Settlement control of highway roadbed under shallow tunnel underpass construction conditions
[Objective]The settlement of roadbeds caused by the construction of shallow large-span tunnels passing under highways in soft strata has always been one of the outstanding technical difficulties encountered in underground engineering.The key to controlling the settlement of highway roadbeds is to restrain the deformation and stress release of tunnel surrounding rock as much as possible.This study thoroughly analyzes the scheme and process of controlling the settlement of roadbeds and the deformation of surrounding rock in the construction of tunnels under highways.Furthermore,it analyzes the settlement pattern and influencing factors of the top arch and roadbed in the tunnel excavation process.[Methods]Based on the Xianglushan Tunnel Project,this study adopts on-site monitoring and numerical simulation and conducts comparative experiments through four design and construction schemes to analyze the influences of systematic support,over-advance support,and construction process on the settlement and deformation of the tunnel vault and roadbed.[Results]The preconvergence deformation of the surrounding rock in front of the palm face of the shallow-buried large-span tunnel accounted for a relatively high percentage of the total displacement of the top arch.It accounted for about 79％ of the total displacement of the top arch in the construction of the Xianglushan Tunnel under the highway.After applying overrun support,the extrusion deformation of the tunnel face and the preconvergence deformation of the surrounding rock in front of the tunnel were substantially reduced,the stability of the surrounding rock of the tunnel was improved,and the highway foundation settlement was reduced.The highway foundation settlement was reduced by about 26.4％,while the tunnel vault settlement was reduced by about 31.3％.Compared with strengthening the initial support parameters,the adjusted construction approach was more effective in controlling the settlement of highway roadbeds.The adjusted construction approach,which included measures such as reserving core soil,temporary arches with vertical supports,and staggered excavation on the same step,effectively restrained the release of surrounding rock stress and deformation,maximized the bearing capacity of the support structure.Compared with the four-step method,the improved construction method resulted in a 9.2％decrease in the roadbed settlement of the Shanghe Highway,a 30％increase in the average axial stress of the anchors of the tunnel system,and a 52.91％increase in the average axial stress of the steel arch.[Conclusions]In summary,the optimized design and construction program achieves better application results,effectively controlling the settlement of the tunnel vault and roadbed of the Shanghe Highway during the construction process of the Xianglushan Tunnel Project.Research findings provide valuable insights into the construction of tunnels beneath highways in similar complex environments.
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