## 1 Introduction
This research acknowledges that these deformations do not always follow the same pattern; indeed, they may even exhibit opposing trends. Focusing on only one direction at a time could lead to incomplete or misleading conclusions, especially when isolation piles have different influences on the two displacement directions. As shown in Figure 1, This study investigates the vertical and horizontal displacements of existing tunnels in soft soils resulting from excavation, utilizing 3D Finite Element Method (FEM) simulations that are validated through field monitoring data. The analysis included soil displacement and tunnel deformation with and without isolation piles. A comprehensive parametric analysis was then conducted. The working mechanism of isolation piles in mitigating both horizontal and vertical displacement of the tunnel was identified by analyzing the influence of isolation piles on the soil displacement and tunnel deformation at different cross-sectional locations. Based on the mechanism, the influences of pile head depth, pile length, and pile-to-pit distance were systematically analyzed. Additionally, the controversies on the influence of isolation pile on tunnel deformation were figured out. This study ultimately emphasizes the importance of considering both vertical and horizontal tunnel displacements in unison. The findings provide valuable insights for optimizing isolation pile design and construction practices, enhancing the effectiveness and resilience of isolation piles. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Influence Factors** & **Methods and Sources** & **Analysis Details** & **Conclusions** \\ \hline \multirow{8}{*}{Pile head depth u} & Range of u: 0–25 m. Pile length: 50 m. & j initially increases, then \\  & Numerical simulation by & Pile position: Close to the tunnel. & decreases, and later increases \\  & [PERSON] et al. [13] & Others: The difference between the & again before ultimately \\  & & depths of pile head and pile toe & decreasing with increasing u. \\  & & is constant. & \\ \hline \multirow{8}{*}{Pile head depth u} & Range of u: 0–6 m. Pile length: 22 m. & \\  & Numerical simulation by & Pile position: The middle section of the & \\  & Numerical simulation by & excavation and tunnel. & j initially decreases and then \\  & [PERSON] et al. [14] & Others: The difference between the & increases with increasing u. \\  & & depths of pile head and pile toe & is constant. \\ \hline \multirow{8}{*}{Indoor tests by [PERSON] et al. [15]} & Range of u: 0–0.2 m. Pile length: 0.7 m. & \\  & Pile position: The middle section of the & \\  & & excavation and tunnel. & \\  & & Others: The difference between the & \\  & & depths of pile head and pile toe & is constant. \\ \hline \multirow{8}{*}{Pile length l} & Range of the length ratio between & \\  & Numerical simulation by & isolation piles and retaining piles: & j initially increases and then \\  & [PERSON] et al. [5] & 0.6–1.1 & decreases with increasing l. \\  & & Pile position: Close to the tunnel. & \\ \hline \multirow{8}{*}{Pile length l} & Range of the length ratio between & \\  & Numerical simulation by & isolation piles and retaining piles: & j initially decreases, then \\  & [PERSON] et al. [17] & 0.2–1.3 & \\  & & Pile position: Close to the & again with increasing l. \\  & & foundation pit. & \\ \hline \multirow{8}{*}{Theoretical analysis by & Range of the length ratio between & \\  & Theoretical analysis by & isolation piles and retaining piles: & j initially decreases and then \\  & [PERSON] et al. [10] & Pile position: Close to the & increases with increasing l. \\  & & foundation pit. & \\ \hline \multirow{8}{*}{Pile-to-tunnel distance d} & Range of d: 0.5–0.8 times & \\  & Theoretical analysis by & excavation-to-tunnel distance. & \\  & [PERSON] et al. [10] & Length ratio between isolation piles & \\  & & and retaining piles: 0.8. & \\ \hline \end{tabular}
\end{table} 
Table 1: Influences of pile head depth, pile length, and pile-to-tunnel distance on the horizontal displacement j of the tunnel.