Numerical analysis of effect of large-scale unloading on underlying shield tunnels

The foundation pit of West Square of Hangzhou East Railway Station is the first project that large-scale unloading is conducted on the top of shield subway tunnels. A 3D FEM model is used to simulate the whole excavation course of the foundation pit, and the effect of various adopted construction measures on underlying shield subway tunnel is studied. The construction measures include step excavation, reinforced cushion and reinforcement of soils surrounding the tunnel. Some conclusions are drawn through the comparison between the calculated and the monitoring results: (1) the heave occupieds a dominant position in the tunnel deformation caused by top excavation, and proper design and construction can lead to an desired tunnel deformation; (2) better results will be got by combining excavation by layers and by blocks with building reinforced cushion simultaneously; (3) the reinforcement around tunnels acts as a protective screen of tunnels in ground displacement field, so the tunnel heave decreases substantially .