# Case Study of an Underpinning Pile Foundation for an Interval Tunnel Crossing an Existing Bridge
## 5 Numerical Simulation Result Analysis
### Deformation Force Analysis of Bearing Platform  
(1) Settlement analysis of existing bearing platform  
Figure 16 gives the settlement curves of the two ends of the existing bearing platform at different construction stages, where P1 and P2 are the settlement of the two ends of the existing bearing platform No. 3 (one existing pile truncated), and P3 and P4 are the settlement of the two ends of the existing bearing platform No. 4 (two existing piles truncated). It can be seen that for P1 and P2 during existing pile truncation construction, the ends of the No. 3 existing bearing platform generated uneven settlement. The bearing platform was tilted to the end of the existing truncated pile, which at this time was not conducive for the existing bearing platform to transfer the upper load to the underpinning bearing platform. The settlement curves of P3 and P4 basically overlap, indicating that the settlement of the existing bearing platform No. 4 is uniform, and it is more favorable for the existing bearing platform to transfer the upper load to the underpinning bearing platform. However, the settlement of the No. 4 existing bearing platform is slightly larger than that of the No. 3 existing bearing platform. In general, the settlement law of the existing bearing platform is similar to the settlement law of the bridge pier in each construction stage.  
(2) Force analysis of underpinned bearing platform  
Figure 16: Settlement curves of existing bearing platform at different construction stages.  
Figure 15: Axial forces of existing piles at different construction stages: **(a)** pile 16-5; **(b)** pile 16-1.  
After the underpinning structure system transformation is completed, the super-structure load is mainly borne by the underpinning bearing and is transferred to the underpinning piles. As shown in Figure 17, the diagram shows the tensile stresses applied to the underpinning bearing platform at each stage of construction (damage to concrete occurs predominantly through tensile damage).  
As can be seen from Figure 17, the underpinning structure began to work after the completion of the truncated construction phase of the existing piles, under the joint action of the upper load and the uneven settlement of the existing bearing platform. There was a sharp increase in the local maximum tensile stress of the underpinning bearing platform, with the local maximum tensile stress of the underpinning bearing platforms No. 1, 2, 3, and 4 being 1.48 MPa, 1.69 MPa, 1.76 MPa, and 1.63 MPa, respectively. The local maximum tensile stress of the underpinning bearing platform appears at the junction between the bearing and the pile foundation, and most of the area is below the design value of C35 concrete tensile strength of 1.52 MPa, except for a very small part of the area where the stress is concentrated (refer to the Design Specification for Highway Reinforced Concrete and Prestressed Concrete Bridges and Culverts (JTG 3362-2018)). After the completion of the pile foundation underpinning, the maximum tensile stress curve on the underpinning bearing platform during the tunnel construction phase changes smoothly, and tunnel construction can be considered to have no significant effect on the underpinning bearing platform.