## 5 Analysis of the Numerical Results
### Longitudinal Settlement of the Ground Surface  
#### 5.1.1 When C/H = 0.764  
Figure 8 displays the final vertical settlement trends of the ground surface corresponding to different D/W values at C/H = 0.764. In the figure, the abscissa represents the construction distance and the ordinate represents the settlement value. Furthermore, A-A represents the model shown in Figure 5a, 10 kPa represents the surface load magnitude, \"htm\" denotes that the existing tunnel was not backfilled, and \"hty\" denotes that the existing tunnel was backfilled. It is noteworthy that the model disallowed octahedral meshing due to the small angle skew of tunnel and the geometrical characteristics of the shallow burial and slope adjacency. Meanwhile, the change in the tunnel position will force a remeshing by the ADINA software since the model enabled the face-link forced-mesh continuity function. In other words, the mesh density was mandatorily and automatically adjusted by the software rather than being specified by manual code. It was thus normal for the simulation curves to have slight fluctuations. As shown in Figure 8a, the surface settlement value at the boundary was rather large due to the boundary effect of the model, which can thus be ignored in the analysis. In the \"htm\" condition, the vertical settlement curves attributed to different surface loads were basically the same in shape. The settlement value increased with the increasing surface load, and the maximum settlements corresponding to 10 kPa, 20 kPa, and 30 kPa loads were 2.7 mm, 5.2 mm, and 7.8 mm, respectively. The settlement values in the \"hty\" conditions were very close to those in the \"htm\" condition, suggesting that the backfilling had a very limited effect on the surface settlement because the contour of the existing tunnel was included in the new tunnel at the initial excavation stage. When the existing tunnel began to intersect with the new tunnel, the backfilling of the existing tunnel directly affected the effective breadth of the tunnel, which thereby affected the adjustment of the secondary stress and displacement fields of the surrounding rocks. Therefore, this area was defined as the sensitive zone. Figure 7: Schematic of data extraction locations. The maximum settlement difference caused by the tunnel being backfilled was located at 65 m (at a tunnel excavation length of 35 m), where the existing tunnel was just separated from the new tunnel, as shown in Figure 7a. For the three surface loads of 10 kN/m\({}^{2}\), 20 kN N/m\({}^{2}\), and 30 kN N/m\({}^{2}\), the settlements following backfilling of the existing tunnel decreased by 63.2%, 48.5%, and 41.5%, respectively, as compared to the non-backfilling condition. This fully proved that the existing tunnel backfilling was effective in controlling the surface settlement. After a further increase in the distance between the existing and new tunnels, the existing tunnel no longer produced a significant effect on the surface settlement; therefore, this area was defined as the unaffected zone. The ground settlement law shown in Figure 8b was basically consistent with Figure 8a. However, since the tunnel began to shift toward the slope direction, the location with a maximum difference in the influence of existing tunnel backfill on surface settlement was precisely the site at which the existing tunnel just separated itself from the new tunnel. At this point, the tunnel excavation distance was approximately 26 m, as shown in Figure 7b. Additionally, the difference in surface settlement caused by the existing tunnel being backfilled for tunnel excavation was more pronounced than that in Figure 8a within an excavation range of 90-100 m due to the slight reduction in the length of the existing tunnel that was included in the excavation face. Figure 8: Longitudinal surface settlement of models A-A to C-C. Finally, a comparison with Figure 8c shows the emergence of a new law regarding the settlement trends.