# The Deformation Law of a Soft-Rock Cavern by Step Excavation in a Pumped Storage Power Station
## 3 Results
### The Influence of Excavation Footage on the Stability of the Surrounding Rock  
According to the study presented in Section 3.2, the support measures proposed to improve the stability of the surrounding rock of the underground cavern can limit the deformation of the soft rock effectively and reduce the maximum displacement of each section and the whole underground cavern efficiently. Therefore, using the finite difference method to simulate the step-by-step excavation of underground caverns and the displacement trend of the surrounding rock after one year under different single excavation footage with the support condition is useful. Combining the simulation results and considering the influence of single excavation footage change on the displacement of underground caverns, even more single excavation footage was obtained.  
The simulation adopted five types of single excavation footage (10, 15, 20, 25, and 30 m). The maximum displacement value and variables under each single excavation footage schemes are shown in Table 9. The total displacement cloud diagram is shown in Figure 25.  
Figure 24: Displacement cloud map of different sections (y = \(-\)156 m, \(-\)76 m, \(-\)30 m) under different support conditions: (**a**) displacement cloud map of y = \(-\)156 m section under unsupported conditions; (**b**) displacement cloud map of y = \(-\)156 m section under supported conditions; (**c**) displacement cloud map of y = \(-\)76 m section under unsupported conditions; (**d**) displacement cloud map of y = \(-\)76 m section under supported conditions; (**e**) displacement cloud map of y = \(-\)30 m section under unsupported conditions; (**f**) displacement cloud map of y = \(-\)30 m section under unsupported conditions.  
According to the displacement cloud map and maximum displacement of the underground cavern under different single excavation footage schemes shown in Figure 25 and Table 9, it can be concluded that the deformation of the surrounding rock of the underground cavern increased with the increase in single excavation footage. A comparison of the maximum displacement is shown in Figure 26.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Length of Single Excavation Footage (m)** & **Maximum Displacement (m)** & **Variables of Maximum Displacement (m)** \\ \hline
10 & 0.18774 & / \\
15 & 0.20121 & 0.01347 \\
20 & 0.21074 & 0.00953 \\
25 & 0.21252 & 0.00178 \\
30 & 0.21424 & 0.00172 \\ \hline \hline \end{tabular}
\end{table}
Table 9: The maximum displacement and variables under different single excavation footage schemes.  
Figure 25: Displacement cloud map of different single excavation footage schemes: (**a**) displacement cloud map of the length of 10 m excavation footage; (**b**) displacement cloud map of the length of 15 m excavation footage; (**c**) displacement cloud map of the length of 20 m excavation footage; (**d**) displacement cloud map of the length of 25 m excavation footage; (**e**) displacement cloud map of the length of 30 m excavation footage.  
Based on Figure 23, it can be observed that, although the maximum displacement of the cavern continuously increases with the increase in single excavation footage, the growth rate decreases. When the excavation footage is longer than or equal to 20 m, the maximum displacement growth rate decreases at a relatively stable level, which can ensure the stable changes in the trend of surrounding rock displacement. For this project, considering both the construction duration and the stability of the surrounding rock of the underground cavern, it was proposed to adopt a length of 20 m as the optimal excavation footage.