## 4 Numerical Realization of Zero-Distance Construction Process
### Deformation and Stability Analysis of the Surrounding Rock on Either Side of the Underground Area  
The lateral deformation curves of the rock and soil masses on either side of the subsurface region are shown in Figure 9. With the unloading caused by the excavation of the soil mass in the undercrossing area, and the additional stress exerted on the rock and soil mass on both sides by the part of the upper existing tunnel extending into the soil mass, the side walls of both sides move to the middle. It can be seen that the lateral displacements of the walls on both sides of Scheme III are the largest, about 4.0 mm and 4.3 mm, respectively; Scheme I takes the second place, about 3.4 mm and 3.5 mm, respectively; and Scheme II is the smallest, at about 2.9 mm and 3.2 mm. The lateral displacement of the strongly weathered sandstone layer in the upper part of the undercrossing area is relatively large, and that of the moderately weathered sandstone layer in the lower part is relatively small. The overall lateral displacement curve is in a \"spoon\" shape, and the inflection point is located at the junction of the moderately weathered and strongly weathered rock layers. Compared with the other two schemes, there is a \"sharp point\" in the lateral displacement curve of Scheme III near \(-\)3 m. This is because Scheme III uses steel support and lacks to support the existing tunnel floor within the range of 0\(-\)\(-\)3.2 m, and no side wall is constructed (as shown in the figure of Step 6 in Table 3). In this range, only the role of soil grouting reinforcement and bolt support is considered.  
Figure 10 shows the profile cloud map of the vertical displacement of the undercrossing area and surrounding rock and soil along the longitudinal direction of the existing tunnel. For the sake of comparison, the cloud graph gradients for each scheme are set to be consistent. It can be seen that after the excavation of the rock and soil mass in the undercrossing area, the rock and soil mass at the bottom have a tendency to have due to unloading, and the amount and influence range of the heave at the bottom of Scheme I are relatively large. In addition, the surrounding rocks on both sides of the undercrossing area and the soil above the existing tunnel show different degrees of settlement, and the settlement range is a \"bowl\" type. We can see that the settlement of the existing tunnel in Scheme I caused a certain disturbance to the rock and soil on both sides, resulting in large settlement of the rock and soil on both sides, while the rock and soil on both sides of Scheme II and Scheme III were affected in a small range. In addition, it can be seen that the floor slab of the main structure of the undercrossing area in Scheme III also has a  
Figure 9: Lateral deformation of rock and soil mass on both sides of undercrossing area.  
large vertical displacement, which is because the bottom of the jack supporting the existing tunnel is supported on the lower main structure, and the main structure bears the load generated by the settlement of existing tunnels and has a collaborative deformation with the existing tunnel.  
As can be seen from the above, the three schemes use almost completely different tunnel support methods and underground excavation methods, resulting in significant differences in the construction effects of each scheme. It is difficult to draw conclusions about the key elements of tunneling settlement control by simple induction. Therefore, it is  
Figure 10: Vertical displacement cloud chart of undercrossing area (unit: m).  
necessary to further investigate the selection and optimization of support and excavation methods for existing tunnels that require zero-distance undercrossing.  
Comparison and Optimization of Support and Excavation Methods for Undercrossing the Existing Tunnels at Zero Distance