## 6 Analysis and Discussion
### Timing of Secondary Lining Support and Exacuation Reserved Deformation
For the design of 0.8 m for Grade III surrounding rock and 1.2 m for Grade IV surrounding rock, the maximum surrounding rock pressure at 0.3 mm cracking is 2.97 MPa and 3.64 MPa, respectively. For Grade III surrounding rock, when 95% and 90% of \(u_{\text{lim}}\) are used as the support timing, the stress on the secondary lining during construction is only 20.2% and 16.4% of the cracking stress, respectively; for Grade IV surrounding rock, when 95% and 90% of \(u_{\text{lim}}\) are used as the support timing, the stress on the secondary lining during construction is only 11.4% and 9.3% of the cracking stress, respectively. For both Grade III and IV surrounding rock, the stress on the secondary lining during construction is relatively small, providing a high safety margin for resisting external water pressure loads. (2) Grade V Surrounding Rock  
The GRC of Grade V surrounding rock and the SCC of the secondary lining under the drill-and-blast construction method are shown in Figure 19. The data on support timing and excavation reserved deformation for Grade V surrounding rock construction are summarized in Table 7. It can be observed that for a 200 m deep tunnel, since 95% of \(u_{\text{lim}}\) and 90% of \(u_{\text{lim}}\) are chosen as the support timing, the load borne by the secondary lining does not change significantly. Therefore, 90% of \(u_{\text{lim}}\) is preliminarily selected for Grade V surrounding rock as the support timing. For tunnels with burial depths ranging from 400 m to 800 m, the support timing is preliminarily selected based on the curve characteristics and experience, with surrounding rock deformations reaching 40 cm, 50 cm, and 60 cm, respectively. However, at burial depths of 600 m and 800 m, the safety reserve against cracking is not high (with a 1.35 m thick lining, \(P_{0.02}\) = 2.07 and \(P_{0.3}\) = 2.87), and the lining is prone to cracking or even failure under external water pressure. Grouting is an effective solution as it enhances the surrounding rock strength, improving the stability and load-bearing capacity of the tunnel structure [51; 52]. However, due to the absence of testing on the effects of grouting reinforcement, the enhancement of surrounding rock strength resulting from grouting has not been considered in the above analysis. Consequently, the support timing and excavation reserved deformation results for Grade V surrounding rock cannot be directly applied to guide construction. For tunnels at a depth of 200 m, 90% of \(u_{\text{lim}}\) can be directly adopted as the support timing. For tunnels with burial depths ranging from 400 m to 800 m, this value may be adopted in trial sections, and the design parameters should be dynamically adjusted based on the analysis of grouting reinforcement effects and monitoring data. For tunnels at greater depths of 1000 m to 2000 m, additional initial reinforcement measures are required to enhance the bearing capacity of the surrounding rock. In areas subjected to high external water pressure, further measures should also be implemented to improve the seepage resistance of the surrounding rock and reduce external water pressures [53; 54]. \begin{table}
\begin{tabular}{c c c c c} \hline
**Bupial Depth (m)** & **Support Timing (cm)** & **Excavation** & **Bearing Stress of** **Secondary Lining** **(\(u_{\text{bar}}\), MPa)** & **Support Timing Selection Strategy** \\ \hline
200 & 15.94 & 16.02 & 0.16 & 90\% \(u_{\text{lim}}\) is selected as the support timing \\ \hline
400 & 40 & 40.45 & 0.63 & Selection is based on experience; safety reserve is considered insufficient; the value can be used in trial sections, with further analysis of grouting reinforcement effects and adjustments made based on monitoring data \\ \hline
1000 & / & / & / & \\
1200 & / & / & / & Excessive support pressure is noted; the risk of lining cracking and failure is considered high; further initial reinforcement measures are required \\
2000 & / & / & / & \\ \hline \end{tabular}
\end{table}