 2.42%, 5.53%, 7.55%, and 10.08%) and acoustic emission (AE) technology. The weakening mechanism of the mechanical property parameters and the crack evolution law, combined with the characteristics of the stress-strain curve, the cumulative AE count, and the cumulative AE energy methods, were used. Therefore, the evolution law of the coal's crack closure (CC) threshold, crack initiation (CI) threshold, and crack damage (CD) threshold was analyzed. The results indicate that coal samples' peak stress and elastic modulus decreased when the water content or inclination angle increased. Peak shear stress decreased as the water content rose, and the overall characteristics gradually rose as the inclination angle increased. The exception was when the ratio was 10.08%; when the inclination angle was 0 degrees, the failure mode of the coal sample progressively changed from tension failure (omega = 0%, 2.42%) to tension-shear composite failure (omega = 5.53%), and finally to shear failure (omega = 7.55%, 10.08%). When the inclination angle was 5 degrees, the coal sample was dominated by tension-shear composite failure at any moisture content. When the inclination angle was 10 degrees and 15 degrees, the coal specimens showed shear failure at any moisture content. The evolution law of tensile and shear cracks determined by AE characteristic parameters was consistent with the macrofracture characteristics of the coal specimens. When the water content or inclination angle increased, the crack closure threshold, crack initiation threshold, and damage threshold of the coal samples decreased. Therefore, these results show that their peak stress ratio does not depend on the water content or inclination angle.