Abstract: In order to study the effect of moisture content on ultrasonic wave velocity and failure characteristics of soft coal, the mechanical testing & simulation (MTS) rock mechanics experimental machine, ultrasonic detector, and digital image correlation (DIC) processing technology were employed to conduct monitoring experiments on the wave velocity and surface morphology of briquette samples with different moisture contents under uniaxial compression. The evolution characteristics of wave velocity and deformation were analyzed. The computed tomography (CT) scanning test system was used to observe the meso-structure of failed coal, and the three-dimensional fissure distribution and characteristic parameters were obtained. The change laws of initial wave velocity and wave velocity of failed coal with moisture content were explored. The results show that: (1) With the increase of axial strain, the wave velocity of coal under different moisture contents exhibits four stages: steady, slowly decreasing, sharply decreasing, and then becoming steady again. As moisture content increases, the onset of the slowly decreasing, sharply decreasing, and then becoming steady stages of wave velocity change is delayed. (2) The initial wave velocity and post-failure wave velocity of coal increase and then decrease with the increase of moisture content, and the decrease in coal wave velocity after loading is linearly related to the moisture content. (3) As moisture content increases, the surface area, volume, and three-dimensional fractal dimension of internal fissures in the damaged coal show a linear increasing trend, and the failure mode of coal transitions from shear failure to tensile failure. (4) The microstructure is the main reason for the difference in initial wave velocity of coal under different moisture contents, while the wave velocity of the damaged coal is determined by both the microstructure and the distribution of meso-fissures. The research results can provide an experimental basis for revealing the acoustic-mechanical response characteristics of water-injected coal and for preventing and controlling instability disasters.

Key words: soft coal, moisture content, wave velocity, microstructure, meso-fissure