**1. Introduction**

With the rapid urban development in China, an increasing number of metro tunnels are being constructed in major cities to alleviate traffic pressure and improve transportation efficiency [1–3]. According to statistics offered by China Association of Metros, there are more than 50 metro cities in China, and the total length of metro lines in operation exceeded 7209.7 km up to December 2021. Benefiting from unique advantages, such as high automation in facilities, fast construction speed, and little disturbances to ground traffic [4–8], the shield technique has been widely used in metro tunnelling. However, since metro lines usually go across densely populated urban areas, shield tunnel excavation inevitably disturbs the surrounding environment, leading to ground uplifting or settlement [9–13], further inducing deformation and even damage of adjacent structures, including buildings, bridges, buried pipelines and existing tunnels [14–17]. Hence, how to minimize the interference of shield tunnel construction process on adjacent structures has been a key issue in

**Citation:** Zheng, F.; Jiang, Y.; Wang, N.; Geng, D.; Xu, C. Investigation on the Influence of Active Underpinning Process on Bridge Substructures during Shield Tunnelling: Numerical Simulation and Field Monitoring. *Buildings* **2023**, *13*, 241. https:// doi.org/10.3390/buildings13010241

Academic Editor: Bartolomeo Pantò

Received: 5 December 2022 Revised: 9 January 2023 Accepted: 11 January 2023 Published: 15 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the field of geotechnical engineering, which has attracted more and more attention from engineers and scholars [18–21].

As one of the most typical engineering cases, once the existing pile foundations of the adjacent buildings and structures intrude into the designed shield tunnel line, the intruded piles are usually cut off and new piles are constructed, acting as an underpinning foundation to continuously support the upper structures [19,22]. In this underpinning process, the overlying load of the upper structures should be smoothly transferred to the underpinning pile foundation to ensure the stability of the buildings and the safety of shield digging [16]. It is a very complex construction process involving complicated interactions between the tunnel, soil, and pile foundation [23], which merit detailed investigation.

In recent years, many efforts have been made by researchers to study the mechanical responses during pile foundation underpinning and the optimization of underpinning schemes through laboratory experiments, numerical simulation, and field tests [16,24]. Stulgis et al. [25] revealed that the pile foundation should be replaced before the destructive settlement of the building by installing grouting micro-piles in the replacement area to ensure the safety of the structures based on the case study of expansion project of St. Joseph Mercy Hospital in Georgetown, Guyana. Ma and Wang [26] theoretically analyzed the bearing capacity of a single pile and the joist deformation according to the structural design and found that the pile foundation underpinning method in the Xi'an Metro shield tunnel project was valid. Yan et al. [27] obtained the formula of shear force of a pile foundation underpinning structure through theoretical analysis and field model tests, which enabled calculation of the shear bearing capacity. Taking Beijing Metro Line 8 as an engineering case, Yao et al. [27] numerically analyzed the pile foundation underpinning scheme using FLAC3D software to improve the effectiveness of the underpinning in reducing the deformation of structures and isolating piles. Xu et al. [22] proposed a pile underpinning technology for shield tunnel cross through group pile foundation in a shield tunnel interval in Shanghai Metro Line 10, and improved the rationality of the scheme by theoretical and numerical analysis as well as field monitoring tests. Park et al. [28] proposed and verified the application of a modified underpinning method in the new subway #9 line in Seoul Metropolitan in South Korea, which was able to reduce construction period by 1.5 times and the construction cost by 1.2 times compared with conventional methods. In addition to the aforementioned studies on pile foundation underpinning schemes, many scholars have focused on the pile cutting process [19,29,30]. Fu [31] numerically investigated the feasibility of the direct shield cutting pile foundation construction technology in cutting plain concrete, glass fiber concrete, and reinforced concrete, based on the mechanical response of the shield cutterhead as well as the tunnelling parameters are analyzed. Chen et al. [32] analyze the effect and mechanism of large-diameter pile cutting process during shield tunnelling and obtained the characteristics of cutting parameters and the damage law of cutting tools through field tests. To achieve stable and effective pile cutting process, a new cutterhead configuration as well as pile cutting scheme were proposed by [33], suggesting that the shield advance rate should not exceed 2 mm/min, and the rotation speed should be controlled at a relatively low level.

In the pile foundation underpinning–cutting scheme, dynamic control of the displacement of superstructure and the underpinning foundation is crucial to the safety and efficiency of this process [16], which still remains to be explored in detail. Moreover, complex geological conditions and differences in the surroundings often make it hard to determine an appropriate underpinning scheme. Considering that several single-column and single-piles in the upper Bayi Bridge invaded the tunnel, the shield tunnel section pile foundations from Hongguzhong Avenue Station to Yangming Park Station of Nanchang Metro Line 2 need to be underpinned, which will inevitably have a great impact on the substructure of the bridge and the surrounding engineering environment, and the construction risk is extremely high. Therefore, in this paper, based on the analysis of on-site working conditions and the comparison of technical solutions, an active gantry type bridge pile foundation underpinning technology was proposed, and the corresponding specific

construction steps were designed. On this basis, for the pile foundation C15 with the most complex working conditions, the 3D finite element ABAQUS numerical simulation was used to analyze the underpinning process. Moreover, the influence of three key stages of the underpinning scheme, i.e., jack lifting, unloading and pile cutting on the bridge pier, underpinning beam and new underpinning pile were studied in detail, and the feasibility and safety of the underpinning technology were preliminarily verified. Finally, by monitoring and analyzing the three important stages of jacking, unloading and pile cutting of the C15 pile, it was further verified that the technical solution is reliable and worthy of further promotion and application.
