## 4 Numerical Simulation Results Analysis  
### Impact of Excavation at Individual Stages on the Retaining Wall  
#### 4.1.1 Surface Subsidence in the Excavation Area  
The formations in the Taipei Basin are mostly composed of soft clay layers. Most of the soft clay soils have a natural moisture content equal to or greater than the liquid limit of the soil, with an SPT-N value between about 1 and 4, and have characteristics such as extremely soft to weak consistency. In this case, subsidence observation points were set up to monitor the amount of surface subsidence caused by excavation in each stage at each observation point. For the south side and north side of the diaphragm wall, the surface subsidence analysis values and the monitoring values were compared after the completion of each stage of construction. The results are shown in Tables 11 and 12, respectively. The surface subsidence was successfully controlled within 10.78 mm, which did not affect the traffic above and the normal use of buildings at ground level. Moreover, the errors between the analyzed values and the monitored values were between 0.3 and 0.78 mm. The results showed that the constructed numerical analysis model had considerable accuracy. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Steps** & **Launching Shaft** & **Receiving Shaft** & **Remark** \\ \hline Initial state & \multicolumn{3}{c}{Establish initial soil stress} \\ Phase 1 & Construction of diaphragm wall and & Construction of steel sheet piles & Reset displacement to zero \\ Phase 2 & Excavation to GL \(-2.8\) m & Excavation to GL \(-2.5\) m & Lower water level to \(-3.8\) m \\ Phase 3 & Erection support at GL \(-1.8\) m & Erection support at GL \(-1.5\) m & \\ Phase 4 & Apply a preload of 60 kN & Apply a preload of 50 kN & \\ Phase 4 & Excavation to GL \(-5.75\) & Excavation to GL \(-5.6\) m & Lower water level to \(-6.75\) m \\ Phase 5 & Erection support at GL \(-4.75\) m & Erection support at GL \(-4.6\) m & \\  & Apply a preload of 130 kN & Apply a preload of 140 kN & \\ Phase 6 & Excavation to GL \(-8.75\) m & Excavation to GL \(-8.1\) m & Lower water level to \(-9.75\) m \\ Phase 7 & Erection support at GL \(-7.75\) m Apply a preload of 220 kN & Erection support at GL \(-7.1\) m & \\ Phase 8 & Excavation to GL \(-11.5\) m & Excavation to GL \(-10.6\) m & Lower water level to \(-12.5\) m \\ Phase 9 & Erection support at GL \(-10.5\) m & Erection support at GL \(-9.6\) m & \\  & Apply a preload of 320 kN & Apply a preload of 350 kN & \\ Phase 10 & Excavation to GL \(-12.15\) m & Excavation to GL \(-12\) m & Lower water level to \(-14\) m \\ Phase 11 & Build the bottom floor & \\ Phase 12 & Remove part of the support (pipe roof application range) & \\ Phase 13 & Construction of the top row of pipe roof & \\ Phase 14 & Construction of the left row and right row of pipe roof & \\ Phase 15 & Construction of the bottom row of pipe roof & \\ Phase 16 & The first round of excavation and box culvert construction & \\ Phase 17 & The second round of excavation and box culvert construction & \\ Phase 18 & Completion of underpass construction & \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Steps in the staged construction. On the other hand, in order to verify the validity of the numerical model, the cumulative values of surface subsidence obtained from the numerical simulation and field measurement in each construction stage were further compared. In order to analyze the surface subsidence caused by the excavation of the diaphragm wall, a section A-A was selected on the south side of the diaphragm wall, and a section B-B was selected on the north side of the diaphragm wall, as shown in Figure 17. The surface subsidence curves for the south and north sides of the diaphragm wall after the completion of each stage are shown in Figures 18 and 19, respectively.