## 6 Analysis and Discussion
### Timing of Secondary Lining Support and Exacuation Reserved Deformation  
The initial stage of the concrete SCC is not affected by the surrounding rock strength, reinforcement ratio, or lining thickness, and the initial slope is mainly determined by the material stiffness. The shape of the SCC changes mainly after concrete cracking. In the  
Figure 15: Influence curve of thickness on \(P_{0.3}\) and \(P_{\rm max}\). design stage, to ensure the stability of the surrounding rock with initial support, sufficient reserved deformation should be adopted to allow the surrounding rock stress to be fully released, followed by the application of the secondary lining. At this time, the secondary lining will almost not bear or will only bear a small amount of the surrounding rock pressure, providing a larger safety margin for bearing internal and external water pressure loads. Therefore, for Grade III, IV, and V surrounding rocks, the preliminary design thickness of the lining should be 0.8 m, 1.2 m, and 1.35 m, respectively, with a reinforcement ratio of 0.6%. Combined with the GRCs for different grades of surrounding rock in Section 5, the timing for the secondary lining support can be determined. 1. Grade III to IV Surrounding Rock  
The GRC of Grade III surrounding rock and the SCC of the secondary lining under drill-and-blast construction are illustrated in Figure 16. To depict the intersection balance point of the GRC and SCCs, only the local cross-section of these curves is presented in the following sections, as shown in Figure 16.  
Figure 16: _Cont._  
point between the GRC and SCC (\(u_{\text{ban}}\)), at which point the maximum stress borne by the secondary lining is \(\sigma_{\text{ban}}\). Furthermore, the support timing, excavation reserved deformation, and load borne by the secondary lining under the drill-and-blast construction method for Grade III surrounding rock are plotted in Figure 17.  
It can be seen that under the drill-and-blast construction method for Grade III surrounding rock, the support timing, excavation reserved deformation, and load borne by the secondary lining almost linearly increase with the burial depth. For the drill-and-blast method, using 95% \(u_{\text{lim}}\) as the support timing, the support timing for the surrounding rock of a 2000 m deep tunnel is 7.13 cm, and the excavation reserved deformation can be taken as 7.35 cm, with the maximum load borne by the secondary lining being 0.30 MPa. Using 90% \(u_{\text{lim}}\) as the support timing, the support timing for the surrounding rock of a 2000 m deep tunnel is 6.75 cm, and the excavation reserved deformation can be taken as 7.25 cm, with the maximum load borne by the secondary lining being 0.51 MPa; increasing the excavation reserved deformation by 5% can effectively reduce the load borne by the secondary lining by 41.17%. The support timing, excavation reserved deformation, and load borne by the secondary lining under the drill-and-blast construction method for Grade IV surrounding rock are shown in Figure 18. It can be observed that when 95% of \(u_{\text{lim}}\) is used as the support timing, the support timing for the surrounding rock of a 2000 m deep tunnel is 31.73 cm. The excavation reserved deformation is 31.82 cm, with the maximum load borne by the secondary lining being 0.18 MPa. When 90% of \(u_{\text{lim}}\) is used as the support timing, the support timing for the surrounding rock of a 2000 m deep tunnel is 30.06 cm, and the excavation reserved deformation is 30.20 cm, with the maximum load borne by the secondary lining being 0.39 MPa. An increase of 5% in the excavation reserved deformation effectively reduces the load borne by the secondary lining by 53.85%. In summary, under Grade III and IV surrounding rock conditions, when 95% and 90% of \(u_{\text{lim}}\) are used as the support timing, the maximum load borne by the secondary lining after the lining is applied is 0.60 MPa and 0.34 MPa, respectively.