A case study on zoned excavation of a large foundation pit in proximity to existing tunnels

摘要

As available ground resource in congested urban areas in Shanghai is scarce, a number of excavations have to be carried out in close proximity to existing tunnels. It is of great concern to not only ensure the safety of excavation itself but also to minimize the adverse excavation-induced influences on adjacent tunnels in services. The performance of a large excavation for a basement covering an area of 23,000 m~2 near existing tunnels is presented in this paper. Zoned excavation technique was utilized to construct the foundation pit. The behaviour of the excavation and corresponding tunnel deformations have been investigated based on three-dimensional finite element analyses and extensive field monitoring. Hardening soil model with small-strain stiffness (HSS model) whose parameters were determined and calibrated against comprehensive laboratory tests was applied. The field monitoring results show that taking advantage of the zoned excavation procedure the lateral wall displacements were smaller along the nearby tunnels than those at other sides. The measured maximum horizontal tunnel movement was found to be 7 mm, which was within acceptable limits. Numerical analyses considering the small strain stiffness of soil give rise to fairly reasonable results as compared with the measured results.