## 6 Conclusions  
(1) When parallel to the tunneling direction, the simulations revealed that a critical interval length must exist between the two tunnel lines such that the longitudinal surface settlement and slope deformation were significantly affected by the existing tunnel backfill. In this study, the area below the critical interval length was referred to as the longitudinal sensitive zone, within which the existing tunnel backfill had a positive effect on reducing the surface settlement. In the present case, the effect of the existing tunnel backfill began to weaken and even disappear after the split distance of two tunnel lines was longer than 2.5 m. To save on project costs, the existing tunnel caverns in this zone may not be backfilled.  
(2) Within the longitudinal sensitive zone perpendicular to the tunneling direction, the surface deformation presented a basically increasing trend with the decreases of the C/H and D/W ratios. When the specific value between the two ratios exceeded 5.2, an opposite trend of intensified surface settlement with an existing tunnel backfill will occur. An example is model C-C (C/H = 0.764 and D/W = 0.16) in the case studied herein. Meanwhile, the simulations revealed that the changes in the equivalent span and burial depth of the two tunnels were essentially attributed to the existing tunnel backfill and the positional variations of the tunnels. This, in turn, affected the shape of the surface settlement trough. When the geometric outline of an existing tunnel with a smaller cross section was contained completely by a larger tunnel, the location of the maximum horizontal surface settlement was controlled jointly by the spatial positions of the two tunnels. After a decrease of the geometric outline overlap zone in the increasing direction of the burial depth, the maximum horizontal surface settlement was controlled primarily by the tunnel with a larger cross section.  
(3) Based on a comprehensive consideration of the upper limit criteria for the surface and slope deformations, model A-A (\"hty\" condition) was identified as the only feasible model among all models. Finally, the simulations of model A-A were compared with the monitored outcomes. As the results show, the tunnel face approaching phase was a decisive stage that affected the deformation of the surrounding rocks and slopes. Due to changes in the surface loads, the monitored outcomes  
Figure 25: Comparison of final road settlement monitoring results.  
of the horizontal surface settlement differed slightly from the simulations in the distribution pattern. Nevertheless, overall, the final maximum settlement completely complied with the construction control requirements. This not only proved the validity of the simulation results, but also suggested an additional redundancy for the degree of project safety.  
Conceptualization, Z.H. and C.L.; methodology, Z.H.; software, C.L.; formal analysis, Z.H.; data curation, Z.H.; writing--original draft preparation, Z.H. and C.L.; writing--review and editing, J.C.; supervision, Y.L.; project administration, Q.H.; funding acquisition, J.C. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Key Program of National Natural Science Foundation of China (51638002).  
The authors declare no conflict of interest.