## 4 Numerical Realization of Zero-Distance Construction Process  
### Numerical Model and Calculation Parameters  
In order to explore the mechanical response law and stability control effect of the new station and the existing tunnel under the three underground excavation schemes and then compare and select the better scheme, the three-dimensional refined numerical calculation models of the station foundation pit and the existing tunnel are established, which have the ability to truly reflect the implementation steps and detailed structure of each scheme. Figure 3 shows the numerical model for Scheme I. The overall models for the other two schemes are similar. The details are described in Section 4.2. In order to eliminate the influence of the boundary effect, the distance between the boundary of the model and the edge of the station shall be controlled to be more than three times the depth of the station. After comprehensive consideration, the model size shall be taken as long \(\times\) wide \(\times\) height = 258.0 m \(\times\) 201.0 m \(\times\) 80.0 m. The model consists of 124,834 nodes and 252,349 zones, with the hexahedron as the primary element. Numerical simulations were performed using Flac3D. The top surface of the model is the free boundary and the rest of the model is fixed with a normal displacement.  
The main structure of the station needs to be modeled separately (as shown in Figure 3). It is introduced into the model after the excavation of the soil mass at the station location in the field model is completed, and the wall and bottom plate of the station structure are coupled with the field model, and then calculated to balance. The main structure of the station consists of 308,682 nodes and 357,635 zones. The relative position of the main structure of the station and the existing tunnel after the import is shown in the upper right panel of Figure 3.  
The formation in the model is established by referring to the geological section of the field in the geological survey report. The site has typical soft soil formations such as mucky soil and clay. In order to better simulate its mechanical properties, the modified Cam-Clay elastic-plastic model is selected for simulation in the model ([PERSON] et al. [39]; [PERSON] et al. [40]; [PERSON] et al. [41]; [PERSON] et al. [42]), and the Mohr-Coulomb elastic-plastic model is used for other rock and soil masses. The constitutive model for the tunnel structure, the foundation pit retention structure, and the main structure of the station is the linear elastic model.  
\begin{table}
\begin{tabular}{c c} \hline
**Steps** & **Sketch Map** & **Explain** \\ \hline \hline \end{tabular} \
\begin{tabular}{c} Excavate part of the soil mass in the middle and construct the remaining part of the top plate, bottom plate, and columns as soon as it is excavated. \\ \end{tabular}
\begin{tabular}{c} The construction process and sequence of earthwork excavation, steel column support, and portals for the remaining exit section was the same as for the entrance section. The position of the jack arrangement after construction is shown in the right figure. \\ \end{tabular}
\end{table}
Table 3: _Cont._The main computational parameters of the strata and structures in the numerical calculations were determined from the site geological survey data and design documents, as shown in Tables 4 and 5.