## 2 Monitoring Program and Result Analysis
### Analysis of Monitoring Results of Vertical Deformation of the Pier and Track  
During the monitoring process, measuring points are strategically placed on 21 piers of the current subway viaduct. The resulting vertical deformation of the piers is seen in Figure 8a. The final vertical deformation number indicates that the pier's vertical displacement is within the range of 3 mm. Points 6, 7, 8, 11, and 12 of the piers at the pile foundation underpinning exhibit a vertical uplift deformation. This is primarily due to the jacking operation causing a larger amount of vertical uplift than the settlement during the late pile cutting. Consequently, the piers continue to experience vertical uplift deformation, with a maximum value of 0.8 mm observed on the north side of Point 7 at the pile foundation underpinning. The ultimate deformation observed on the southern side of measuring Point 5, as well as Points 9, 10, and 13 near the road foundation pit, is the settlement. In this context, settlement deformation [26] refers to the phenomenon of displacement or deformation of an object in the vertical direction, usually manifested as a reduction in elevation. The highest settlement value recorded is \(-0.3\) mm, which is observed at measuring Point 13 near the road foundation pit, specifically at the bridge pier. The ultimate deformation observed at measuring Points 4, 14, 15, and 16 of the other pier is settlement deformation. However, the ultimate deformation observed at measuring Point 3 of the pier is vertical uplift deformation, primarily resulting from manual measurement mistakes. During the underpinning stage of the pile foundation, the vertical deformation of the pier at the underpinning point is measured to be an average of 0.65 mm, indicating a vertical uplift deformation. The mean value of the ultimate vertical displacement at the pier measuring site adjacent to the road foundation pit is \(-0.17\) mm, indicating settlement deformation. The mean value of the ultimate vertical displacement of the remaining piers is \(-0.08\) mm, indicating settlement deformation. It can be seen that the influence of pile foundation underpinning on the vertical deformation of pier measuring points of the existing subway viaduct is as follows: pier measuring points at pile foundation underpinning > pier measuring points near the road foundation pit > other pier measuring points. Figure 8b displays the ultimate vertical displacement of the current subway tracks 1 and 2 on the top line, as well as Track 3 and Track 4 on the lower line, as determined from the track vertical deformation monitoring outcomes. Based on the figure, during the pile foundation underpinning stage, the vertical deformation curves of Track 1 and Track 2 on  
Figure 8: Final vertical deformation of the piers of existing subway viaducts [25]. (**a**) The vertical deformation of the piers. (**b**) Comparison of vertical displacement of track. the upper subway line align, while the vertical deformation curves of Track 3 and Track 4 on the lower line exhibit a similar pattern. Additionally, the vertical deformation values for all tracks fall within the range of 3 mm. The vertical deformation trend of the track at the upper and lower lines is essentially identical. The vertical deformation of the upper track, specifically at Piers 6\(\mu\), 7\(\mu\), 8\(\mu\), 11\(\mu\), and 12\(\mu\), is characterized by uplift. This is due to the vertical uplift deformation of the piers caused by the jacking operation at this location, resulting in the uplift deformation of the track structure. The greatest vertical deformation of the rail structure takes place at Pier 7\(\mu\), where the piling foundation is reinforced, aligning with the location of the most vertical distortion. The vertical deformation of the pier and track structure of the current subway viaduct bridge are directly related. This means that the vertical deformation of the pier may accurately represent the vertical deformation of the upper track structure.