# Case Study of an Underpinning Pile Foundation for an Interval Tunnel Crossing an Existing Bridge
## 1 Introduction  
With the development of urban rail transit and underground tunnel engineering in China, tunnel engineering has gained popularity as an effective way to relieve the pressure of surface traffic. During the construction process of a large number of urban tunnels, there will be situations where construction will need to be carried out through the bottom foundation of an existing building or bridge. Pile foundation underpinning technology is often used in engineering to ensure the safety of the original superstructure in the construction process, specifically referring to the expansion of the foundation of existing buildings or foundation reinforcement. In particular, research on tunnel underpass bridge pile foundations has been a hot topic in recent years [1]. Previous studies on pile foundation underpinning have focused on theoretical empirical formulations [2; 3; 4; 5; 6], indoor similar model tests [7; 8; 9; 10; 11; 12], and numerical simulations [13; 14; 15; 16; 17; 18; 19]. Specifically, [PERSON] [2] proposed a new excavation scheme for urban high-rise buildings, and the theoretical equations of lateral frictional resistance and end resistance of existing piles influenced by adjacent boreholes were derived. [PERSON] [3] developed a computational model for the effect of foundation pits on the dynamic impedance of supported piles, considering the effect of stress history. [PERSON] [7] conducted indoor model tests based on the pile foundation underpinning design scheme. The changes in existing piles and underpinning piles(pile vertical displacement, pile axial force, and lateral friction resistance) before and after underpinning were studied. [PERSON] [10] simulated the construction process by indoor model tests, studying the pile foundation underpinning the construction process and the pile bearing characteristics. [PERSON] [13] used a three-dimensional finite element method to compare and analyze the effectiveness of the protection scheme in mitigating the impact of tunnel excavation on the adjacent group piles. The results show that the pile foundation underpinning scheme is effective in reducing the structural settlement and ground deformation of the bridge. [PERSON] [16] detailed the key technologies for pile foundation underpinning construction in tunnels, and conducted theoretical analysis, numerical calculation, and field monitoring to verify the reasonableness of the method. [PERSON] [19] used the engineering example of a double-line shield tunnel crossing Fengqi Bridge in Hangzhou Metro Line 2, discussed strengthening options for Fengqi Bridge, and analyzed the monitoring data before and after the shield crossed the bridge. There have also been many studies on underpinning structural load-bearing systems. [PERSON] [20] used the beam wrapped pile cap method for pile foundation underpinning construction. Due to the limited technical conditions at that time, the effect of the connection between the old and new concrete after its construction could not be effectively monitored. He [21] concluded that the passive pile foundation underpinning project is feasible and reasonable when applied in an eight-story frame structure building. [PERSON] [22] used numerical simulation and theoretical derivation to investigate the reasonable excavation exposure length of pile foundations during the construction of pile foundation underpinning, the force conversion mechanism of a piled raft system, and the influence of shield cut piles on the superstructure. [PERSON] [23] used underpinning beams as the underpinning structural load-bearing system; at the same time, in order to prevent the cracking of the underpinning bearing platform concrete, several prestressing steel strands were placed inside it. [PERSON] [24] used an underpinning beam as a load-bearing structure for the underpinning structure. Numerical simulations of pile foundation underpinning construction and tunnel crossing construction processes were carried out using finite element software.