## 2 A Brief Introduction of the Model Test
In the three model test samples, the number of strain sensors in the intact rock area was more than that of the stress sensors, and the range of the strain sensors was also larger. Therefore, the strain time history curves recorded at the strain measuring points at different distances from the explosion source were used to calculate the stress wave velocity through the arrival time difference of the waves. In each model test sample under different biaxial static loads, based on the time difference \(\Delta t\) corresponding to the jumping point in the time history curves of strain measuring points 13 and 16 in the intact rock, and the distance between the two measuring points \(\Delta l\), the value of the stress wave velocity \(c\) of the intact rock can be calculated by the following equation:  
\[c=\frac{\Delta l}{\Delta t} \tag{2}\]  
Through the above equation, the average propagation velocities of stress waves in the intact rock section of each model test sample under different biaxial static loads were calculated, which were 1990 m/s, 2077 m/s, 2099 m/s, and 1898 m/s when the static loads were 0, 0.75 MPa, 1.5 MPa, and 3 MPa. The reason for this phenomenon was that when the static load was small, the initial microcracks in the intact rock started to close with the growth of the static load, resulting in the increase in the wave velocity with the elastic modulus. When the static load was raised to a critical value, the initial microcracks in the intact rock were completely closed, and when the static load continued to rise, new microcracks were initiated, resulting in the decrease in the wave velocity, and this critical value of the static load can be obtained by a subsequent analysis. Meanwhile, it can be seen that when the static load was 0 MPa, the average stress wave velocity in the intact rock was about 1990 m/s. Meanwhile, the ultrasonic wave velocity of the intact rock, similar to the material measured in the model test, was 1980 m/s. The results showed that when the amplitude of the stress wave was not large enough, its propagation speed in the rocks was about the same as that of an elastic wave, which is consistent with the conclusion that when the stress wave amplitude is small under the combined action of dynamic and static, the initial damage to the rock is mainly caused by the application of the static load [40]. To derive the variation law of stress wave velocities in the rocks under biaxial static loading, the average stress wave velocities in the intact rock under different static loads were plotted, as seen in Figure 4. It should be noted that for the subsequent initial damage analysis, the stress wave velocity in the intact rock was assumed to drop to zero when the biaxial static load reached the biaxial compressive strength of 7.826 MPa. From the above Figure 4, it can be seen that the stress wave velocity in the intact rock increased and then decreased with the increase in the static load, which was also similar to the variation law of the physical attenuation of stress waves in the intact rock with the biaxial static load derived from the model test results, reflecting the stress sensitivity of the evolution of the microcracks in rocks. Numerous studies have shown that there is a close connection between the wave velocity and the intrinsic damage of a medium, so the damage evolution of the propagation medium can be described by the change of wave velocity. Combining the fitted static load versus the stress wave velocity curve in Figure 4, the maximum wave velocity of the intact rock was 2109 m/s, and the corresponding biaxial static load was 1.23 MPa, which was about 15.7% of the biaxial compressive strength, indicating that the microcracks inside the rocks were in the fully compacted stage at this static load level. Meanwhile, the new microcracks had not yet started to initiate.