## 5 Conclusions  
This research was motivated by two unresolved issues: (1) controversies regarding the influence of key parameters (e.g., pile head depth, pile length, and pile-to-pit distance) and (2) the neglect of both horizontal and vertical displacement control effects of isolation piles, used as protective measures to mitigate excavation-induced tunnel displacements. To address these issues, this study proposed a hybrid validation method combining a three-dimensional finite element model (HS-Small constitutive model) with field monitoring data. Additionally, the concept of \"control efficiency\" was introduced to quantify the effectiveness of isolation piles, and a parametric sensitivity analysis framework was established. Building on this foundation, this study integrated the mirror image method and statistical theory to reveal the mechanism by which isolation piles control tunnel deformation, clarify the underlying causes of controversial influencing factors, and conduct a comprehensive and integrated analysis of both horizontal and vertical control efficiencies, emphasizing their critical significance for engineering applications. The following conclusions can be drawn:  
1. Theoretical innovation: dual-path control mechanism  
Tunnel deformation is governed by the synergistic interaction between the displacement blocking effect of isolation piles and the geometric reconfiguration of the tunnel. The dual-path control mechanism operates as follows:  
1. Displacement blocking effect:  
The blocking effect primarily suppresses soil deformation at the location of the isolation piles, while also modifying the surrounding soil's displacement field. 2. Geometric reconfiguration:  
The tunnel's geometric configuration undergoes obvious changes, including increased ellipticity and rotation. Maximum vertical deformation occurs near the tunnel crown, while maximum horizontal displacement is observed near the tunnel springline. 3. Combined influence: Under the combined influence of these two factors, the isolation piles exhibit the following control characteristics: 1. Pile head depth: For a fixed pile length, both the vertical and horizontal control efficiencies of the isolation piles generally decrease with increasing pile head depth or follow an initial increase-then-decrease trend, depending on the pile length and the distance between the pile and the excavation. 2. Optimal control: The optimal control effect is achieved when the pile head depth is zero and the length ratio of the isolation piles to retaining piles is approximately 1. 3. Pile-to-excavation distance: For a given pile length, the control efficiencies exhibit varying trends (e.g., initial decrease-then-increase, initial increase-then-decrease, or monotonic trends) as the distance between the isolation pile and the excavation increases. This dual-path control mechanism provides a comprehensive framework for understanding the complex interactions between isolation piles, soil displacement, and tunnel deformation, offering valuable insights for optimizing isolation pile design in tunnel engineering. 2. Controversy resolution (1) Pile head depth effects: The control efficiency exhibits three patterns--monotonic, unimodal, and oscillatory--depending on the pile head depth and its position relative to the foundation pit and tunnel. These findings reconcile most existing controversies, except for studies with limited experimental data. (2) Pile length effects: The control efficiency trends are determined by the length ratio of isolation piles to retaining piles. Discrepancies in previous studies are attributed to differences in pile length ranges and its position relative to the foundation pit and tunnel. (3) Pile-to-tunnel distance effects: The horizontal control efficiency is influenced by both pile length and pile head depth, with six distinct categories identified. The results provide a clear explanation for existing controversies.