## 4 Design and Stability Analysis of Surrounding Rock-Supporting Structure of Shaft
### Support Plan Design Optimization  
Shaft excavation alters the original stress equilibrium regime of plutons, and the rock side pressure gradually decreases during excavation unloading. As the excavation surface continues to advance, the rock surface pressure decreases to zero by losing the excavation surface restraint effect. The rock stress is simultaneously realigned to depth to produce a stress concentration, and the maximum release peak can reach 30 MPa in rock stress release owing to the high ground stress nature associated with deep pluton formation. The plastic failure zone sharply expands upon the stress release. This rock stress state may not be improved by timely reinforcement of the rock without supporting measures. The rock failure extension trend, thus, cannot be controlled, which produces large deformation failure. However, in the course of shaft support, the application of the high-strength and high-stiffness support structure to the rock after excavation is a support measure to effectively restore the stress balance state of plutons and slow the extent of rock fracture extension to enhance its self-bearing capacity. Although this passive type of support can ensure rock stability, it requires inaccessible requirements for the load-bearing capacity of the adopted support structure, Thus, in the case of \"three highs and one disturbance\", the support structure design should focus on the concept of high resistance and release pressure by means of reinforcement to alter the rock stress state.  
It is important to effectively control the discontinuous deformation of rock and enhance rock monoliths using high-strength anchors for the release pressure support. A high-strength anchor is used to improve the extension rate of continuous rock deformation, and part of the rock stress is released to reduce the rock pressure to the support structure. In practical field situations, a high-strength anchor is used as the initial support to provide sufficient stress release space for the rock. When the surrounding rock stress reaches the appropriate release level, a well ring and shotcrete are used for high-strength and high-stiffness secondary support to meet the surrounding rock-stability requirements.  
The two original care programs were optimized and improved. Building on these results, two options were proposed for optimizing the care design. The optimized supporting structure settings are shown in Table 8.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Initial Support**} & \multicolumn{3}{c}{**Secondary Support**} \\ \hline  & \multicolumn{3}{c}{Resin Anchor Rod (End Anchor)} & \multicolumn{3}{c}{Well Circle} & Concrete \\ \hline  & Diameter \(\Phi\) & Row Spacing d\({}_{2}\times\) d\({}_{3}\) & Length L & Shed Distance d\({}_{1}\) & Thickness t \\  & (mm) & (m) & (m) & (m) & (mm) \\ Original plan & 22.0 & 1.0 \(\times\) 1.2 & 2.5 & 1.5 & 600 (C30) \\ one & 22.0 & 1.0 \(\times\) 1.2 & 2.5 & 1.2 & 700 (C30) \\  & & & & & \\  & & & & & \\  & Diameter \(\Phi\) & Row spacing d\({}_{2}\times\) d\({}_{3}\) & Length & Length of anchoring section & Shed distance d\({}_{1}\) & Thickness t \\  & (mm) & (m) & L\({}_{1}\) (m) & L\({}_{2}\) (m) & (m) \\ Optimization & 22.0 & 1.0 \(\times\) 1.2 & 8.0 & 3 & 1.5 & 600 (C30) \\ plan one & 22.0 & 1.0 \(\times\) 1.0 & 8.0 & 3 & 1.2 & 700 (C35) \\ Optimization & 22.0 & 1.0 \(\times\) 1.0 & 8.0 & 3 & 1.2 & 700 (C35) \\ plan two & & & & & \\  & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 8: Optimized support design scheme.