5.2.3. Underpinning Pile Cutting Stage

As shown in Figure 13, in the process of two piles cutting of C15, the maximum change rate of the bridge pier displacement is 0.06 mm/h, and the cumulative maximum value is 0.18 mm, showing an upward trend. This is because in the jacking stage, due to the existence of the old piles, the jacking force will generate prestress between the old piles and the bridge piers. When the old piles are truncated, the stress is gradually released, increasing the displacement of the bridge piers, but the displacement of the bridge piers tends to be stable at the end, and the magnitude of the change is also within a safe range. The maximum change rate of the pier inclination angle is 0.038◦/h, and the cumulative maximum value is 0.046◦. The maximum change rate of the displacement of the underpinning beam is −0.1 mm/h, and the cumulative maximum value is −0.42 mm, which are both within the safe range. Combined with the analysis results of numerical simulation and field measurement data, it can be seen that the active gantry type bridge pile foundation underpinning technology scheme is highly feasible, and it is worth popularizing

in pile foundation underpinning projects of single-column and single-pile bridges in similar narrow-space curved bridge sections.

**Figure 11.** Displacement monitored during jacking-up stage: (**a**) displacement of new pile; (**b**) displacement of underpinning beam; (**c**) displacement of bridge pier; and (**d**) pier inclination.

**Figure 12.** Displacement monitored during unloading stage: (**a**) displacement of underpinning beam; (**b**) displacement of bridge pier; and (**c**) pier inclination.

**Figure 13.** Displacement monitored during pile cutting stage: (**a**) displacement of underpinning beam; (**b**) displacement of bridge pier; and (**c**) pier inclination.

#### **6. Conclusions**

In this paper, the shield tunnel between Nanchang Metro No. 2 Hongguzhong Avenue Station and Yangming Park Station was taken as the engineering background, and an active gantry-type bridge pile foundation underpinning technology and its specific construction sequence were developed based on the technology comparison and analysis of in-situ engineering conditions. On this basis, for the C15 pile foundation with the most complex working conditions, ABAQUS finite element software was used to simulate the pile foundation's jacking, unloading and pile cutting process, and the displacement variation law of the bridge pier, underpinning beam and new pile during the whole construction process were analyzed. The application of this newly proposed technology was verified through the analysis of the practical construction process on-site monitoring data, and the following main conclusions were drawn:

(1) It can be seen from the changing trend of the displacement of the new piles through numerical simulation that were the same as the bridge pier and the underpinning beam, that there is a relatively stable growth of the displacement in the first five times of jacking, and after the 6th jacking, especially at the new pile near the old pile side, the settlement value in the last jacking was increased by 40% compared with the previous one. The maximum settlement value of the new pile on the side away from the old pile was 2.12 mm, while the maximum settlement value of the new pile was

3.48 mm, which reached the early warning value during monitoring, but did not reach the control value; thus, the focus should be on monitoring the settlement changes of the new piles near the old piles during the jacking process of the jack underpinning the pile foundation;


However, it should be noted that the above conclusions are case-based findings, which are directly related to the engineering conditions. The shield tunnelling parameters and the underpinning schemes should be evaluated and optimized according to the actual engineering conditions of certain cases when the proposed active gantry bridge pile foundation underpinning technology is adopted.

**Author Contributions:** Y.J.: Conceptualization, funding acquisition and writing; N.W.: Data curation, formal analysis and funding acquisition; D.G.: Conceptualization and funding acquisition; F.Z.: Data curation, formal analysis and writing. C.X.: Conceptualization and funding acquisition. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research work is supported by the National Natural Science Foundation of China (Grant No. 42267022, 51768021), the Natural Science Foundation of Jiangxi Province (No. 20202ACBL214016), and the State Key Laboratory of Performance Monitoring and Protecting of Rail Transit Infrastructure Foundation (No. HJGZ2021102).

**Data Availability Statement:** The general data are included in the article. Additional data are available on request.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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