## 3 Effect of Pit Construction on the Deformation of Existing Structures  
### Finite-Element Modeling  
To assess the impact of an instance of road-deep foundation pit building on the existing subway viaduct bridge and track structure, a finite element model of the construction process is created. The numerical model includes a shared node connecting the surrounding pile and the soil, as well as a contact element linking the bridge pile and the soil. The numerical model's boundary conditions consider the surface as the free boundary, with normal constraints applied around the model and at the bottom. The dimensions of the model are 390 m in length, 185 m in width, and 100 m in height. The material specifications can be found in Tables 7 and 8. To simplify the model calculation, certain parts of the deep foundation pit project for the new road are simplified. The remaining sections of the project, except for the initial ground construction and the removal of the temporary road surface system, are constructed using the paving method. The construction process for these sections is the same as that for the pile foundation underpinning section and the adjacent subway bridge pile section. Consequently, the modeling process does not take into account the installation and removal of the military beam temporary pavement system. The model simulation employs the equivalent stiffness method to equate the retaining piles with diaphragm walls. The pile foundation supporting the section pit retaining piles can be considered equivalent to a 768 mm diaphragm wall with a length of 16.5 m. Similarly, the pile foundation supporting the section pit adjacent to the subway can be considered equivalent to a 768 mm diaphragm wall with a length of 24.2 m. The road's deep foundation pit engineering model exclusively utilizes the Solid45 material. In Ansys, the calculating model is partitioned into 1,029,972 elements and 175,323 nodes. Figure 11 displays the comprehensive 3D finite element model. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Name** & **Density (g/cm\({}^{3}\))** & **Angle of Internal Friction** & **Cohesion (kPa)** & **Poisson’s Ratio** &
\begin{tabular}{c} **Modulus of** \\ **Elasticity (MPa)** \\ \end{tabular} \\ \hline Miscellaneous fill & 1.75 & 5 & 10 & 0.3 & 12.0 \\ Powder clay & 1.96 & 18 & 25 & 0.3 & 30.0 \\ Chalky soil & 2.01 & 23 & - & 0.33 & 32.0 \\ Fine-to-medium sand & 2.08 & 30 & - & 0.24 & 80.6 \\ Clay & 1.84 & 14.5 & 23.6 & 0.26 & 21.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Physical parameters of the soil layer. Figure 10: Ultimate transverse deformation of existing viaduct piers [25]. **(a)** Final transverse deformation map of the measured points of the subway viaduct abutments. **(b)** Final lateral deformation map of the track structure. The construction process is comprised of six steps. Firstly, the deep foundation pit of the road in the pile foundation underpinning section is excavated until reaching the bottom plate. Secondly, the deep foundation pit of the road in the neighboring subway section and the remaining section are excavated to a depth of 7.0 m. Thirdly, the deep foundation pit of the road is excavated to a depth of 4.2 m. Fourthly, the deep foundation pit of the road is excavated to a depth of 3.0 m. Fifthly, the deep foundation pit of the road is excavated down to the bottom plate. The entire building procedure utilizes the \"kill\" element to replicate the excavation of the road's deep foundation trench. Comparison and Analysis of Monitoring Results and Simulation Results of Bridge Piers and Track Structures  
(1) Comparative analysis of vertical deformation  
The numerical simulation results in Figure 12a display the final vertical deformation of the bridge pier during the pile foundation underpinning, which is compared to the actual vertical deformation observed in the field. The observed maximum vertical uplift deformation value of the pier in the field is 0.8 mm.