## 3 Results
### Three-Dimensional Model Mesh Simulation Results
#### 3.1.2 Single and Twin Tunnel Completed Numerical Simulation Results  
In the construction step simulation, the y direction dimensions were 50 m, comprising a total of 10 excavation steps of 5 m each. Using the left tunnel as an example for the single tunnel part, the right line of the twin tunnel was simulated using the same construction steps as the left line. The final displacement cloud map for the completed single and twin tunnels is depicted in Figure 5.  
Figure 4: Displacement cloud map for the second construction stage. (**a**) the displacement values in the x, y and z directions after the initial support construction; (**b**) the displacement values in the x, y and z directions after the lower half of the gravel construction; (**c**) the displacement values in the x, y and z directions after the tunnel lining segment construction; (**d**) the displacement values in the x, y and z directions after the upper half of the gravel construction with a half symbol.  
As can be seen from the above figure, following the completion of the left tunnel construction, the maximum displacement in the x-direction occurred at the horizontal axis position of the initial excavation, with \(\mathrm{x_{max}}\) = 3.834 mm. The maximum displacement in the y-direction occurred at the top and bottom of the tunnel in the vertical axis position. The maximum displacement occurred at the initial excavation position, with \(\mathrm{y_{max}}\) = 0.244 mm. The maximum displacement in the z-direction occurred at the top of the tunnel at the initial excavation position, with \(\mathrm{z_{max}}\) = 6.044 mm. All the aforementioned displacement values were within the allowable range. With the completion of the right tunnel construction, the maximum displacement in the x-direction occurred at the horizontal axis position of the initial excavation, with \(\mathrm{x_{max}}\) = 4.114 mm. The maximum displacement in the y-direction occurred at the top and bottom of the tunnel in the vertical axis position, with the maximum displacement occurring at the initial excavation position \(\mathrm{y_{max}}\) = 0.19 mm. The maximum displacement in the z-direction occurred at the top of the tunnel at the initial excavation position, with \(\mathrm{z_{max}}\) = 6.653 mm.  
Figure 5: Tunnel x, y and z direction displacement values after single tunnel completion. (**a**) x direction displacement value of the tunnel after the completion of the left and right tunnel construction; (**b**) y direction displacement value of the tunnel after the completion of the left and right tunnel construction; (**c**) z direction displacement value of the tunnel after the completion of the left and right tunnel construction.  
In this study, the limits of Schedule B.0.2 of the Technical Specification for Safety Protection of Urban Rail Transit Structures (CJJ)/T202) [35] were adopted for the displacement control requirements of this model, as listed in Table 2. According to the simulation results, the displacement values are within the permissible range. Based on the simulation results presented above, it can be observed that during the construction of a single tunnel, the maximum displacement tends to occur at the top, bottom, and waist of the tunnel after completing the initial support, segmental lining installation, and gravel backfilling. The ground settlement gradually increases as the construction progresses, whereas the uplift at the bottom of the tunnel decreased after the tunnel was sealed by installing a segmental lining. Therefore, minimising the duration of the tunnel-sealing process is beneficial for controlling the uplift. Given that the net distance of the tunnel in this project is greater than one tunnel in diameter, after the single-hole and double-hole construction is completed, the location of the maximum displacement shifts; however, the relative location and absolute value of the displacement are closer to the results of the single tunnel.