Deformation and Control of Bridge Pile Adjacent to Excavation Under Superimposed Disturbances of Shield Tunneling and the Top-Down Method

The construction of deep foundation pits for urban metro systems, particularly when in close proximity to existing sensitive structures, can easily induce unacceptable settlement and deformation. When shield tunneling and the top-down method are superimposed in both space and time, the resulting disturbance to the surrounding environment is more complex. A metro station project in Shenzhen was selected as the case study. Field monitoring and numerical simulation were combined to analyze the deformation of nearby bridge piers during different construction stages. The settlement-control effects of different reinforcement measures were also compared. The results indicate that the maximum settlement of the bridge piers throughout the monitored construction period reaches 31.81 mm. In the third stage, the cumulative settlement exceeded 10 mm. After the left-line shield passed through the station, the differential settlement between adjacent piers exceeded 5 mm. In the fourth stage, upon completion of the south section of the negative third floor, the differential settlement within the same pair of piers exceeded 4 mm. The maximum differential settlement between adjacent piers on the side closer to the foundation pit is 8.92 mm, whereas that on the side farther from the pit is 12.53 mm. The maximum values of both the differential settlement between adjacent piers and that within the same pair of piers occur during stages where multiple construction processes overlap. The connections between successive construction procedures are thus identified as weak links in deformation control. The discrepancy in cumulative settlement between symmetrically located piers arises primarily from the different reinforcement methods adopted. Supplementary pile reinforcement provides better performance than sleeve-valve-pipe grouting reinforcement. Because its reinforcement depth is insufficient to extend below the foundation pit excavation surface, the latter method fails to effectively restrain deformation in deep soils. The results describe the spatio-temporal development of pier deformation under combined construction disturbances. They also help identify critical construction stages and compare the settlement-control effects of different reinforcement measures. In addition, the numerical results provide a possible interpretation of pile load-transfer behavior under the combined effects of shield tunneling and top-down excavation. These findings serve as a practical reference for the design, construction sequencing, and risk management of similar metro station projects adjacent to bridges.