Abstract:

A highway tunnel in Chongqing is a twoway fivelane urban tunnel. The portal section of the tunnel passes through the loose deposits composed of urban construction backfill. During the construction of the left tunnel, the tunnel crown collapsed, and there are some adverse phenomena such as large deformation, support cracking, and surface crack. To explore the causes of the accident and verify the rationality of the modified scheme, the field monitoring results of the original scheme are analyzed. On this basis, the numerical simulation software FLAC3D is used, and the calculation results are compared with the field monitoring data to verify the reliability of the results. Finally, the displacement around the tunnel, the stress of the support structure, and the plastic zone induced by different construction schemes are analyzed. Some conclusions are drawn as follows. (1) In original construction scheme, threebench and sevenstep construction method is adopted, and the support cannot be closed to form a ring in time; whereas the modified construction scheme can effectively control the surrounding rock displacement, and improve the coordination of deformation around the tunnel. (2) In original construction scheme, the primary support shows obvious stress concentration phenomenon, the maximum tensile pressure exceeds the allowable value of the structure, and the surrounding rock stress is large; whereas the advance reinforcement scope is enlarged in the modified scheme, which can reduce the surrounding rock pressure of the primary support and improve the stress state of the support structure. (3) The plastic area of the modified construction scheme is obviously smaller than that of the original one, and the damaged area of the soil is small; the modified construction scheme can reduce the impact of tunnel construction on the surrounding environment. The research results are applicable to the excavation of tunnel portal section under the condition of loose deposits.

Key words: urban tunnel, large crosssection tunnel, tunnel portal section, loose deposits, construction plan adjustment, numerical simulation