## 5 Conclusions  
The mechanical properties and deformation behavior of limestone under different initial saturation and hydraulic loading modes were studied using self-developed hydraulic loading test equipment. The effects of high hydraulic pressure on the stress-strain curves, strain rate, and deformation modulus of saturated and unsaturated rocks were systematically analyzed, and the effects of the combined action of external water pressure and pore water pressure on the internal pore structure of rocks were revealed. The main conclusions of this study are as follows:  
1. Unsaturated limestone always underwent compressive deformation during the linear increase in external water pressure, while saturated limestone changed its deformation state from compression to expansion during the linear decrease in external water pressure. The stress-strain curve of limestone showed apparent hysteresis characteristics in the process of increasing and decreasing external hydraulic pressure cycles. With the increase in the number of hydraulic loading cycles, the strain of limestone had a tendency to become smaller under the same water pressure, and the ability of the saturated rock to resist this tendency was stronger than that of the unsaturated rock. 2. Overall, the rock strain rate decreased with the increase in external water pressure, and the two values showed an apparent negative correlation, indicating that the hydraulic pressure had an inhibitory effect on the deformation rate of the rock. In the process of hydraulic loading, the strain rate of the saturated rock had a smaller range of variation. 3. Overall, the deformation modulus of the rocks gradually increased with the increase in external water pressure, and the two values showed a certain positive correlation, indicating that hydraulic pressure can enhance the ability of rocks to resist deformation. In the process of hydraulic loading, the transient deformation modulus of the saturated rock had a smaller range of change. The cyclic loading of water pressure weakened the deformation modulus of limestone to a certain extent. 4. When limestone was subjected to cyclic external water pressure and pore water pressure at the same time, there was a strong water-rock coupling with water. The cyclic change of the pore water pressure produced stress concentration and stress fatigue at the tip of the fissure, which promoted the generation and expansion of the fissure and aggravated the deterioration of the internal structural system of limestone. And, the external water pressure effect made the pore closed and the pore squeeze, which hindered the deterioration of the pore structure by water.  
The periodic changes in water pressure has significant influence on the pore pressure, stress distribution, and rock fracture mechanism. With the repeated change in water pressure, the rock mass will experience the process of elastic deformation, plastic deformation, and even fracture. Under this background, the deformation law of the valley deformation plane may show more complex nonlinear characteristics. Further study of this deformation rule can provide more accurate prediction models for hydraulic engineering, underground energy storage, and groundwater resource development.  
Conceptualization, D.T. and C.H.; methodology, D.T.; software, D.T.; validation, C.J., C.H. and Y.L.; formal analysis, D.T., Y.L. and H.Z.; investigation, D.T. and Y.Z.; resources, D.T.; data curation, D.T.; writing--original draft preparation, D.T.; writing--review and editing, D.T. and H.C.; visualization, D.T. and C.H.; supervision, Y.Z. and C.J.; project administration, C.J.; funding acquisition, H.C. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Research Project of China Yangtze Power Co., Ltd., with the contract number Z412302020.