# 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
#### 3.4.2 Analysis of Additional Stresses on Existing Tunnels by Double-Line Shield Penetration  
The construction of new tunnels beneath the existing line will cause a redistribution of stress in the surrounding rock of the existing tunnel. This redistribution results in additional deformation of the existing tunnel, along with additional stress, which may lead to cracking of the tunnel's two-lining structure, track misalignment, and significantly affect the normal operation of the existing line. Therefore, this section extracts the Von Mises stresses in the lining structure of the existing tunnel during key construction stages for analysis, as shown in Table 3.  
Figure 8: Vertical deformation rule of left line monitoring point.  
Table 3 shows that the additional stress generated by the excavation of the new tunnel on the secondary lining of the existing tunnel is entirely tensile. As construction progresses, these tensile stresses continue to increase, ultimately reaching 3164 kN/m\({}^{2}\). For the maximum Von Mises stresses, the value rises as shield tunneling progresses, and by the end of the excavation of the new tunnel, the maximum additional Von Mises stress in the existing tunnel reaches approximately 1470 kN/m\({}^{2}\) (1.47 MPa). The minimum Von Mises stress decreases gradually as the left tunnel is excavated, then increases slowly during the excavation of the right tunnel. This is due to the increased tensile stress on the secondary lining of the existing tunnel, which intensifies as the excavation of the right lane disturbs the upper strata further.  
The secondary lining of the existing tunnel is constructed using C50 concrete, with standard compressive and tensile strengths of 32.40 MPa and 2.64 MPa, respectively. Comparing the additional stress values to the tensile strength, it is evident that the increased stress is still below the standard tensile strength. Therefore, the secondary lining continues to meet the tensile load-bearing requirements. However, during the excavation of the new right-lane tunnel, special attention should be paid to monitoring any potential cracking of the secondary lining to ensure the safety of the tunnel's operation. Further analysis of the Von Mises stress values at key positions, including the arch top, arch bottom, and both arch waists at the intersection of the existing left and right lines and the centerline of the new left tunnel, shows that the patterns at these positions remain consistent at each construction stage, as depicted in Table 3.  
Taking the existing right tunnel as an example, during the excavation of the new tunnel, the overall stress is primarily tensile. At the vault position of the existing right tunnel, the stress value increases with the progress of excavation, ultimately reaching 1619 kN/m\({}^{2}\). The stress at the bottom of the arch shows minimal change. The stress at both sides of the arch waist initially decreases, then gradually increases. Therefore, during the construction process, special attention should be given to the secondary lining at the sides of the arch waist and vault to prevent cracking caused by excessive stress.