## 3 Results  
### Stress-Strain Relationships in Limestone  
In order to investigate the deformation behavior of limestone under the action of high water pressure, the strain and stress (external water pressure) data measured on the rock samples during the test are plotted in Figure 3. Positive strains in Figure 3 represented the expansion deformation of the rock samples and the negative strains represented the compression deformation of the rock samples. The hydraulic loading patterns of the S1 and S2 samples was linear, and their stress-strain curves were approximately straight lines, indicating that the rocks were elastically deformed but did not undergo significant plastic deformation. The unsaturated S1 sample showed a gradual increase in strain during the increase in water pressure from 0 MPa to 1.4 MPa and was always in a state of compressive deformation. The strain of the saturated S2 sample decreased and then increased during the reduction in water pressure from 1.4 MPa to 0 MPa, and the deformation state changed from compression to expansion. The hydraulic loading mode of samples S3 and S4 was cyclic loading, and their stress-strain curves showed apparent hysteresis characteristics. A rise in the water pressure increased the deformation of the rock, while a fall in the water pressure  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test No.** & **Sample** & **Initial Water Pressure** & **Hydraulic Loading Mode** \\ \hline
1 & S1 & 0 MPa & Linear pressurization \\
2 & S2 & 0.5 MPa & Linear pressure drop \\
3 & S3 & 0 MPa & Cyclic loading \\
4 & S4 & 0.5 MPa & Cyclic loading \\ \hline \hline \end{tabular}
\end{table}
Table 2: Classification of the test conditions.  
decreased the deformation of the rock, and the rock was always deformed elastically. The rock was kept in a state of compressive deformation under the pressure of circulating water from 0.6 MPa to 1.4 MPa. At the end of cyclic loading, the water pressure was gradually reduced to 0 MPa, and the rock deformation state changed from compression to expansion. The saturated S4 samples ended up with less expansion deformation compared to the unsaturated S3 samples. As the number of loading cycles increased, there was a tendency for the rock strain to become progressively smaller under the same water pressure. The saturated S4 sample was more resistant to this trend than the unsaturated S3 sample. As shown in the curves S3 and S4 in Figure 4, under cyclic load, when the water pressure rose, the absolute strain value of the rock gradually increased, while when water pressure dropped, the absolute strain value gradually decreased with strong linear characteristics. Therefore, it can be considered that the rock was still in an elastic state under this water pressure. When the water pressure was loaded to the maximum value of 1.4 MPa, the test began to carry out cyclic loading at '0.6 MPa-1.4 MPa'. As can be seen from Figure 4c,d, although the stress-strain curve gradually moved to the right during cyclic loading, the strain was still negative in numerical terms, and the rock still suffered compressive contraction and deformation. When the water pressure was completely discharged, the stress-strain curves of the two specimens became positive, which indicated that the rock had expanded and deformed at this time. Compared with the data shown by the S3 and S4 rocks, the deformation of the S3 specimen was significantly larger than that of the S4 specimen when the water pressure was completely unloaded, and the S4 specimen was in the initial saturated state. Therefore, it can be concluded that 'compared with the unsaturated specimen, the expansion deformation of the saturated specimen under the cyclic load of water pressure is smaller'.  
Figure 3: Stress-strain curves for the limestone samples. (**a**) Specim 1. (**b**) Specim 2. (**c**) Specim 3. (**d**) Specim 4.