## 4 Numerical Realization of Zero-Distance Construction Process
### The Mechanism of Influence of the Excavation Method  
Figure 15 shows the vertical deformation curves of the existing tunnel floor at each stage of the excavation under the four working conditions. For the convenience of the analysis, for conditions 1 and 4, since the full-face two-bench method is used for excavation, it is considered Step 1 when the soil is excavated through the middle wall of the existing tunnel, and Step 2 for the end of the excavation. In Condition 2 and Condition 3, which chose the double-pilot tunnel method of excavating the two pilot tunnels first and the middle soil later, the excavation of two pilot tunnels passing through the middle wall and the end of excavation are Step 1 and Step 2, respectively, and the excavation of the middle soil mass passing through the middle wall and the end of excavation are Step 3 and Step 4, respectively. Figure 14: Deformation mechanism of existing tunnel during tunnel support. It can be seen that at the end of the tunnel support stage, the settlement of the tunnel in Condition 1 and Condition 2 with the L-type beam is about 0.45 mm, and that in Condition 3 and Condition 4 with the pier column is about 0.25 mm, which has a better settlement control effect. For the full-face excavation method (Condition 1 and 4), the final settlement is relatively larger (both about 0.3 mm larger) than the conditions of different excavation methods of the same tunnel support method, and the final settlement is about 3.50 mm and 2.75 mm, respectively. When the excavation exceeded the middle wall of the existing tunnel (Step 1), the settlement of the existing tunnel exceeded half of the total settlement. It can be seen that the settlement of the existing tunnel developed rapidly during the initial stage of excavation. In Conditions 2 and 3, the double pilot tunneling method was used for excavation, and the final settling was about 3.1 mm and 2.5 mm, respectively. When the pilot tunnels are excavated (Step 1 and Step 2), the middle soil mass is not excavated, and the structure near the center of the existing tunnel is still supported by the lower soil mass with small settlement. By comparing the difference between the settlement values at the end of tunnel support and the end of excavation, it is apparent that when the L-type beam is used, the settlement of the existing tunnel during the excavation of the undercrossing area is greater than that of the existing tunnel when the pier column is used. It is also known that when the earth is excavated, the pier columns also have a good effect on the settling control of the existing tunnels. The above analysis can indicate that the L-type beam has a relatively weak ability to resist deformation, and its deformation control effect on the existing tunnel during the tunnel support stage and underpass excavation stage is worse than that of the pier column. The pier columns, however, exhibit a much stronger stiffness, and can effectively support the existing tunnel and prevent further deformation during the excavation of the earth for the underpass. Figure 16 shows the deformation curve of the lateral wall of the underground area during excavation. Since the lateral displacements on the south and north sides are almost symmetric, only one side displacement is shown in the figure. The meaning of Step 1 to 4 in the figure is the same as Figure 15. We can see that the maximum lateral displacement of the side wall under the four working conditions in the figure is almost the same, which is about 3.5 mm. Figure 16a,d shows the conditions of full-face excavation. When the soil is excavated to the middle wall of the existing tunnel, the deformation of the side wall reaches about 2.5 mm, which exceeded half of the total deformation. Figure 16b,c shows the  
Figure 15: Deformation curve of existing tunnel during excavation (longitudinal). exavation conditions of the double-pilot tunnel method. By the time the excavation reaches Step 2, two pilot tunnels are excavated.