## 2 Discussion on Water Pressure Reduction Coefficient of Sub-Sea Tunnel Lining
### Field Test on External Water Pressure of Initial Support of the Sub-Sea Tunnel  
In order to verify the previous research on the external water pressure of the initial support of the sub-sea tunnel, the real-time on-site detection of the external water pressure of the initial support was carried out in the sea area of the Daqing section of Qingdao Metro Line 8. At the same time, to guide the construction, the monitoring sections are selected in the important and difficult construction section of the F5 structural fracture zone. A total of three monitoring sections are selected (see Figure 2). In Figure 2, F5 (red line) is an active fault, and the regional scope (ZDK41+990-ZDK42+200) is the area affected by the F5 fault. The surrounding rock in this area is broken under the influence of the F5 fault. The surrounding rock is grade IV-V, all of which are soft and broken surrounding rock. The classification of surrounding rock in Table 2 refers to the Chinese tunnel design code [23]. The total head height of the tunnel vault is 51-55 m. Pre-grouting water blocking measures are taken for all three sections. The statistics of the monitoring sections are shown in Table 2. The physical and mechanical parameters of the surrounding rock are shown in Table 3. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Monitoring Points** & **1** & **2** & **3** & **4** \\ \hline Water pressure/kPa & 11.53 & 12.66 & 18.74 & 31.54 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The calculation results of initial support external water pressure. Figure 2: Local geological profile near the tunnel monitoring section. It can be seen from Figure 5 that the external water pressure of the initial support of ZDK42+330 quickly is located in an F5 fracture zone. There was strong volatility in the early stage, although the external water pressure on the initial support of the two sections gradually stabilized after more than two months. ZDK41+943 and ZDK42+092 are located in the F5 bedrock fracture zone, although the two sections finally reach a stable state. However, there was strong volatility in the early stage, which gradually stabilized after two months. On the 14 th day of monitoring, the external water pressure of the initial support of ZDK41+943 increased as a whole. The maximum value appears in the inverted arch, reaching 61 kPa, about 11% of the total head. After the water pressure of each part is maintained for about 1 week, it gradually drops to the normal level. Combined with the on-site construction, it is found that the reason is the poor drainage in this area. After the  
Figure 4: The pore water pressure sensor and field installation drawing: **(a)** pore water pressure sensor; **(b)** data acquisition instrument; **(c)** drill hole; **(d)** layout of the pore water pressure sensor. Figure 5: The initial support external water pressure: **(a)** the time-history curve of water pressure of the monitoring section ZDK41+943; **(b)** the time-history curve of water pressure of the monitoring section ZDK42+092; **(c)** the time-history curve of water pressure of the monitoring section ZDK42+330; **(d)** the water pressure distribution along the section. (The dates are in the form of “in the form of “day/month”.) external water pressure of the lining is stable, the water pressure at the inverted arch is the largest. The water pressure at the tunnel invert for ZDK 41+943 and ZDK 42+092 is about twice the one at the other parts of the tunnel. The initial support water pressure of the three monitoring sections finally stabilized below 50 kPa, which is equivalent to the hydrostatic pressure generated by a 5 m water head. The total water head at the vault of the three sections of the tunnel is between 51 m and 55 m. It can be seen that the external water pressure of the initial support measured on site is less than 10% of the total water head.