## 7 Conclusions and Prospects  
Field tests have been conducted to measure ground stress distribution in the study area to ensure the long-term safe service performance of deeply buried water conveyance tunnel groups. Numerical simulations have been employed to obtain the GRC and SCC of deeply buried water conveyance tunnels under various conditions, including surrounding rock grades, tunnel spacings, and burial depths. Factors affecting the normal service  
Figure 19: GRC and SCC of the drilling and blasting method of Grade V surrounding rock: (**a**) 200 m; (**b**) 400 m; (**c**) 600 m; (**d**) 800 m; (**e**) 1000 m; and (**f**) 1200 m.  
and ultimate state-bearing capacity of the secondary lining, such as surrounding rock strength, reinforcement ratio, and lining thickness, have been analyzed. Additionally, the timing of secondary lining support, excavation reserved deformation, and load borne by the secondary lining have been derived through theoretical analysis. The main research findings and conclusions are summarized as follows:  
1. Under a burial depth of 400 m, the effect of tunnel spacing (D \(\geq\) 35 m) on the displacement of tunnels with Grade III and IV surrounding rock is negligible. The tunnel group effect for Grade V surrounding rock is more pronounced when D \(\leq\) 50 m and becomes less significant when D \(>\) 50 m. Furthermore, at D = 65 m, the tunnel group effect for Grade V surrounding rock is not significant at a 1600 m burial depth. 2. The SCC of the secondary lining exhibits an initial rapid linear increase, followed by a phase of slow growth, and finally levels off as displacement increases. Compared to Grade III surrounding rock, the \(P_{\text{max}}\) and \(P_{0.3}\) values for Grade V surrounding rock decrease by 1.95% and 11.52%, respectively, with the surrounding rock grade having minimal impact on \(P_{\text{max}}\). As the reinforcement ratio and thickness increase, both \(P_{0.3}\) and \(P_{\text{max}}\) of the secondary lining increase almost linearly. \(P_{0.3}\) increases by an average of 5.40% for each 0.1% increase in reinforcement ratio and 11.18% for each 0.1 m increase in lining thickness. 3. The timing of secondary lining support, excavation reserved deformation, and the load borne by the secondary lining for Grade III and IV surrounding rocks with burial depths ranging from 200 m to 2000 m under the drill-and-blast construction method have been proposed. The maximum load borne by the secondary lining after construction is 0.60 MPa and 0.34 MPa for Grade III and IV surrounding rocks, respectively, when 95% \(u_{\text{lim}}\) and 90% \(u_{\text{lim}}\) are selected as the support timing. During construction, the stress on the secondary lining is only 20.2% and 11.4% (9.3%) of the cracking stress for Grade III and IV surrounding rocks, respectively, indicating that the stress during construction is relatively small, providing a high safety margin for resisting external water pressure loads. 4. For tunnels with a 200 m burial depth in Grade V surrounding rock, 90% \(u_{\text{lim}}\) is recommended as the preliminary support timing. For tunnels with burial depths ranging from 400 m to 800 m, the support timing is selected based on curve characteristics and experience, with surrounding rock deformation reaching 40 cm, 50 cm, and 60 cm, respectively. However, at 600 m and 800 m burial depths, the safety reserve against cracking is insufficient, and design parameters should be dynamically adjusted based on growing reinforcement effects and monitoring data. For tunnels with burial depths between 1000 m and 2000 m, additional initial reinforcement measures are required to enhance the bearing capacity of the surrounding rock. In areas with significant external water pressures, measures are also necessary to improve the surrounding rock's seepage resistance and reduce external water pressures.