Two-stage method for analyzing effects on adjacent metro tunnels due to foundation pit excavation

Unloading of foundation pit excavation will no doubt induce the soil deformation and have result in uneven settlements of adjacent metro tunnels. The excessive settlements may induce the crack of tunnel segments and eventually may affect the safety use of metro trains. Two-stage method is presented for determining the longitudinal deformation of metro tunnels caused by adjacent foundation pit excavation. The soil unloading effects of the bottom plan and the surrounding walls induced by the foundation pit excavation can be considered by this method. Firstly the additional stress due to adjacent foundation pit excavation is calculated. Secondly the governing differential equation is built up based on Winkler model. Then the differential equation is deduced to the finite element equation by the Galerkin method; and vertical displacement and internal forces along longitudinal axis are obtained. Furthermore, the parametric analysis of existing tunnels is carried out; such as, the different depths, the distance from the excavation site, the different geologic conditions and the different outer diameter of the tunnel. Finally, the results from present approach are compared with that from 3D numerical simulation and measured data; good agreement is obtained. It may provide certain basis to draw up correctly protective measures of metro tunnels influenced by adjacent excavation.