Abstract: The acoustic characteristics of rock mass are closely related to the stress state and the degree of damage of rock mass. It is an effective engineering measure to analyze the stress state and evaluate the stability of rock mass through the change of acoustic properties of rock mass. The axial and transverse ultrasonic synchronous tests during the uniaxial loading tests of sandstone were carried out and the evolution laws of wave velocity with stress in different directions during loading were obtained. Experimental results showed that the wave velocity in axial direction increased with the increase of stress, while the wave velocity increased first and then decreased in transversal direction. Since the acoustic test results were different in different directions, the influence of fracture direction on ultrasonic velocity was verified by the acoustic test of gypsum specimens with pre-existing cracks with different inclinations. The wave velocity achieved the maximum value when the fissure direction was consistent with the direction of sound wave propagation, and the wave velocity was minimum when the fissure direction was perpendicular to the propagation direction. In addition, in order to analyze the correlation between the wave velocity and stress state, the relationship between wave velocity and volumetric strain was established. The results showed that the average velocity gradually increased with the increase of volumetric strain, and reached the maximum value at the peak of volumetric strain, and then began to decrease with the decline of volumetric strain. According to the relationship between stress and wave velocity during the loading process, an exponential fitting formula of stress and wave velocity was obtained. As a result, the wave velocity obtained by field test could be used to predict the stress range of the surrounding rock and evaluate the stability of the rock mass.

Key words: ultrasonic wave, wave velocity, uniaxial loading, crack inclination, volumetric strain, damage