## 2 Analysis of Construction Impact on Existing Lines  
The area adjacent to the parallel section of the Beijing-Shanghai and Shanghai-Nanjing lines was selected as the on-site test section. The test section is located at DK95 + 650 of the newly built Shanghai-Nanjing Intercity Railway (corresponding to K1243 + 100 of the existing Beijing-Shanghai line), and the old and new lines share a drain groove, as shown in Figure 1. The subgrade of the test section is covered with silty clay ranging from 2 to 6 m.  
Its strata are as follows: (1) \(\text{Q}_{4}\text{al}+\text{pl}\) (Quaternary Holocene alluvial-pluvial layer): silty clay; brownish gray; hard to soft plastic; thickness 0-2 m, \(\sigma_{0}=\) 120 kPa; (2) \(\text{Q}_{4}\text{al}+\text{pl}\) (Holocene alluvial-pluvial): muddy silty clay; gray, soft plastic; thickness 0-11.6 m, \(\sigma_{0}=\) 80 kPa; (3) \(\text{Q}_{3}\text{al}\) (Fourth series update system): silty clay; brownish yellow, hard plastic; thickness 1.1-29.1 m, \(\sigma_{0}=\) 200 kPa; (4) grandoiorite, light yellow and gray, fully weathered. The specific physical and mechanical properties of the soil layer are shown in Table 1.  
Figure 1: Engineering field. (**a**) Subgrade pit; (**b**) adjacent existing lines.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c} \hline \hline
**Soil Layer Name** & \begin{tabular}{c} **Formation** \\ **Thick-** \\ **ness/m** \\ \end{tabular} & \begin{tabular}{c} **Moisture** \\ **Capacity** \\ **\(\omega\)/\%** \\ \end{tabular} & \begin{tabular}{c} **Density** \\ **Peg/cm3** \\ \end{tabular} & \begin{tabular}{c} **Void** \\ **Ratio e** \\ \end{tabular} & \begin{tabular}{c} **Liquid** \\ **Limit** \\ **\end{tabular} & \begin{tabular}{c} **Plastic** \\ **Limit** \\ **\end{tabular} & \begin{tabular}{c} **Plasticity** \\ **Index I\({}_{p}\)** \\ **Index I\({}_{k}\)** \\ \end{tabular} & \begin{tabular}{c} **Impacity** \\ **\(\omega\)/MPa** \\ **\end{tabular} & \begin{tabular}{c} **Constained** \\ **Modulus** \\ **E\({}_{c}\)/MPa** \\ **\end{tabular} & \begin{tabular}{c} **Cohesion** \\ **Friction Angle** \\ **Resistance** \\ **P\({}_{\omega}\)/\({}^{c}\)** \\ **P\({}_{\omega}\)/\({}^{c}\)** \\ \end{tabular} & \
\begin{tabular}{c} **Internal** \\ **Refistance** \\ **P\({}_{\omega}\)/\({}^{c}\)** \\ **P\({}_{\omega}\)/\({}^{c}\)** \\ \end{tabular} &
\begin{tabular}{c} **Specific** \\ **Restistance** \\ **P\({}_{\omega}\)/\({}^{c}\)** \\ **P\({}_{\omega}\)/\({}^{c}\)** \\ **P\({}_{\omega}\)/\({}^{c}\)** \\ \end{tabular} \\ \hline (1) Silty clay & 0.6 & 25.9 & 1.99 & 0.72 & 31.3 & 19.3 & 11.97 & 0.55 & 0.29 & 5.97 & 27.0 & 11.8 & \\ (2) Muddy silty clay & 8.8 & 35.0 & 1.90 & 0.99 & 32.4 & 19.7 & 12.64 & 0.82 & 0.41 & 4.68 & 16.0 & 7.6 & 0.39 \\ (3) Silty clay & 11.7 & 23.1 & 2.02 & 0.66 & 32.7 & 19.7 & 13.04 & 0.27 & 0.14 & 12.8 & 29.2 & 12.5 & 1.8 \\ \hline (a) Full regolith & \(11.05\) & & & & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: The physical and mechanical properties of the soil.  
In order to meet the post-construction settlement control requirements for the high-speed railway, the Shanghai-Nanjing Passenger Dedicated Line adopted a new type of pile-raft foundation. The foundation reinforcement at the adjacent positions of the two lines uses grouted a gravel pile, and the pile top is a raft structure.  
The existing Beijing-Shanghai line was built in the 1970s, and the foundation has essentially been consolidated after many years of operation. During the construction of the new Shanghai-Nanjing Passenger Dedicated Line, a shallow foundation pit was excavated on the side of the Beijing-Shanghai subgrade, which changed the existing line stress field and displacement field; the release of stress had an inevitable impact on the output of the existing subgrade. In addition, the soil-squeezing effect during the pile-forming process and the additional stress generated by the filling of the new line foundation caused different degrees of disturbance to the existing line foundation. The process of the influence is shown in Figure 2.