## 3 Interpretation of Water Pressure Characteristics Based on Dual Medium Model
### Establishment of Dual Media Model  
When a shaft is built in a rock mass with large fissures, the reduction of the water pressure outside the lining is closely related to the existence of large fissures. Generally speaking, when the fracture is larger, the hydraulic conductivity of the rock mass is larger, and the hydraulic conductivity of the stone body formed after grouting is larger. The osmotic pressure formed by the hydrostatic pressure on the outside of the grouting ring after passing through the grouting ring, that is, the osmotic pressure on the outside of the lining, is also greater. The dual medium model is a combination of porous and fractured media. This model can generate fractures of different sizes and well-simulate large, fractured rock masses. In this paper, the random discrete fracture network (DFN) embedded in FLAC3D is used to establish a dual medium model. In the fractured rock mass, the Monte Carlo and statistical methods are generally used to generate the same fracture network as the actual fracture. Specific steps: collect on-site fracture parameters (including the number of groups and geometric parameters), perform statistical analysis on the fracture parameters, and use simulation software to generate a fracture network according to the fracture statistics. Figure 10 shows the fractures revealed during the construction of the Gaoligongshan tunnel shaft site. Since only part of the on-site fissure parameters can be collected through the on-site excavation of the shaft, the fissure parameters inside the rock mass, where the grouting circle is located cannot be collected, use the method of on-site sub-drilling to collect. A total of 66 sub-drilling holes were drilled at the site, with a drilling depth of 771 m. According to the situation revealed by on-site drilling, the fracture parameters were counted. The on-site drilling revealed the situation, as shown in Figure 11. \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline \multirow{2}{*}{**Position**} & **Elastic** & \multirow{2}{*}{**Modulus**} & \multirow{2}{*}{**Poisson**} & \multirow{2}{*}{**Cohesion**} & **Internal** & **Tensile** & \multirow{2}{*}{**Density**} & **Hydraulic** & \\  & & & & **Friction** & **Strength** & **(g/m\({}^{3}\))** & **Conductivity** & **Comductivity** \\  & & & & **Angle (\({}^{\circ}\))** & **(MPa)** & **(m/s)** & **(m/s)** & **(m/s)** \\ \hline S1 ZK0 + 660 & 30,000 & 0.23 & 20.00 & 52.00 & 26.00 & 2.66 \(\times\) 10\({}^{-6}\) & 8.60 \(\times\) 10\({}^{-9}\) & 0.3 \\ S1 ZK0 + 735 & 30,000 & 0.23 & 20.00 & 52.00 & 26.00 & 2.66 \(\times\) 10\({}^{-6}\) & 8.60 \(\times\) 10\({}^{-9}\) & 0.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Model geological parameters. Figure 10: On-site construction cracks revealed. (**a**) On-site fracture picture 1; (**b**) On-site fracture picture 2. The field revealed that there are about three groups of fractures in the field [38]. Considering the actual situation on the site, with the increase of the excavation depth and influence of geological conditions, such as faults, the changes of the deep in-situ stress field are more complicated. The occurrence of fissures is closely related to the distribution of in-situ stress, so it is necessary to supplement the original basis to investigate the distribution of deep strata fissures. Through the investigation of deep shafts, bottom wells, and strata, it is found that there are still three groups of obvious fissures in deep strata, including two groups of small fissures and one group of large fissures. Considering the computational efficiency and research focus, the research only focuses on the influence of large fractures and ignores small fractures. According to the research results on the geometric parameters of rock mass fractures, the geometric parameters of rock mass fractures generally obey one or several probability density distributions.