## 2 Discussion on Water Pressure Reduction Coefficient of Sub-Sea Tunnel Lining
### Bearing Capacity of Secondary Lining under Different Water Pressure Reduction Coefficients  
Based on the above research results, it is inferred that the initial support can form a closed structure with strong impermeability, which can bear large water pressure, while the water pressure on the secondary lining is very small. However, the long-term erosion of primary support shotcrete by seawater may seriously decline its impermeability and cannot play a good role in water plugging [24,25], which may increase the water pressure of the secondary lining. The initial support is shotcrete and includes steel arch, reinforcement mesh, and other structures, which is easy to produce weak sealing parts and even form cavities in them [26]. As the last line of defense of the tunnels, the secondary lining needs to consider the above situations in the design and must have high impermeability and bearing capacity as a safety reserve. When calculating the bearing capacity of the lining structure, in order to ensure the safety of the tunnel lining structure, this paper assumes that the initial support is not waterproof. Considering that all the external water pressure of the initial support studied above acts on the secondary lining, the internal force of the lining structure is calculated, and then the reinforcement and safety factors are calculated. The calculation is mainly based on Code for Design of Metro [27] and Code for Design of Railway Tunnel [23]. This part of the calculation still takes the F5 fracture zone in the Daqing section of Qingdao Metro Line 8 as the engineering background and selects the representative class V surrounding rock section ZDK42+092 section to establish the model. Due to the influence of the fracture zone, the fractured rock at this section is extremely developed. The whole stratum is in loose to columnar structure, the surrounding rock is mainly jointed tuff, and the groundwater is bedrock fissure water. The total water head at the vault of the tunnel is 53.4 m. The secondary lining of section ZDK42+092 is 45 cm thick with C50 reinforced concrete structure. The physical and mechanical parameters of surrounding rock and concrete are shown in Table 7[23], and the structural dimensions of the lining are shown in Figure 7. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \(t_{2}\) (m) & \(\mathbf{k_{r}/k_{g}}\) & \(\mathbf{5}\) & \(\mathbf{10}\) & \(\mathbf{50}\) & \(\mathbf{100}\) & \(\mathbf{200}\) \\ \hline
1 & 1.00 & 0.95 & 0.85 & 0.75 & 0.70 \\
3 & 1.00 & 0.80 & 0.35 & 0.30 & 0.25 \\
5 & 1.00 & 0.65 & 0.30 & 0.25 & 0.20 \\
7 & 0.95 & 0.60 & 0.25 & 0.20 & 0.15 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Recommended value of water pressure reduction coefficient of class C surrounding rock. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \(t_{2}\) (m) & \(\mathbf{k_{r}/k_{g}}\) & \(\mathbf{5}\) & \(\mathbf{10}\) & \(\mathbf{50}\) & \(\mathbf{100}\) & \(\mathbf{200}\) \\ \hline
1 & 0.95 & 0.90 & 0.75 & 0.65 & 0.60 \\
3 & 0.95 & 0.60 & 0.25 & 0.20 & 0.15 \\
5 & 0. Midas GTS NX finite element calculation software established the Load-structure model. The central line of the second lining is taken as the contour line of the model, and the contour line is a five center circle. The calculation load mainly considers the surrounding rock pressure, water pressure, structural self-weight and formation elastic resistance. The elastic resistance of formation is simulated by radial spring. The lining water pressure is calculated by the reduction coefficient method. The surrounding rock load calculation method is shown in the Code for Design of Railway tunnel [23]. After calculation, the lining load is shown in Table 8, in which the actual water pressure shall be reduced according to the working conditions. The surrounding rock load distribution and the calculation model are shown in Figure 8. The earth pressure at the bottom of the tunnel is simulated by the foundation reaction provided by the foundation spring (see Figure 8 outside the tunnel).