## 4 Comparison between the Monitoring Data and the Results of 3D FEM Analysis
### Comparison with the Monitoring Data of the Railway Track Subsidence Point  
The railway track subsidence point (SR) was installed on the track surface to understand the amount of subsidence caused by the excavation. Since the construction range of the retaining column was close to the railway track, the warning value and the action value of the vertical and horizontal displacements according to the design specifications were 5 and 7 mm, respectively. In order to avoid any unexpected situations on the railway track, the resulting time-dependent flow of the railway track is not affected by the presence of the excavation. Figure 12: Iso-value of the total displacement of the retaining piles obtained with the 3D FEM analysis for (**a**) the first-stage stratum excavation and (**b**) the second-stage stratum excavation. Figure 13: Iso-value of the second deviatoric stress of the retaining piles obtained with the 3D FEM analysis for (**a**) the first-stage stratum excavation and (**b**) the second-stage stratum excavation. track, the monitoring system used 24 h automatic monitoring, and if the system's settings reached a warning value, it would send a text message to notify the construction unit. As shown in the left part of Figure 4, there were a total of nine railway track subsidence points in the south area, of which the three circled in red were taken as examples, and their numbers were SR-003, SR-006, and SR-007. Table 2 shows the observation data of the railway track subsidence points at various construction stages when the adjacent railway tracks were used as retaining piles. From the observation data in the table, it can be seen that the maximum value of the railway track subsidence points appeared during the construction of the retaining piles and the first and second stages of excavation. The maximum value was 3.92 mm and did not exceed the warning value (5 mm). After the pouring of the concrete for the vertical wall and retaining piles was completed, the subsidence value changed from a negative value to a positive value, and the subsidence amount was gradually transferred from the adjacent rail side to the excavation surface. In addition, the track subsidence values of the first and second stages of excavation had an obvious trend of increasing toward the excavation surface. This monitoring-recorded phenomenon showed a consistent trend, as previously described in the 3D finite element analysis results in Figure 11.  
The results of the 3D finite element analysis and the monitoring data (SR) were compared, as shown in Figure 14. For the results of the subsidence analysis along the railway track's direction (the AB direction in Figure 14a), in the first stage of excavation (GL-3.5 m), the maximum displacement of the ground in the numerical analysis was 2.27 mm, and the maximum subsidence in the monitoring data was 2.38 mm. In addition, in the second stage of excavation (G.L.-6.75 m), the computational and monitoring values were 2.53 and 2.58 mm, respectively. Moreover, for the results of the subsidence analysis perpendicular to the direction of the railway track (the CD direction in Figure 14b), in the first stage of excavation, the maximum displacement of the ground in the numerical analysis was 2.19 mm, and the maximum subsidence in the monitoring data was 2.31 mm. In addition, in the second stage of excavation, the computational and monitoring values were 2.42 and 2.5 mm, respectively. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Construction Stage** & \begin{tabular}{c} **SR-003** \\ **(mm)** \\ \end{tabular} & \
\begin{tabular}{c} **SR-006** \\ **(mm)** \\ \end{tabular} &
\begin{tabular}{c} **SR-009** \\ **(mm)** \\ \end{tabular} \\ \hline Water removal operation and vertical wall construction & 3.5 & 3.58 & 3.92 \\ Construction of retaining piles, and during the first and second stages of excavation & 3.8 & 3.88 & 3.94 \\ Pre-stressed ground anchor construction & 3.6 & 3.29 & 3.72 \\ Foundation floor construction & 2.72 & 3.08 & 2.57 \\ \hline \hline \end{tabular}
\end{table}