## 5 Results Analysis  
### Analysis of Different Foundation Reinforcement Schemes  
#### 5.1.1 Foundation Settlement  
Figure 8 shows the foundation settlement after twin-line tunnel excavation under two different foundation treatment conditions. It can be observed from Figure 8 that the two different reinforcement schemes have a minimal impact on foundation settlement. The maximum foundation settlement under different foundation treatment conditions  
Figure 7: Ground surface settlement curves of tunnel excavation.  
differs by only 0.7%. This also implies that the reduction coefficient in the reinforcement scheme has a minimal effect on foundation settlement. Figure 8 shows that the maximum foundation settlement occurs at the tunnel crown, approximately 16 mm. The maximum foundation uplift occurs at the tunnel bottom, around 14 mm. The maximum settlement of the road surface without culverts is 9 mm, occurring in Area I, while the maximum settlement of the road surface in the culvert section is within 2 mm, occurring in Area II. The presence of culverts effectively reduces road surface settlement.  
Figure 9 shows the variation in settlement at the foundation bottom beneath the axis of the steel corrugated pipes with the distance from the tunnel excavation. It can be observed from the figure that there is no significant difference in settlement between the two reinforcement schemes. The difference in settlement caused by a 10% variation in the deformation modulus between the two schemes is within 1 mm. This is because the JG-0 design reinforcement scheme has a higher composite foundation-bearing capacity, and the cover soil above the steel corrugated pipe arch and the self-weight load of the culvert are not significant. Therefore, reducing the deformation modulus by 10% in the JG-1 reinforcement scheme does not result in significant settlement changes. It can be seen from Figure 9 that the settlement at P1 ZD is greater than at P4 ZD for both conditions. This is because the P1 ZD point is located at the edge of the steel corrugated pipes and is more influenced by the shield tunnel excavation. For the P1 ZD monitoring point, when the tunnel excavation reaches a distance of 7.3 m from the monitoring point, there is a sudden change in settlement at the monitoring point. After the tunnel excavation passes through, the settlement at the monitoring point slows down when it reaches a distance of 4.4 m from the point. This is because that the foundation modulus is different on both sides of the P1 ZD monitoring point, with the foundation modulus in the reinforced area being higher than in the non-reinforced area. Therefore, the influence range of tunnel excavation on the weaker foundation is greater when it is above the reinforced area.  
Figure 10 shows the settlement at the foundation bottom along the excavation profiles of the 1-hole and 4-hole (axis profile locations indicated by the green lines in Figure 6). The maximum settlement at the 1-hole is approximately 2.5 mm greater than at the 4-hole. The settlement trough in the area enclosed by the red dashed lines does not follow a Gaussian distribution. This is because this location is situated at the edge of the steel corrugated pipes, where the soil treatment methods on either side are different. The difference of physical properties in soil leads to the appearance of a steep slope of settlement. Therefore, the maximum settlement has shifted by approximately 5 m to the right tunnel after the completion of the twin-line tunnel excavation, essentially aligning with the axis of the right tunnel.  
Figure 8: Foundation settlement after twin-line tunnel excavation under two different foundation treatment conditions. (**a**) Condition JG-0; (**b**) condition JG-1.