The use of pre-support in shallow tunnels is increasing in urban regions, with the purpose of improving excavation stability and reducing ground settlements induced by tunneling. In addition, in the shallow tunnels, evaluation on optimal pre-support system will be considered intend of stabilization of face tunnel and decreasing of caused by settlement. The Concrete Arc Pre-Supporting System is one of the new methods that it uses in some tunneling project now. In this paper, 3D modeling for Tohid twin tunnels using FLAC3D was performed. This analysis was carried out to investigate:
the convergence at the tunnel cavity and the deformation at the tunnel face, and
surface settlement after the installation of the Concrete Arc. The numerical results indicate that the tunnel roof formed by the Concrete Arc Pre-Supporting System provide a restraining effect on the tunnel induced displacement and helps to reduce the tunnel deformation and corresponding ground surface settlement. Ground settlement directly above the crown could reduce by 50–60% of the settlement produced without any Concrete Arc reinforcement.
Many tunnels must be constructed in weak materials, such as sands and clays. These tunnels tend to be at shallow depths and near urban infrastructure so the construction method must minimize the surface settlements. Ground loss at the tunnel is consequentially translated into an equivalent surface depression especially in cohesive soil and tunneling in shallow ground (Attewell et al., 1986). The effect of settlement due to shallow and soft ground tunneling is hazardous to nearby buildings, infrastructures and existing services. New methods of reinforcing tunnel excavation and reducing the effect of ground loss at the tunnel are to create a boundary of high stiffness between the ground surface and the tunnel. A new method of reinforcing tunnel excavation and reducing the effect of ground loss at the tunnel is to create a boundary of high stiffness between the ground surface and the tunnel. The Concrete Arc Pre- Supporting System is one of methods that it uses in some urban tunneling project now. This method based on used of rib in roof and around tunnels. The numerical results indicate that the tunnel roof formed by the Concrete Arc Pre- Supporting System provide a restraining effect on the tunnel induced displacement and helps to reduce the tunnel deformation and corresponding ground surface settlement (Bengt, Stillborg, 1979)
Due to recent city developments with limited available land to build on, more and more public facilities are developed under the ground surface. Complex underground constructions may cause serious damage to existing structures if predictions do not forecast the ground behavior during excavation.