## 3 Effect of Pit Construction on the Deformation of Existing Structures
### Finite-Element Modeling
The calculated maximum vertical uplift deformation value of the pier through numerical simulation is 1.904 mm. This occurs specifically in Pier 7# of the pier in the pile foundation underpinning. The numerical simulation of the vertical deformation value of the abutment is approximately 1.4 times greater than the vertical deformation value observed through site monitoring. The reason for this is that the field jacking operation involves graded loading and displacement control as a criterion. In the process of jacking and cutting the piles, displacement is monitored and adjusted in real-time. However, this process cannot be accurately represented in numerical simulations. The numerical simulation results for the pier at both ends of the bridge show a slight deviation from the deformation trend observed on-site. However, the vertical deformation values for both are minimal, fluctuating within the range of \(\pm\)0.3 mm. These discrepancies can be attributed to measurement errors in the instruments used. Figure 11: Three-dimensional finite element overall model [25]. \begin{table}
\begin{tabular}{c c c c c} \hline
**Serial Number** & **Name** & **Natural Density (g/cm3)** & **Poisson’s Ratio \(v\)** & **Modulus of Elasticity (MPa)** \\ \hline
1 & Spinning pile shotcrete & 2.1 & 0.18 & 25.0 \\
2 & Retaining piles, diaphragm walls & 2.5 & 0.20 & 30.0 \\
3 & Bridge structures & 2.3 & 0.22 & 33.5 \\ \hline \end{tabular}
\end{table} 
Table 8: Physical and mechanical parameters of the existing subway viaduct structure and road-deep foundation pit engineering enclosure structure. Figure 12b illustrates the comparison of the final vertical deformation obtained from numerical simulation with the final vertical deformation of the track structure in the upper and lower lines of the existing subway, as well as the final vertical deformation observed in the actual monitoring. The vertical deformation trend observed in the numerical simulation closely matches that of the field monitoring. The deformation occurs at the same location, specifically directly above Pier 7#, where the pile foundation is providing support. The vertical deformation of the bridge track differs between the numerical simulation results and field monitoring data at both ends. This discrepancy is due to the minimal impact of the pile foundation underpinning construction on the structure of the bridge track at these ends, resulting in a small absolute value of vertical deformation. However, the vertical displacement of the track structure is significantly influenced by the dynamic load of the subway train at this time. The model calculation of the train load is based on static load considerations, which leads to a larger influence on the track structure at the two ends of the bridge region. Hence, the simulation outcomes for vertical deformation at both ends of the bridge diverge from the on-site monitoring data. The model calculations show that the vertical deformation of the track directly above the piers, at the location of the pile foundation underpinning, is greater than the deformation observed in the monitoring data. This discrepancy can be attributed to the fact that in the actual pile foundation underpinning the project, the jacking force is applied gradually, and displacement is used as a control criterion. Additionally, the vertical deformation of the track is continuously monitored and controlled in real-time by the ground support system during the jacking process. These real-time control measures cannot be replicated in the numerical simulation. However, it should be noted that the model's calculations for the vertical deformation of the track above the piers with pile foundation underpinning are not accurate. Still, the model's calculations of vertical deformation and the measurements of vertical deformation from the monitoring data at the abutment above the swapping location do not go over the limit that was set. Therefore, it is safe to continue operating the existing subway.