Safety Management For A Covered-Excavation Tunnel Traversing Existing Bridges

With the development of city modernization, it is more and more inevitable for the new tunnel crossing the existing bridge. Because the ground deformation and stress relieving generated by the excavation of tunnel are bound to affect the bridge and threaten its safety, it is very important to study on the safety control theory and method of tunnel construction nearby existing bridges.In this paper, based on the domestic and foreign documents, taking the Beijing Subway Line7project as example, the safety control theory and method of tunnel construction nearby existing bridges are studied deeply by the method of combination of theoretical analysis, numerical simulation and field testing. The research results are as follows:(1) Based on the study of the interaction relationship between stratum deformation and single pile foundation, the paper has a detailed description on the deformation characteristics of pile group. Then the shielding effect of pile group is studied with the method of numerical simulation, which shows that the deformation model and stress characteristics of pile group are markedly different from the single pile.(2) The paper elaborates the particularity of the pile group from the two aspects of pile cap and soil arching. When the tunnel is under construction, on the one hand, the pile cap can redistribute the pile load equally and change the load transfer path, on the other hand, the soil arching can make the stress transferring from the soil to the pile and lead to the local stress concentration.(3) Considering the effect of tunnel excavation, the paper establishes the performance function of the pile bearing capacity and calculates the failure probability of the pile group with the matlab program.(4) According to the analysis of the bridge characteristics under differential pile settlement, the paper comes up with a new method of determining the bridge settlement standard considering the damage of the bridge.(5) The safety control theory of tunnel construction nearby existing bridges is elaborated from the aspects of traditional and active protection. Meanwhile, the paper proposes the new interaction relationship of tunnel-soil-pile-superstructure, and the active protection method which is based on the bridge jacking technology.(6) The dynamic control system of tunnel construction nearby existing bridges is established. The implementation technical process of the management system is also formed. The management process emphasizes the determination of the risk status and the confirmation of the response measures, which can help us determine the risks in advance and take measures immediately.(7) The traditional protection measures cannot compensate the early settlement of the pile foundation, so the paper comes up with the new complete technology of the bridge jacking. The new bridge jacking technology can be divided into five steps, namely, the design of the lifting program, the layout of lifting system, the arrangement of the monitoring system, bridge lifting, and the clean-up of the site.(8) Faced with the characteristics of the shallow tunneling method, the paper proposes the new dynamic control method of back-analysis, which combines the deformation distribution method and the back-analysis method. The new dynamic control method can realize the scientific warning, hierarchical control and active coping, which can avoid the risk accumulation greatly.