# Influence of Approaching Excavation on Adjacent Segments for Twin Tunnels
## 6 Conclusions  
A series of numerical simulations and on-site monitoring have been conducted in this paper to analyze the influence of approaching excavation on adjacent segments for twin tunnels in Beijing, China. The effects of the approaching excavation on the ground displacement and segment internal forces were discussed. The contrastive analysis was performed with two construction schemes. The achievements of the present study are possible to be drawn as follows:  
1. The deformation of the measured segment is asymmetric with respect to the tunnel axis due to the influence of the overcutting. Under the influence of lateral pressing effect induced by approaching excavation, a pronounced asymmetric and increased ovalization is presented on the measured segment when the second tunnel approaches the measured cross-section.
2. As the second tunnel approaches and keeps away from the measured cross-section, the peak value of the ground settlement at the monitoring cross-section always shifts toward the second tunnel, and the ground nearby the tunnel axis on both sides always moves outward. Under the influence of construction loadings, soil loss and ovalization of the segment, the approaching excavation reduces the horizontal displacements of the ground between twin tunnels and increases the ground settlements as well as the horizontal displacements of the ground on both sides.
3. When the second tunnel is far from the measured cross-section, the profiles of internal forces in the measured segment are asymmetric with respect to the curved tunnel axis, which is mainly affected by the overcutting. After that, the asymmetric characteristics become more striking due to the approaching excavation. As the second tunnel approaches and keeps away from the measured cross-section, the normal force increases gradually, whereas the bending moment first decreases and then increases. The maximum increments of normal force and bending moment are both located at the regions near the second tunnel.
4. For the curved tunnel and straight-line tunnel required for excavation in this project, the tunneling case of first along the curved trajectory and then along the straight-line trajectory will result in smaller internal forces of the segment. However, the construction case of first along the straight-line trajectory and then along the curved trajectory will result in a smaller horizontal displacement of the ground between twin tunnels.  
Figure 16: Comparisons of safety factors.  
* The variations of strain increments obtained by numerical simulation agree well with the monitored results. The impact of the approaching excavation is significant on the measured segment at the tunnel springline and shoulder regions, which are mainly subject to the circumferential compression and axial tension.  
[PERSON] and [PERSON] prepared the concepts and designs; [PERSON] provided the original data; [PERSON] conducted the on-site monitoring: [PERSON] analyzed the data and performed the numerical simulation. All authors participated in edited, reviewed, and improved the manuscript. All authors have read and agreed to the published version of the manuscript.  
The financial support for this work was provided by the National Natural Science of Foundation of China (Grants No. 51978018 and 51978019), which are gratefully acknowledged.  
The authors appreciate [PERSON] and [PERSON] and [PERSON] at the Beijing University of Technology for providing the facility and assistance.  
The authors declare no conflict of interest.