## 4 Numerical Simulation and Analysis
### Effect of the Water-Resisting Curtain  
The foundation pit of this project adopts a bottom-down water-resisting curtain to cut off the connection between the groundwater inside and outside the foundation pit. Owing to the high groundwater level and the thick confined aquifer at the site, the construction method pile should be fully extended into layer 2, composed of silty clay in the design, so that the confined aquifers inside and outside the foundation pit are completely separated. However, owing to the complex geological conditions at the site, a part of the water-resisting curtain on the west side of the foundation pit (the frame selection range in Figure 12) failed to extend into the lower aquiclude, and a 0.6-1.3 m confined coarse sand layer was present below the pile bottom. After the construction of the elevator shaft, the maximum seepage velocity of the soil layer on the west side of the foundation pit, where the water-resisting curtain is not fully extended into the aquifuge, reaches 248.1 mm/day, which is 40 times greater than the seepage velocity (approximately 6.2 mm/day) at the bottom of the water-resisting curtain opposite the east side. Therefore, the seepage velocity and flow rate at the defect position are significantly greater than those at the other positions. Figure 11: Comparison of the deep horizontal displacements of the foundation pit: (**a**) CX1 monitoring point; (**b**) CX2 monitoring point; (**c**) CX3 monitoring point; (**d**) CX4 monitoring point. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Monitoring Point** & CX1 & CX2 & CX3 & CX4 \\ \hline
**RMS Error** & 5.37 mm & 1.67 mm & 3.07 mm & 2.17 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 3: RMS Error of the deep horizontal displacement of the foundation pit. In addition to the seepage velocity and flow rate, the direction of groundwater seepage also changes with the excavation process of the foundation pit, particularly near the defect of the water-resisting curtain. Figure 13 compares the groundwater seepage near the defect location when the foundation pit is excavated to different depths. Before excavation to 8.5 m, because the confined aquifer in the defect location is not horizontally distributed, the groundwater has a low seepage velocity to the outside of the pit, and the seepage velocity is approximately 6.4 mm/day, so the groundwater seepage in the foundation pit is relatively stable. When the excavation reached 9.2 m, the self-weight stress of the soil in the foundation pit and the length of the seepage path were further reduced. The influence of the difference between the head pressures inside and outside the pit on the seepage direction reaches a critical state, and the direction and velocity of groundwater seepage near the defect location change. Owing to the high head of confined water, the maximum seepage velocity at this position reaches 78.8 mm/day, posing a threat to the safety of the foundation pit. The foundation pit inrush accident occurred at the bottom of the elevator shaft in the foundation pit. At the time of the accident, the construction of the foundation pit cushion for the elevator shaft had been completed. According to the field geological survey report, the clay layer at approximately 1-1.5 m in the underground part of the elevator is the main aquifuge, and a confined aquifer of approximately 10.3 m is located beneath it. Therefore, the water barrier layer of the elevator shaft is thin and prone to surge and other damage. Figure 14 shows the seepage before and after the steel sheet pile is pulled out in a typical section of the elevator shaft position of the foundation pit. Before the steel sheet pile is  
Figure 12: Seepage situation of the defect location of the supporting structure. Figure 13: Seepage of the defect location in different excavation stages: (a) excavation to 8.5 m; (b) excavation to 9.2 m.  
pulled out, the seepage flow at the bottom of the elevator shaft increases.