Numerical Investigation on Effects of Deep Excavation's Position on Existing Metro Tunnels in Urban Areas

Nowadays deep excavations are needed for construction of foundation of high rise buildings, providing space for parking and etc. In some cases deep excavations may be constructed in the vicinity of the subway tunnels and causes unpredicted extra displacements and internal forces in tunnel lining of tunnels which were not designed for them and consequently affect serviceability of tunnel. Therefore in order to avoid structural damages, comprehensive studies should be carried out when the excavation is located in adjacent of tunnel and necessary precautions should be adopted. In this paper 2D FDM software FLAC 2D was used for modeling and investigating the effects of excavations on existing tunnels in sand layer. In this analysis it is assumed that the cross section of tunnel is circular and tunnel exists before deep excavation and also stress relaxation in the soil body around tunnel was taken into account before installation of tunnel lining. In this study different locations of excavation with respect to the tunnel were taken into account and effects of excavation on induced displacements and changes of internal forces of tunnel lining have been studied. Diaphragm wall is used for retaining of excavation and in the analyses Mohr- Coulomb constitutive model is used for soil body. The results of 2D numerical analyses show that when tunnel is located in the outside of the excavation, because of the shielding effect of the diaphragm wall, excavation has less effect on tunnel in comparison with the tunnel which is located below the excavation.