# 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
The slightly larger settlement at the intersection of the existing right line and the new tunnel is due to the same factors that contribute to the settlement at the top of the arch. The settlement trend of the arch bottom of the existing line follows the same pattern as the settlement of the arch top during the excavation of the new tunnel. In this section, we take the existing left line as an example. When the new tunnel first reaches directly underneath the existing right line, the bottom of the arch of the existing left line experiences a slight settlement of 1.05 mm due to the impact of the excavation. As the palm face moves closer directly beneath the existing left line, the settlement rate increases. Once the excavation reaches directly underneath the existing left line, the settlement reaches its first peak of 1.92 mm, and it continues to increase, reaching 4.30 mm by the end of the excavation of the new left line. As the new right tunnel is excavated, the surrounding  
Figure 7: Settlement of existing tunnels. rock stress is redistributed, and the settlement trough begins to show an asymmetric \"W\" distribution, with the peak settlement of 2.16 mm occurring at the intersection of the new tunnel and the existing left tunnel. By the end of the excavation, the settlement at the intersection of the new right tunnel reaches 5.12 mm, and the overall settlement trough forms a \"W\" shape. Further comparative analysis of the settlement values at the arch top and bottom of the existing tunnels, as well as the settlement trough, shows that the final settlement of the arch bottom is larger than that of the arch top. This is because the bottom plate of the existing tunnels is closer to the arch top of the new tunnels. Additionally, the curvature of the elevated arches of the existing tunnels disperses some of the additional stresses, resulting in smaller settlements in areas not directly crossed by the new tunnel. Figure 8 clearly shows the vertical deformation curves of the monitoring points below the left line of the existing tunnel interval under different shield construction steps by the construction of the new left line. According to Figure 8, the monitoring points of each major section show similar deformation patterns. Before the shield excavation of the left line reaches the monitoring point, the vertical deformation of the existing tunnel shows upward floating; when the shield palm surface is more than 30 m away from the existing tunnel interval, it is in the natural excavation state, and the data of the monitoring section are relatively smooth. When the shield machine of the left and right lines advanced to the bottom of the existing tunnel, the monitoring points showed an obvious vertical settlement trend. To control the settlement of the existing tunnel structure, measures such as secondary grouting and radial grouting reinforcement were taken after the shield excavation was completed. Subsequently, due to the installation of tube sheets and grouting construction, the existing structure appeared to be slightly lifted, but as the shield continued to advance, the existing tunnel settlement occurred again until the shield moved away from the existing line, and the vertical deformation of the structure gradually stabilized.