## 3 Wave Velocity and Mass Variation of Unloaded Samples under Dry-Wet Cycle  
### P-Wave Velocity  
We use NM-4B non-metallic ultrasonic testing analyzer and ultrasonic pulse technology to calculate the P-wave velocity of rocks according to the ratio of the length of the  
Figure 5: Unloading stress–strain curve at 80% unloading amount.  
sample to the round-trip time of the ultrasonic wave through the sample. The velocity of P-wave can reflect the existence state of internal structure and crack to a certain extent. If there are micro-cracks or weak planes in rock, the propagation speed of the ultrasonic wave will be greatly reduced, and the reduction degree of sound speed is closely related to the number, compactness, and continuity degree of cracks. Figure 6 shows the variation curve of the P-wave velocity with cycles under the dry state of unloaded samples with different initial confining pressures at 80% unloading amount.  
It can be seen from Figure 6 that the variation trend of wave velocity of unloaded samples with dry-wet cycle in the selected unloading amount and three confining pressure ranges shows three stages of rapid decrease-slow decrease-stability. In stage I (the first eight cycles), the wave velocity decreases non-linearly, the sample with initial confining pressure of 9 MPa has the largest wave velocity variation, and the cumulative reduction percentage is 60.94%. The wave velocity variation range of the sample with initial confining pressure of 12 MPa and 15 MPa is similar; the overall trend is that the larger dry-wet cycles, the smaller the reduction percentage. In stage II (8-15 cycles), the wave velocity changes more slowly than that of stage I, and the cumulative average reduction percentage is 31.36%. In stage II, the maximum reduction percentage was 12.14% for the sample with initial confining pressure of 9 MPa, and the minimum was 9.97% for the sample with initial confining pressure of 15 MPa. In stage III (15-20 cycles), the wave velocity of each sample is basically maintained at a steady state.  
It can be seen that the damage of unloaded samples caused by the dry-wet cycle is a cumulative process. On the one hand, the damage crack caused by early unloading provides more crack channels for the dry-wet cycle, thus water easily infiltrates along the crack channels, which increases the pore water pressure, inter-cambium expansion, and inter-particle expansion and increases the crack. At the same time, water molecules slowly infiltrate into the sample along the cracks, causing the dissolution of soluble minerals and cements and causing the closed pores formed by the presence of cements to gradually transform into open pores. In stage I, the high content and solubility of soluble cement leads to continuous expansion and penetration of microcracks, and the number and volume of cracks increases rapidly. As the dry-wet cycle continues, various dissolved substances of the sample are transported and precipitated. In stage III, the content of soluble substances in the sample decreases, which leads to the weakening of the crack enlargement effect caused by the dissolution of cements, thus showing that the wave velocity changes tend to be stable.  
Figure 6: Variation curves of mean wave velocity of unloaded samples with cycles.