# The Deformation Law of a Soft-Rock Cavern by Step Excavation in a Pumped Storage Power Station
## 4 Conclusions  
1. Through a petrographic analysis, uniaxial compressive strength tests, and shear tests, the basic engineering characteristics of the study stratum were identified. Subsequently, the stress-strain curves of mudstone in natural and saturated states under different surrounding rocks at 0.5, 3, and 6 MPa were obtained through triaxial creep tests, and the relevant calculation parameters were fitted according to the constitutive equation.  
2. By analyzing the calculation results for the cavern excavation under the conditions of support and no support, it can be observed that the maximum deformation area of the cavern surrounding rock was located in the exposed area of the soft-rock stratum on the left side of the cavern. This was because, on the left side of the cavern, the stratum was inclined, while on the right side of the cavern, the stratum was a reverse inclined stratum, which meant that the main control factor of the cavern's surrounding rock was the distribution of the soft rock in the composite stratum. Then, we observed that, under the condition of no support, the displacement of the left wall was greater than that of the right wall, while under the condition of support, in the position across the center of the underground cavern, the displacement of the right wall was greater than that of the left wall, that is, the support measures lead to the offset of the maximum displacement area.  
3. Through the research conducted on five different excavation distances of 10, 15, 20, 25, and 30 m, we found that as the excavation advance increased, the deformation of the surrounding rock also gradually increased; however, the growth rate of the surrounding rock deformation decreased. This can be clearly divided into the rapid and slow increase areas of the surrounding rock's deformation. The shorter the excavation footage, the better the stability of the surrounding rock. However, beyond a certain range, the increase in excavation footage would have less of an impact on the stability of the surrounding rock.  
4. Based on the single excavation advance of 20 m, the support was conducted with linings of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 m. By comparing the maximum intermediate principal stress and the variation in the intermediate principal stress under different lining thicknesses, it was found that the increase in the lining thickness reduced the lining stress, and with the increase in thickness, the stress reduction could be clearly divided into two parts: the effective stress reduction area and the low-efficiency stress reduction area. Therefore, for the lining, the increase in the lining thickness could effectively reduce the pressure on the lining at the initial stage of the thickness increase. After a certain thickness level was exceeded, the change in the lining thickness had little effect on the pressure on the lining. Considering the economic benefits comprehensively, a lining thickness of 1.5 m was chosen as the optimal lining thickness value.  
Investigation resources and validation, N.S., S.L., D.H., M.H., Z.L. and F.Z.; writing, N.S., X.L., M.D. and Z.L.; software, X.L., S.L.; Z.L. and Y.Z.; creep test, Z.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Power Construction Corporation of China, Scientific and Technology project (No. DJ-ZDXM-2015-22).  
The data for this study was designed for a nationally important hydroelectric facility, so we are sorry that we cannot provide public data.  
**Acknowledgments:** We express our sincere thanks to Zhongnan Engineering Corporation Limited for their strong support of our research. And we would like to express our sincere gratitude to the Power Construction Corporation of China for its support of Scientific and Technology Funding (No. DJ-ZDXM-2015-22) in this study. In addition, we are especially grateful to [PERSON] and [PERSON] for their helpful work and discussions on the topics related to this paper.  
**Conflicts of Interest:** The authors declare no conflict of interest.