## 4 Numerical Realization of Zero-Distance Construction Process
### Mechanism of Influence of Tunnel Support Structure  
Figure 12 shows the vertical deformation curve of the bottom plate in the transverse direction of the existing tunnel for four conditions after the construction of the tunnel support structure is completed, with negative values indicating downward deformation. It is obvious that the existing tunnel floor in Condition 1 and Condition 2 is an inverted \"U\"-shaped deformation, with large vertical displacement near the walls on both sides and small vertical displacement in the middle. The reason for this can be attributed to the fact that the L-type beam scheme required excavating the earth at the base of the two side walls of the existing tunnel, making that suspension, and then constructing the concrete beam. During this process, large vertical displacements occur on both sides of the existing tunnel, while the displacement in the middle is small due to the earth-jacking. The deformation of the existing tunnel floor in Condition 3 and Condition 4 is relatively gentle and the overall settlement is small. Although the pier column scheme also required earthwork excavation, only the soil of the six-column area is excavated, and the horizontal area is small. Therefore, the disturbance of the jacking effect of the underlying soil on the existing tunnel is not obvious. \begin{table}
\begin{tabular}{c c c c} \hline
**Condition** & **Supporting Methods** & **Excavation Methods** & **Simplified Expression** & **Remark** \\ \hline
1\({}^{\#}\) & L-type beam & Full-face excavation method & L-beam + F-face & Scheme I \\
2\({}^{\#}\) & L-type beam & Double-pilot tunnel method & L-beam + D-pilot & Compare group \\
3\({}^{\#}\) & Pier column & Double-pilot tunnel method & P-column + D-pilot & Scheme II \\
4\({}^{\#}\) & Pier column & Full-face excavation method & P-column + F-face & Compare group \\ \hline \end{tabular}
\end{table} 
Table 7: Numerical simulation conditions and its simplified expression. Figure 11: Numerical simulation condition design diagram. Figure 13 shows a plot of the minimum principal stress cloud in the transverse direction near the center point of the existing tunnel at the end of the tunnel support phase. We can see that there is a clear compression near the bottom of the middle wall for the four working conditions; this is because after the excavation of the soil on both sides, the middle soil acts as a jack on the existing tunnel, preventing the overall settlement of the existing tunnel. In the presence of gravity, the middle wall is subjected to greater pressure. In Condition 1 and Condition 2 of the L-type beam, due to the large free area of the tunnel floor after the excavation of soil on both sides, the middle part bears a large tunnel structural load, and the bottom of the middle wall is subject to a large pressure of about 1.85 MPa, while Condition 3 and Condition 4 are relatively small, at about 1.6 MPa. The deformation mechanism of the existing tunnel in the tunnel support process is shown in Figure 14. The main structure of the station is concealed in the figure for the convenience of observation, and only the existing tunnel, the rock and earth mass of the underground area, and one side of the boundary of the foundation pit are drawn. At the tunnel support stage, the L-type beam scheme needs to excavate a certain area of soil on both sides of the existing tunnel, resulting in a large area of free face under the bottom plate of the walls on both sides of the tunnel, as shown in the axonometric drawing on the left side of Figure 14a. There is no soil support under the free face, but only the unexcavated soil plays a supporting role in the middle of the tunnel structure. The deformation of the tunnel  
Figure 12: Vertical deformation curve of existing tunnel floor at the end of tunnel support stage (transverse). Figure 13: Cloud chart of minimum principal stress at the end of tunnel support stage. is shown in the diagram on the right.