Mechanical Properties and Damage Characteristics of Coal Samples under Water Immersion Pressure

Abstract

Abstract Uniaxial compression tests were conducted on four groups of coal samples under dry condition and 0, 3, and 5 MPa water immersion pressure using acoustic emission, scanning electron microscopy, and a digital scattered spot deformation monitoring system to investigate the mechanical properties and damage characteristics of coal samples under the action of water pressure. The results showed that as the water immersion pressure increased, the water–rock interaction intensified and aggravated the internal damage of the samples. The uniaxial compressive strength, elastic modulus, and peak strain decreased by 71.4%, 43.3%, and 34.74%, respectively. The deformation localized zone of the samples first appeared at the original crack in the deformation and failure process. As the water immersion pressure increased, the coal sample deformation localization zone appeared earlier, the displacement dislocation momentum of the deformation localization zone increased, and the deformation failure gradually increased, furthermore, the samples transitioned from brittle to plastic failure, and their failure mode changed from tensile to tensile–shear hybrid failure. The acoustic emission activity of the samples corresponds to their failure processes. The initial compaction stage of the samples was prolonged as the water immersion pressure increased. In the stage before the peak point, the plastic damage to the samples caused a gradual increase in the intensity and frequency of the acoustic emission signal, and multiple sudden increase points appeared. After the peak point, the active degree of the acoustic emission decreased, and the signal changed from “single tremors” to “swarm tremors” type fluctuation. The water immersion pressure promoted the water–rock interaction in the samples, the water flow continuously eroded the samples, and many water marks appeared on the fracture surface. The average porosity of the fracture surface increased, and the defect size of the coal samples increased.