Stability comparison of twin and single tube tunnels in line 1 and 2 of Tabriz metro using 3D numerical modeling

Increasing urban development and social needs for transportation systems bring about construction and extension of urban transportation systems. Considering the fact that the important part of costs relates to excavation and maintenance of tunnels, choosing the single or double tube tunnels is one of the most important decisions in urban tunneling. In case of using single tube tunnel, it should be large enough to accommodate a pair of trains and their appurtenances. However, in case of using twin tunnels, considering that each tunnel is meant to be used for one train, the sections of the tunnels can be smaller. The aim of this paper is to use 3D numerical models for line 1 twin tube tunnels and line 2 single tube tunnel of Tabriz metro to study the various aspects of using twin or single tube tunnels in urban areas. Various criteria have been considered such as: stability of the single and twin tube tunnels, subsidence occurred on the surface and nearby buildings, forces and moments exerted on the twin and single tube tunnels and their factors of safety. In this paper finite difference numerical method (FLAC 3D) has been used to model the tunnels of both lines. The results show that when considering the viewpoint of displacements occurring on the walls of tunnels, using a single tube tunnel with big section will be recommended. However, when the viewpoint of subsidence control and forces and moments acting on segmental linings is taken into account, the use of the twin tube tunnels will be advantageous.