## 4 Impact of Pile Foundations Underpinning Construction on Existing Structures  
### Finite Element Modeling  
During the construction of the pile foundation underpinning, the upper subway continues to operate normally. However, due to the strict deformation requirements of the  
Figure 14: Comparison of the final transverse deformation of the pier on the north side. (**a**) Comparison of lateral deformation of bridge abutments. (**b**) Comparison diagram of transverse deformation of track. existing structure, active underpinning is used. This involves designing and calculating the jacking force of the piers and columns. Upon calculation, the maximum design load of the single pile of the underpinning pile is 11,400 kN, that is, about 1140 t. The hydraulic jack's tonnage should exceed the load it bears by approximately 30%. Thus, this project suggests using four jacks to replace the single pile simultaneously. Each jack should have a tonnage greater than 370.5 t. Because multiple jacks will be used simultaneously for jacking, there may be an uneven load distribution. Therefore, it is necessary to have a sufficient margin of jacking force. Hence, this project proposes using four 400-ton jacks to jack the single pile. Additionally, square pads with a side length of 400 mm are recommended for this purpose. When using a jack, Figure 15 displays the arrangement and design of the jacks. Based on the jack's plan layout, a finite element model of the pile foundation underpinning construction was created. Analyze the deformation impact of the underpinning project on the existing subway's viaduct structure and track structure to verify the feasibility of the design parameters and jack layout design. (1) The current subway is only designed for normal use and does not take into account earthquake or human defense conditions during tunnel excavation and construction. (2) The existing subway structure is assumed to behave like a linear elastic material. (3) It is assumed that there is proper coordination of deformation between the pit structure, the existing subway structure, and the surrounding soil. (4) The construction is carried out under well-controlled conditions. The parameter values are comparable to those in Section 3. The soil's physical-mechanical parameters are presented in Table 7, while the structural physical-mechanical parameters are displayed in Table 8. Given the impact of spatial factors in construction and the positional relationship between the foundation pit's abutment underpinning and the existing subway viaduct, it has been concluded that the current model's dimensions are 390 m \(\times\) 185 m \(\times\) 100 m. Given the strong correlation between the deformation of the track structure and the deformation of the bridge girder, a specific cross-section is chosen to investigate the deformation of the subway track structure. The numerical model includes a shared node between the surrounding pile and the soil, as well as a contact element between the bridge pile and the soil. To streamline the model calculation, the current project is partially simplified. The numerical model's boundary condition designates the ground surface (the uppermost surface of the model) as the free boundary. Normal constraints are applied around the model and at the bottom. All pile foundation underpinning construction models utilize the Solid45 element. In Ansys, the calculation model is comprised of a total of 476,275 elements and 82,954 nodes. The 3D finite element model closely resembles the schematic diagram presented in Section 3. Additionally, Figure 16 illustrates the finite element model of the subway viaduct structure. The construction process is segmented into four distinct stages: The construction involves the use of retaining piles and pile foundation underpinning. A \"kill\" element is employed to simulate the excavation of the pit. Additionally, the replacement piles under the replacement abutment are subjected to an upward design top force to simulate the jacking operation.