## 3 Establishment of Numerical Model and Selection of Calculation Parameters
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Region** & \(\mathbf{E}_{0}\) & \(\mathbf{E_{50\text{ref}}}\) & \(\mathbf{E_{\text{oedref}}}\) & \(\mathbf{E_{\text{urref}}}\) \\ \hline Beijing & \(\beta E_{\text{s}}\) & \(\beta E_{\text{s}}\) & \((0.5\text{--}1.0)E_{50\text{ref}}\) & \((2\text{--}4)E_{50\text{ref}}\) \\ Shanghai & \(\beta E_{\text{s}}\) & \((0.9\text{--}1.0)E_{\text{s}}\) & \((0.7\text{--}1.2)E_{50\text{ref}}\) & \((4\text{--}9)E_{50\text{ref}}\) \\ Tianjin & \(\beta E_{\text{s}}\) & \((1.5\text{--}2.0)E_{\text{s}}\) & \((1.0\text{--}1.5)E_{50\text{ref}}\) & \(5E_{50\text{ref}}\) \\ \hline \hline \end{tabular} Note: \(E_{0}\): deformation modulus; \(E_{\text{s}}\): compression modulus; \(E_{50\text{ref}}\): triaxial test secant stiffness; \(E_{\text{urref}}\): unloading elastic modulus; \(\beta\): conversion coefficient, \(\beta=\frac{1-2\beta^{2}}{1-\
u}\) (\(\
u\) Poisson ratio); \(E_{\text{oedref}}\): primary consolidation loading test tangent stiffness.  
\end{table} 
Table 3: Modified Mohr-Coulomb model parameter empirical value table. The existing station is C30 concrete. In light of the structural strength reduction during operation, the concrete strength is valued by the actual strength multiplied by the safety factor 0.8, and the elastic modulus is 24 GPa. The model excavation is carried out in line with the actual construction procedure, with simultaneous symmetrical excavation on both sides, the 3-dimensional model is shown in Figure 4.  
The simulation of groundwater is achieved by setting the head boundary conditions. Phreatic water is simulated by setting the total head and the confined water by setting the pressure head. As for the simulation of dewatering processes, it can be defined by the percolation boundary function. The dewatering in the pit is applied for the shallow ground water and confined water to simulate the water level drop by passivation head boundary conditions; the deep confined water is pumped by dewatering well to simulate the dewatering process [30]. The layer is stratified, and different stratum parameters are set according to different aquifers. At the same time, the corresponding interface unit is added between the aquifer and the water barrier layer according to the permeability of the weak water permeable layer to simulate the effect of the water barrier layer. Based on the geotechnical exploration report of Ciqu station, the values of the permeability parameters of the water-bearing layer and the water-resisting layer are shown in Table 6.  
Figure 4: Model diagram of foundation pit excavation next to existing station: (**a**) Overall model diagram; (**b**) Foundation pit and station location. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Structure** & \begin{tabular}{c} **Constitutive** \\ **Model** \\ \end{tabular} & \begin{tabular}{c} **Elastic Modulus** \\ **E/MPa** \\ \end{tabular} & \
\begin{tabular}{c} **Severe/kN/m3** \\ \end{tabular} &
\begin{tabular}{c} **Poisson Ratio** \\ \end{tabular} \\ \hline Diaphragm wall & Elastic & 24,000 & 25 & 0.167 \\ Crown beam & Elastic & 24,000 & 25 & 0.167 \\ Concrete support & Elastic & 24,000 & 25 & 0.167 \\ Steel support & Elastic & 212,000 & 78.5 & 0.31 \\ Column & Elastic & 212,000 & 78.5 & 0.31 \\ Waist beam & Elastic & 24,000 & 25 & 0.167 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Table of mechanical parameters of supporting structures.