## 5 Analysis of Monitoring Results
### Vertical Displacement of Diaphragm Wall Top  
Figure 12 presents four typical monitoring points (QDC-1, QDC-3, QDC-5, and QDC-7, located at \(0^{\circ}\), \(90^{\circ}\), \(180^{\circ}\), and \(270^{\circ}\) orientations), systematically documenting the evolutionary patterns of vertical displacement at the wall top throughout the entire excavation period of the foundation pit. As shown in Figure 12, during the initial excavation phase, the wall top exhibited uplift displacement, with a maximum uplift of +3 mm (at monitoring point QDC-5), attributed to soil unloading rebound caused by shallow excavation in the soft soil area. Between 17 and 21 days, the wall top began to settle, reaching a maximum settlement of approximately \(-3\) mm (at QDC-1). Subsequently, the wall underwent multiple \"increase-decrease\" cycles in the vertical direction but displayed an overall upward fluctuating uplift trend. The maximum uplift of 8 mm occurred after the completion of pit excavation.  
Based on the excavation sequence (Table 2) and inner lining configuration (Figure 2), the vertical displacement evolution in Figure 12 is attributed to multi-phase soil-structure interactions. Initial shallow excavation (0-8 d) induced +3 mm uplift (QDC-5) due to soil unloading rebound, and was amplified by localized 5 m range soil removal prior to first-layer lining placement. Subsequent first-layer lining casting (8-10 d) increased structural self-weight, slightly suppressing displacement. Second-phase excavation (10-17 d) within the first lining zone triggered renewed rebound (\(-3\) mm settlement at QDC-1). The 17-21 d transition to settlement reflects combined effects of second-layer lining loading and dewatering-induced pore pressure dissipation, consistent with [PERSON]'s  
Figure 12: Vertical displacement at wall top.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & & \multicolumn{2}{c}{**Design Safety Grade of Foundation Pit**} \\
**Monitoring Item** & **Structure Type** & \multicolumn{2}{c}{**Grade I**} \\  & & \multicolumn{2}{c}{**Cumulative Value**} \\  & & **Absolute Value (mm)** & \multicolumn{2}{c}{**Control Value Relative to**} \\  & & & **Design Depth H** \\ \hline Vertical Displacement at Wall Top & Diaphragm Wall & 10–20 & 0.1\%–0.2\% \\ Deep Horizontal Displacement & Diaphragm Wall & 30–50 & 0.3\%–0.4\% \\ \hline \hline \end{tabular} Note: The cumulative value is taken as the smaller of the absolute value and the relative control value of the foundation pit design depth.  
\end{table}
Table 3: Monitoring alert values for soil foundations (GB 50497-2019).  
principle: reduced groundwater levels elevated effective stresses, driving recompression of over-consolidated clays. The final 8 mm uplift post-excavation demonstrates the dominance of cumulative soil rebound over structural constraints, highlighting the soil vertical displacement mechanisms due to joint effects of foundation pit excavation and foundation pit dewatering in soft soil environments.  
According to Table 3, the allowable vertical displacement for the diaphragm wall is 10 mm. The maximum measured vertical deformation on-site is approximately 8 mm, equivalent to 0.04% of the excavation depth (H), which complies with the control criteria.