## 3 Analysis of Water-Bearing Characteristics in Water-Rich Region
### Result Analysis  
When the drainage rates are 25%, 33%, 50%, and 66.7%, the distribution characteristics of the confining pressure water pressure in the east-west and north-south directions of the tunnel are shown in Figure 11.  
When the segment structure is drained by the drainage hole, the seepage field distribution of the surrounding rock around the tunnel changes significantly, and a more obvious \"dewatering funnel\" is formed above the vault. However, the change of the seepage field only occurs in the local area around the tunnel and does not spread to the overall seepage field of the surrounding rock.  
When the drainage rate is 25%, a pressure head area of 50 m is formed above and around the tunnel, and the external water pressure below and in front of the tunnel is still 60 m. When the drainage rate is 33.3%, there is a small range of low-pressure area above the tunnel, but the external water pressure near the whole tunnel is about 50 m. When the drainage rate is 50%, the low-pressure area above the tunnel increases and the external water pressure near the tunnel is about 40 m. When the drainage rate is 66.67%, there is a large area of pressure head area above the tunnel with a middle of 30 m and an edge of 40 m. The external water pressure of the tunnel is about 30 m. When the contrast drainage rate is 25%, the external water pressure near the tunnel is reduced by about 40 m. The size of the external water pressure is closely related to the drainage rate. The external water pressure decreases gradually with the increase of the drainage rate. By using the quadratic equation fitting, \(\mathrm{R}^{2}=0.98228\) (Figure 12).  
With the increase of drainage rate, the external water pressure decreases, which indicates that in the process of excavation of the Earth pressure balance shield machine, when the drainage rate reaches a certain degree, the effect of pressure reduction can be fully exerted, and the external water pressure of tunnel can be greatly reduced. When the water inflow of the Shiling-Baihe 15-25 m section discharge tunnel is 66.67%, that is, when the drainage volume reaches 37.43 m\({}^{3}\)/d, the external water pressure of the tunnel can be reduced to 0.3 MPa, which can ensure the safety of the Earth pressure balance shield machine in the process of excavation (Table 3).  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Water Inflow at Zero Water Pressure in Tunnel** & **Tunnel Drainage Rate** & **Water Inflow** \\
**m\({}^{3}\)/d** & **(\%)** & **m\({}^{3}\)/d** \\ \hline \hline \multirow{4}{*}{\(-\)55.19} & 25.0 & \(-\)14.029 \\  & 33.3 & \(-\)18.707 \\ \cline{1-1}  & 50.0 & \(-\)28.059 \\ \cline{1-1}  & 66.7 & \(-\)37.43 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Water inflow in excavation section of tunnel under different drainage rates.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Drainage Rate (\%)** & **Tunnel External Water Pressure (m)** \\ \hline \hline
25 & 60 \\
33.3 & 50 \\
50 & 40 \\
66.67 & 30 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Relation between drainage rate and external water pressure.  
Figure 11: Cont.  
Figure 11: The water pressure distribution of the surrounding rock of the tunnel under different drainage rates: (**a**) drainage rate of 25%; (**b**) drainage rate of 33%; (**c**) drainage rate of 50%; (**d**) drainage rate of 66.7%.