## 4 Numerical Results and Discussion
### Stability Analysis of Small Clear-Distance Tunnels under Different Excavation Schemes
#### 4.1.2 Deformation Analysis of the Surrounding Rock and Lining  
The target section was selected to analyze the difference in vault deformation between the main line and ramp. Figure 7 shows the vertical displacement variation curve of the main line and ramp vault with the tunnelling process. A rapid decline followed by levelling was observed in the vertical deformation of the tunnel vault, which corresponded to the complete excavation of the main line and ramp in the target section. In terms of displacement, the order was CD method \(<\) double wall heading method \(<\) step method \(=\) full section method, indicating that compared with the full section method, although the CD method increased the construction step sequence, the initial settlement occurred slowly, and the absolute settlement was lower. The double wall heading method could maintain a platform with long construction steps before the curve drop. The comparison of the settlement difference between the main tunnel and the ramp at the vault revealed that the deformation of the main tunnel excavated first was greater than that of the ramp. Figure 8 shows the horizontal displacement of the lining of the main line and ramp under each excavation scheme after the tunnel was penetrated. Figure 8a,b indicate that the horizontal displacement under the step and full section methods was mainly concentrated at the inner hance of the two tunnels. The displacement at the front of the tunnel was large. With the gradual construction of the lining, the horizontal displacement at the rear of the tunnel decreased gradually. Figure 8c,d show that in the schemes with temporary support, such as the CD method and double wall heading methods, the overall horizontal displacement of the lining structure was mainly concentrated on the temporary structural deformation, and the distribution on the primary lining was uniform, which was conducive to the overall stability. Figure 6: Overall horizontal displacement of surrounding rock (unit: m): (**a**) Step method; (**b**) Full section method; (**c**) CD method; (**d**) Double wall heading method. The monitoring points in the target section marked in Figure 9 were selected to measure the deformation of the hance of the two tunnels. Figure 10 presents the horizontal displacement curves of the monitoring points along the main tunnel and ramp as the construction progressed. The horizontal displacement curve of the main line tunnel in the step, CD, and double wall heading methods initially declined and then rose. The analysis showed that when the excavation reached the section near the monitoring point of the main line tunnel, the rock mass near the hance could not be fully excavated to release the stress, and the hance was squeezed during the deformation of the lower rock mass. With the excavation and lining of the lower rock mass, the hance displacement could be controlled after forming a closed loop. The section could be quickly closed due to the one-time  
Figure 8: Horizontal displacement of the lining (unit: m): (**a**) Step method; (**b**) Full section method; (**c**) CD method; (**d**) Double wall heading method. Figure 7: Variation curve of vertical displacement of the vault in the small clear-distance tunnel. excavation and support, and the fluctuation of the horizontal displacement of the hance was low. Given that the double wall heading method involved the temporary support in the horizontal direction, it had the best control effect on the horizontal displacement, and the final displacement was close to 0. The ramp horizontal displacement curve presented a downward trend with the construction step; that is, a trend of left horizontal displacement was observed at the hance. In terms of the final horizontal displacement, the order was double wall heading method \(<\) CD method \(<\) step method = full section method. Figure 10: Horizontal displacement of hance in small clear distance section: (**a**) Step method; (**b**) Full section method; (**c**) CD method; (**d**) Double wall heading method.