Analysis of metro tunnel deformation by upper excavation unloading considering spatial effect in soft soil

The spatial effect of the foundation pit on the tunnel has a great influence on the tunnel deformation. In this paper, a new model of unloading ratio considering spatial effect is developed. It can comprehensively consider the influences of the depth, width and longitudinal length along the tunnel of the upper foundation pit. The three-dimensional finite element method and field measurement method are employed to determine the influence of spatial effect of the upper foundation pit on the tunnel deformation. Numerical result and field monitoring data show that the deformation of the shield tunnel is mainly caused by upper excavation unloading in the main overburden area over the shield tunnel, the construction of foundation pits outside the main overburden area has little effect on the maximum uplift deformation of tunnel, and in the main overburden area the maximum uplift deformation of tunnel is approximately linear with the unloading ratio proposed in this paper. In soft soil area the uplift deformation of the tunnel by upper excavation unloading can be controlled by controlling the unloading ratio.