The ultrasonic P-wave velocity-stress relationship of rocks and its application

When a rock sample is extracted from an underground rock mass, it is subject to unloading, which will cause changes in the physical and mechanical properties. This article describes a laboratory experiment to determine the change of P-wave velocity of rock samples during a uniaxial compression test. It was found that the P-wave velocity vs. stress curves (V-S curves) of the rock samples could reflect three stages of bulk volume deformation commonly observed in a uniaxial compression test of rocks. When the applied stress was less than sigma (c) (about 0.25-0.33 of the uniaxial compressive strength), the P-wave velocities increased rapidly with the increase of stress; this part of the V-S curves could be fitted with a power function. When the stress was greater than sigma (c), the P-wave velocities of rock samples increased more slowly and gradually approached the peak before decreasing dramatically near failure; the V-S curves above sigma (c) could be fitted with a polynomial function of the second degree. During the V-S experiment, it could be also observed that the increasing rate of P-wave velocity decreased dramatically when the applied stress reached the overburden stress. An unloading index was defined as the ratio of the P-wave velocity under in situ overburden stress to the P-wave velocity at free stress and could be calculated from the measured V-S curves. Based on the calculated unloading index, the calculation of the intactness index of rock mass could be modified, and then an improvement of the basic quality (BQ) classification method of rock masses, which is used widely in China, was made.