## 3 Numerical Simulation of Open Excavation and Added-Stories Construction
### Deformation of Existing Stations and Forced Action on Them  
The maximum displacement of the existing station in the lower region was closely related to the depth of the foundation pit created by open excavation. The deformation of an existing station under large unloading becomes complex. Therefore, to determine the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material** & **\(\gamma\)/(kN\(\cdot\)m\({}^{3}\))** & **E/MPa** & **\(\upsilon\)** & **\(\alpha\)/kPa** & **\(\varphi\)/\({}^{c}\)** \\ \hline Silty Clay & 17.5 & 7 & 0.30 & 10 & 10 \\ Pebble\(\geq\) & 19.5 & 12 & 0.26 & 0 & 25 \\ Pebble\(\leq\) & 21.0 & 15 & 0.22 & 0 & 40 \\ Pebble\(\geq\) & 21.5 & 40 & 0.18 & 0 & 45 \\ Pebble\(\geq\) & 22.0 & 140 & 0.18 & 0 & 50 \\ Pebble\(\geq\) & 22.0 & 150 & 0.26 & 0 & 50 \\ Sandstone & 28.3 & 100 & 0.35 & 100 & 35 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mechanical parameters of soil. Figure 4: Schematic diagram of retaining structure and existing station. (**a**) Top view of existing station; (**b**) schematic diagram of retaining structure. deformation law of the existing station and ensure its safety, it is necessary to analyze the effect of open excavation and story-adding on the station below the foundation pit. A cloud diagram of the overall vertical displacement of the existing station was extracted, as shown in Figure 5a. Observe that the station exhibits obvious uplift deformation near the foundation pit. The uplift value in each excavation stage of the station top plate on line B-B' in the model was extracted and drawn in curves, as shown in Figure 5b. With an increase in the upper foundation pit depth, the uplift in the station top plate gradually increased. The uplift in the station top plate is within the longitudinal uplift range of the station top plate in the excavation area and 30 m area on both sides of the foundation pit. The vertical and horizontal displacement cloud diagrams of section A-A' of the existing station before and after foundation pit excavation were extracted by simulation, as shown in Figure 6. Observe from Figure 6a,b that the station top and bottom plates deformed toward the inside of the station before excavation. After excavating the foundation pit above the existing station, the existing station top and bottom plates were uplifted to different degrees, with maximum uplift values of 2.61 and 1.35 mm, respectively. The total uplift of the top plate is greater than that of the bottom plate. Figure 6c,d depict the horizontal displacement of the existing station. Observe from the figure that the horizontal displacement of the sidewall of the station was small; moreover, the sidewall of the station moved horizontally to the soil side before excavation. After excavating the foundation pit, the sidewall showed a tendency of internal crowding. The maximum horizontal displacement of the sidewalls before and after excavation was 0.51 mm. A comparison cloud diagram for the vertical and horizontal stresses in section A-A' before and after the foundation pit excavation was extracted is shown in Figure 7. Notice that after the foundation pit excavation, the stress state of the station top plate changes from the upper compression and lower tension states to the upper tension and lower compression states, respectively. The stress state of the station bottom plate before excavation is that the lower part is compressed and the upper part is tensioned, whereas the stress state of the station bottom plate after excavation remains unchanged but the stress value decreases. After foundation pit excavation, the stress state of the upper sidewall changes from external tension and internal compression to external compression and internal tension, respectively. The change in the horizontal load on the lower sidewall was small; although the stress state remained unchanged, the stress value decreased. The maximum tensile stress of the station structure before and after excavation was 1.28 MPa; the station was not damaged. Figure 5: Schematic diagram of longitudinal uplift of station.