# Analysis of the Impact of the New Two-Lane Shield Tunnel Underpass on the Existing Tunnels
## 3 Numerical Simulation of Underpass Construction
### Analysis of Simulation Results
#### 3.4.1 Influence of Two-Lane Shield Underpass on Vertical Settlement of Existing Tunnels  
Figure 6 shows the overall vertical deformation distribution of the existing structure after the completion of the new tunnel underpass construction. As can be seen from the figure, the existing tunnel shows a trend of settlement, the maximum settlement occurs in the center of the tunnel, and the size of the settlement deformation gradually decreases from the center of the tunnel to both ends of the tunnel. By intercepting the deformation data from different locations in the longitudinal direction, we analyzed the influence of the shield tunnel on the existing tunnel structure during the construction process of the near-connection underpass. As shown in the figure, the horizontal coordinate is the distance from the center line of the new double-lane tunnel, and the vertical coordinate is the settlement value of the arch top and arch bottom of the existing tunnel. In Figure 7, (1) to (6) represent the six important construction stages in the shield construction process. For the vault, whether it is the left line or the right line, its settlement trend is generally similar, and ultimately shows a normal distribution; the settlement groove curve is 'U' shape, the left and right lines of the maximum settlement are in the center line of the new two-line tunnel position, and the final settlement is 6.07 and 5.85 mm, respectively. The reason for the settlement displacement of the left tunnel arch being slightly larger than that of the right line is that the soil under the existing right line was excavated first, and its additional displacement value is naturally larger, which is also consistent with the actual engineering situation. At the beginning of the excavation, no significant settlement change was observed at the vault of the existing right line. As the palm face advanced, the settlement trend became more apparent. When the excavation of the new right line reached directly below the existing left line, the settlement displacement at the center of the tunnel of the new left line peaked at 1.23 mm. As the palm face moved further, the displacement accumulated slowly, and by the time excavation reached the lowest point of the existing right line, the settlement reached 1.57 mm. With the excavation of the new right line, the soil above the existing tunnel began to show signs of biased pressure, causing the settlement trough of the existing tunnel to shift from the axis of the new right line toward the axis of the new left line. Simultaneously, the settlement of the vault of the existing right line continued to increase. When the new right line reached directly below the existing tunnel, the settlement increased to 4.00 mm, and by the end of the new tunnel excavation, it reached 5.32 mm. Figure 6: Cloud image of sedimentation deformation. Additionally, analysis of the existing right line vault settlement data showed that as the palm face reached directly below the existing right line, the additional displacement accumulated gradually, and when excavation reached the lowest point of the existing right line, it amounted to 1.57 mm. The settlement rate was larger at the right line, so the construction process should focus on monitoring the lining deformation at the intersection of the axes of the new and existing tunnels to ensure the smooth progress of construction and the safety of operations. For the vertical displacement analysis of the existing line's arch bottom, the deformation trends of the left and right tunnels are largely similar. By the end of the excavation, the arch bottom settlement exhibits a \"W\" distribution, with the largest settlement values occurring at the intersection of the existing right line and the left and right axes of the new tunnel. The values are 5.17 mm and 5.08 mm, respectively, while the settlement values for the existing left line are 5.12 mm and 5.04 mm.