## 2 Test Design and Result Analysis
### Analysis of Monitoring Results
#### 2.3.3 Deformation of the Track Structure  
To ensure the normal operation of subway vehicles, real-time follow-up displacement monitoring of station track structures is carried out in the process of foundation pit construction. The measured vertical displacement data of the right track structure at each measuring point at different construction time nodes are analyzed, as shown in Figure 13. During the construction period from October 2018 to February 2019, the deformation of the track lines was small, and there was a downward trend. During the subsequent construction period, the deformation of the track line considerably increased, and the overall deformation of the line showed a trend of small deformation at both ends and large deformation in the middle, which was consistent with the deformation trend of the station. The maximum cumulative vertical displacement of the track structure was 1.24 mm.  
Figure 11: Vertical displacement time\(-\)history curve of entrance and exit structures.  
Figure 12: Horizontal displacement time\(-\)history curve of the entrance and exit structures.  
The time-history displacement data of the right track measuring points YGD-4, YGD-7 and YGD-12 are analyzed, as shown in Figure 14. The right track has a stable subsidence trend in the construction stage of the foundation pit retaining structure, and the subsidence error of each measuring point is small. When the vertical displacement of the track is in excavation stages 1 and 2, that is, the excavation depth of the foundation pit is 3.5 and 6.5 m, the vertical displacement of the track maintains a stable downward trend. In excavation stages 4 and 5, when the excavation depth is less than 14.5 m, the vertical displacement of the track has an obvious upward trend. In the pouring stage of the bottom slab, the cumulative displacement increases first and then decreases, and the vertical deformation of the right track is consistent with the deformation trend of the station measuring point.  
Figure 15 shows that the horizontal displacement of the right track continues to increase at all stages of foundation pit excavation construction, and the horizontal displacement deformation trend of each measuring point shifts to the pit. The maximum horizontal displacement of the track is 1.4 mm.  
Figure 14: Vertical displacement time\(-\)history curve of the right track.  
Figure 13: Vertical displacement time\(-\)history curve of each monitoring point of the right track.  
In the process of foundation pit excavation, by extracting the vertical displacement data of the left and right tracks and subtracting the left and right rail data, the horizontal time-history changes of the left and right rails are obtained. As shown in Figures 16 and 17, the horizontal deviation of the right rail is between \(-0.8\) and \(0.6\) mm, and the maximum deviation occurs in the fourth excavation stage of the foundation pit (that is, the excavation depth is \(20.5\) m), which is located at the position of the YGD-8 measuring point (that is, the middle of the right rail). The horizontal deviation of the left rail is \(-0.4\) to \(0.6\) mm, and the maximum horizontal deviation occurs in the fifth excavation stage of the foundation pit (that is, the excavation depth of the foundation pit is \(25.5\) m), which is located at the ZGD-4 measuring point (that is, the middle of the left rail).  
Through the analysis of the manual monitoring data of the track distance of the left and right lines, the maximum deviation value of the track distance in the construction process is \(+2\) mm, less than the deformation control requirements, which meets the allowable deformation control requirements of the track geometry. In the process of foundation pit construction, the deformation control of the track structure is better, and the safe operation of subway trains is not affected in the process of foundation pit construction.  
Figure 16: Right track level.  
Figure 15: Horizontal displacement time\(-\)history curve of the right track.