# Case Study of an Underpinning Pile Foundation for an Interval Tunnel Crossing an Existing Bridge
## 5 Numerical Simulation Result Analysis
### Bridge Superstructure Deformation Characteristics  
(1) Settlement analysis of bridge piers  
As shown in Figure 9a, the vertical deformation of the bridge pier caused by each construction stage is analyzed (where pier 1 indicates the pier corresponding to the No. 1 underpinning bearing platform, as shown in Figure 4). It can be seen from the figure that the excavation of the foundation pit causes the piers to bulge, with a maximum bulge of 0.3 mm (occurring during the foundation pit 1 excavation phase). During the construction stage of the underpinning bearing platform, the settlement of the bridge pier was caused by the self-weight of the underpinning bearing platform, and the maximum settlement was 0.64 mm (bridge pier No. 2). During the existing pile truncation construction stage, the lower pile bearing capacity suddenly reduced, which caused the bridge superstructure to deform, and then the underpinning structure system transformation was completed, in which the existing pile truncation construction caused a large change in pier settlement. The settlement of piers 1, 2, 3, and 4 caused by the existing pile truncation construction stage accounted for 41%, 42%, 43%, and 43% of the total construction stage settlement, respectively. During the tunnel construction phase, the settlement of piers 1, 2, 3, and 4 was stabilized at around 1.6 mm, 2.4 mm, 1.9 mm, and 2.5 mm, respectively, which can be seen to have little effect on the bridge superstructure during this phase. Four piers showed the same settlement trend, among which the superstructure loads of piers 1 and 3 were mainly shared by the underpinning system (underpinning bearing platform and underpinning pile) and the existing piles that were not truncated, so the settlement was smaller, being only 1.98 mm. The existing pile foundation corresponding to pier 2 and 4 needs to be truncated completely, and the entire upper load is transferred to the underpinning bearing platform and underpinning pile, so the settlement value is greater compared with pier 1 and 3. The maximum settlement value appeared in pier 4, with a value of 2.55 mm (<8 mm), meeting the settlement control requirements. (2) Deformation analysis of the bridge deck  
Similarly to Figure 10, which shows the settlement of the bridge deck during the construction of the underpinning pile foundation, it can be seen from Figure 9a that the diaphragm wall and the underpinning pile construction have minimal impact on the superstructure of the bridge; this is therefore not repeated below. From Figure 10b, it can be seen that soil loss during foundation pit excavation causes soil rebound inside the foundation pit, which drives the uplift deformation of the superstructure, and the maximum uplift of the bridge deck is approximately 0.37 mm; as can be seen in Figure 10c, when the underpinning bearing was poured, the new bearing settled under its own weight and  
Figure 9: Settlement of bridge piers at different construction stages: (**a**) calculated value; (**b**) measured value. the maximum settlement of the bridge deck was approximately 0.6 mm. From Figure 10d, it can be seen that the existing pile truncation construction leads to the direct transfer of the superstructure load to the underpinning bearing platform, and that due to the loss of the original pile bearing capacity, the settlement of the bridge deck becomes dramatically large, with a maximum settlement of 2.55 mm. From Figure 10e,f, it can be seen that the backfilling of the foundation pit (equivalent to soil loading) further causes a slow increase in superstructure settlement after the completion of the underpinning system, while the tunnel construction phase has almost no effect on the superstructure. The final settlement of the bridge deck was approximately 3 mm maximum (<8 mm), which satisfied the settlement control requirements.