## 3 Midas GTS Simulation Process and Results  
### Geometric Model  
In this study, MIDAS GTS NX finite element software was used to establish the analysis model. According to relevant specifications [43], the potential impact zone of foundation pit engineering is 2.0-3.0 H, where (H) represents the excavation depth of the foundation pit. Given that H = 16.7 m in this case, the boundaries of the model's length and width must exceed 50.1 m from the edge of the foundation pit structure. The existing tunnel's roof is buried at a depth of 16.35 m, which is less than 3D (D is the span of the tunnel; D = 6.2 m in this case). According to relevant specifications [44], generally, the affected zone of the tunnel is 1.0-2.0 h (h represents the buried depth of the tunnel bottom plate during external shield operations; h = 22.55 m in this case), beyond which it is unaffected by tunnel construction. Consequently, the numerical model's boundary must extend beyond the study object's outer edge for at least 45.1 m, the boundary range diagram of the model is illustrated in Figure 3.  
According to the literature [45,46], a numerical model can effectively eliminate the effect when the tunnel's length surpasses 120 m. Furthermore, according to the relevant  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Soil** & \begin{tabular}{c} **Natural Unit Weight** \\ \(\gamma\) **(kN/m\({}^{3}\))** \\ \end{tabular} & \begin{tabular}{c} **Cohesion** \\ \(c\) **(kPa)** \\ \end{tabular} & \
\begin{tabular}{c} **Angle of Friction** \\ \(\varphi\) **(\({}^{0}\))** \\ \end{tabular} &
\begin{tabular}{c} **Soil Thickness** \\ **(m)** \\ \end{tabular} \\ \hline Miscellaneous fill & 17.5 & 8 & 15 & 3 \\ Powdered sand & 19.3 & 5 & 33 & 8.4 \\ Slit with sandy silt & 19.2 & 5 & 33 & 4.0 \\ Sandy silt & 19.6 & 6 & 28 & 3.4 \\ Silty clay & 19.1 & 28 & 18 & 4.3 \\ Clay & 19 & 25 & 19 & 4.8 \\ Puddingstone & 20.2 & 3 & 42 & 16.5 \\ Strongly weathered rock & 25 & 50 & 37 & 3.1 \\ Moderately weathered rock & 25.2 & 100 & 38 & 12.5 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Soil physical and mechanical parameters after simplification.  
Figure 3: Boundary range diagram of the model.  
specifications [44], any distance exceeding 3.0D of the outer diameter of a tunnel of the urban rail transit structure constructed by the shield method falls within the unaffected zone. Consequently, the new tunnel's length within the numerical model should extend at least 18.6 m beyond the intersection with the existing tunnel.  
The subsurface conditions mainly determine the model's lower boundary extension depth. A hard soil layer beneath can serve as the model's lower boundary. Since soil stiffness increases with depth, a lower boundary situated well below the foundation pit's bottom exerts a minimal impact on the calculated results [47]. Given that the level below 47.5 m in this instance is a moderately weathered rock layer, the height of the model was set to a depth well beyond the foundation pit, specifically at 60 m.  
To mitigate the boundary effect on the numerical simulation results, the model's length was established at 150 m for the new tunnel, extending approximately 80 rings beyond the existing tunnel point. The width was set to 78 m at the foundation pit's outer edge, with approximately 70 rings from the new to the existing tunnel's crossing point. The height of the model was determined to be twice the depth of the underground diaphragm wall, totaling 60 m, the finite element model is showed in Figure 4.