## 4 Monitoring Results and Analysis
For W-2, the corresponding deformation values are \(-0.105\), \(-0.924\), \(-1.618\), and \(-2.186\) mm, respectively. For W-3, the corresponding deformation values are \(-0.069\), \(-0.304\), and \(-0.714\) mm, respectively. The deformation values of level gauges W-1 and W-4 are smaller on the downlink side. When the composite foundation is under construction on the downlink side, the level gauges W-1 and W-3 on the downlink side show a similar pattern to that of W-2 and W-3 during composite foundation construction on the uplink side. Both display large bulge deformation in the level gauge on the uplink side and small uplift deformation in the level gauge of the transition section. When the culvert extension structure is under construction on the downlink side, the deformation of the level gauge on the uplink side decreases abruptly with the construction of each stage. Until the completion of the culvert extension structure under construction, the deformation values of W-1 to W-4 are \(-3.092\), \(-2.451\), \(-0.859\), and \(-1.184\) mm, respectively. Due to the differences in the composite foundation treatment area, culvert extension structure, and construction time on both sides, the deformation values of the level gauge on the upper side of the culvert (W-1, W-2) and transition section positions (W-3, W-4) on both sides are different. The maximum differences appear at the end of the composite foundation treatment on the downward side, and the maximum differences are \(2.873\) and \(0.706\) mm, respectively. Transverse symmetric construction was adopted for the construction of the culvert extension structure at Site 2, and longitudinal and transverse sym  
Figure 10: Relation curve of level gauge deformation with the construction time of each stage (Site 1): (a) composite foundation treatment of uplink side; (b) culvert extension structure construction of uplink side; (c) composite foundation treatment of downlink side; (d) culvert extension structure construction of downlink side. employed to fill the transition sections at Sites 1 and 2. These methods reduced the influence of the deformation value of the culvert's upper side and both sides of the transition section on the existing track horizontal value (shown in Figure 11). The relation curve of level gauge deformation with the construction time of each stage at Site 2 is shown in Figure 12. The analysis shows that the deformation of level gauges W-1 and W-2 on the upper side of the culvert fluctuates with the construction of the composite foundation and shows an overall upward trend until the completion of the composite foundation. The deformation values are 0.676 and 0.877 mm, respectively. The deformation of level gauges W-3 and W-4 increases first and then decreases in the transition section position with the construction of the composite foundation. At the late stage of composite foundation construction, due to the material accumulation in the transition section near the downlink side, the deformation values of the level gauge on the downlink side are lower than those on the uplink side for the upper side of the culvert and the transition section. When the culvert extension structure is constructed, the deformation values of W-1 to W-4 decrease abruptly under the pouring stage of the bottom slab, side slab, top slab, and splayed wall. Until the culvert extension structure is completed, the deformation values of W-1 to W-4 are \(-\)2.636, \(-\)3.009, \(-\)1.109, and \(-\)1.310 mm, respectively, and the deformation values of W-1 and W-2 are significantly higher than those of W-3 and W-4. The maximum difference values of the upper side of the culvert and the transition section position are 0.373 and 0.201 mm, respectively. Combined with Figure 10, the analysis shows that lateral symmetrical construction can significantly reduce the influence of the difference in deformation values between the upper side of the culvert and the transition section position on the horizontal values of the existing line track for culvert extension section construction.