# Case Study of an Underpinning Pile Foundation for an Interval Tunnel Crossing an Existing Bridge
## 6 Conclusions  
This paper investigates the effect of using passive underpinning construction on existing bridges when undercrossing existing bridges in interval tunnels. The main findings are as follows:  
1. By combining numerical simulation and measured data, we compared the settlement of the foundation pit and bridge piers caused by the main construction stages. The numerical simulation results and the measured data results were consistent, which proves the accuracy of the numerical model. 2. The settlement change law of the bridge deck is expressed as follows: the foundation pit excavation stage, equivalent to the foundation pit bottom unloading; the foundation pit bottom uplift drives the bridge superstructure to uplift; and the underpinning bearing platform construction, existing pile truncation, and foundation pit backfill stages cause the bridge deck to sink, with the existing pile truncation construction stage causing the largest settlement of the bridge deck, accounting for approximately 40% of the total settlement during the entire construction process. At the same time,  
Figure 17: Maximum tensile stress of underpinned bearing platform at different construction stages. tunnel undercrossing construction removes obstacle piles, the impact on the bridge superstructure is very limited, and the settlement of the bridge deck tends to stabilize, so the impact of tunnel construction on the existing bridge can be ignored. 3. Through the comparison of the settlement of existing piles requiring underpinning and existing piles not requiring underpinning in the same existing bearing, we found that the settlement of the truncated existing piles was larger than the settlement of the untruncated existing piles, which caused uneven settlement on both sides of the existing bearing platform. In the comparison of underpinning one existing pile and underpinning two existing piles, we found that the superstructure load of the former is shared by the existing pile without underpinning and the underpinning structure; the settlement of the latter is larger, and the superstructure is completely borne by the underpinning bearing and the underpinning pile. 4. The settlement law of the existing bearing platform and that of the bridge pier are similar. Further analysis of the forces in the underpinning bearing reveals that the maximum tensile stress is generated during the truncated construction phase of the existing pile, with a maximum local tensile stress of 1.76 MPa. It appears at the junction of the existing bearing and existing pile, and most of the area is lower than the design value of C35 concrete tensile strength of 1.52 MPa. The reinforcement at the junction of the existing bearing and existing pile should be strengthened during construction to meet the safety requirements for the force of the underpinning bearing platform. 5. When using passive underpinning and an underpinning bearing platform as construction solutions for underpinning the structures of pile foundations, the settlement of the upper bridge structure, the internal force of the underpinning bearing platform, and the settlement of the surrounding pipelines are all within safe ranges. It is therefore reasonable and feasible to use passive underpinning for similar projects. Conceptualization, Y.Z. ; methodology, Y.Z. and Y.L. ([PERSON]); software, Y.Z. ; validation, Y.Z. and Y.L. ([PERSON]); investigation, Q.C. ; resources, Y.L. ([PERSON]); writing--review and editing, Y.Z. ; visualization, X.O. and Y.L. ([PERSON]); supervision, Y.L. ([PERSON]). All authors have read and agreed to the published version of the manuscript. This work is supported by the Guiyang Metro Line 3 Phase I Engineering Research Project (Project No. GD3-FW-YJ-05-2020-13-ZB, to [PERSON]), the Science and Technology Planning Project of Guizhou Province (Qianke Basic Project ZK [2022] General Project 082, Qiankehe Basic Project [2019] No. 1057, and Qiankehe Major Special Project [2018] 3011), and the Regional First-Class Discipline Construction Project in Guizhou Province (QYNYL [2017] 0013).