# Case Study of an Underpinning Pile Foundation for an Interval Tunnel Crossing an Existing Bridge
## 5 Numerical Simulation Result Analysis
### Deformation Force Analysis of Existing Piles  
As can be seen from above, the most significant impact on the deformation of the bridge superstructure during the construction of the pile foundation underpinning is the truncation phase of the existing piles. It is essential to study the deformation and axial force changes during the truncation of existing piles. Since the existing piles in this paper are mainly located in medium-weathered limestone, which can limit the displacement of the piles in the horizontal direction, this paper focuses on the vertical displacement of existing piles. We take the 16-3 (uncut) and 16-4 (cut) existing piles as an example to compare the deformation law of existing piles without underpinning and with underpinning construction. Taking the 16-5 and 16-1 existing piles as an example, we also compare the deformation and force change law of existing piles when underpinning one existing pile and underpinning two existing piles. (1) Settlement analysis of existing piles 16-3 and 16-4  
Figure 11: Settlement change curve of the rainwater and sewage pipeline. Figure 12: Settlement change curve of the gas pipeline. From Figure 13a, it can be seen that the settlement values of the pile tops of the existing pile 16-3 (untruncated) in the excavation of foundation pit 2, the construction of the underpinning bearing platform, the construction of the truncated existing pile, and the backfilling of foundation pit 2 are 0.1 mm, \(-0.55\) mm, \(-1.22\) mm, and \(-1.58\) mm, respectively. The pile end settlement values are 0.13 mm, \(-0.49\) mm, \(-1.15\) mm, and \(-1.42\) mm, respectively. The pile settlement is continuously varying; the settlement at the top of the pile is greater than the settlement at the end of the pile because it is in the upper-soft and lower-hard stratum (miscellaneous fill and medium-weathered limestone). The 16-3 existing pile settlement changes the most in the existing pile truncation construction stage; this stage of settlement accounts for 0.42% of the total settlement. From Figure 13b, it can be seen that the settlement change in the existing pile (truncated) No. 16-4 in the excavation stage of foundation pit 2 and the construction stage of the underpinning bearing platform is similar to that in Figure 13a, while in the construction stages of the truncation of the existing pile, the backfilling of foundation pit 2, and the breaking of the upper and lower step pile foundation, etc., the settlement change in the pile is discontinuous and can be divided into two parts from the truncated position of the existing pile, with settlement occurring at the top and uplift at the bottom. The maximum settlement of 1.81 mm occurs at the top of the pile. Comparing the maximum settlement of the existing piles in Figure 13a,b, the settlement of the truncated existing piles is slightly larger than the settlement of the untruncated existing piles, and uneven settlement occurs on both sides of the existing bearing platform. (2) Settlement analysis of existing piles 16-5 and 16-1  
Comparing Figure 14a,b, it can be seen that the excavation phase of the foundation pit caused the upward displacement of the existing piles, with pile 16-5 bulging by approximately 0.35 mm and pile 16-1 bulging by approximately 0.2 mm; during the construction stage of the underpinning bearing platform, the existing piles settled under the self-weight of the underpinning bearing platform, and the settlement of the existing piles slowly decreased from top to bottom due to the action of pile side friction. The maximum settlement of the top of the existing pile is approximately 0.54 mm and the maximum settlement of the end of the existing pile is approximately 0.35 mm. With the truncation of the existing piles, the loss of bearing capacity at the end of the existing piles leads to the deformation of the superstructure; at this time, the upper load produces stress redistribution and is transferred to the underpinning structure, which mainly bears the upper structure load.