Numerical Modeling for Construction of Large Span Bifurcations of Niayesh Tunnel, Iran Maziar Hosseini Assistant Professor, Post Graduate School, Department of Civil Engineering, Azad University, Tehran South Branch, Iran; email: maziarhosseini121@yahoo.com Aliakbar Golshani Assistant Professor, Department of Civil Engineering, Tarbiat Modares University, Tehran, Iran; email: hgolshani@hotmail.com Sina Majidian Ph.D. candidate, Department of Civil Engineering, Tarbiat Modares University, Tunnel design engineer, P.O.R. Consulting Co, Iran; email: sina_en412000@yahoo.com ABSTRACT The main goal of this research is to study influence of rib elements in construction of bifurcations and increasing the construction safety for the bifurcation with large span in one of major tunnelling projects in Tehran (Niayesh tunnel). The bifurcation structure and the surrounding soil are modelled together simultaneously. Analysis were performed in two different Scenarios (1- With rib elements; 2- without rib elements) in order to reach a safe design. Both the 2D and 3D approaches are considered for simulation. The results of numerical modelling show that construction of rib elements accompanied by NATM stages for tunnel excavation reduces the surface settlement and also mobilized forces in initial lining. Therefore, combination of rib elements and NATM tunnelling stages was chosen as the design method for bifurcations in the Niayesh tunnel. It is found that the critical section for the design of the bifurcation is not the largest section. Also, due to the construction of ribs and existence of soil column at the location of connection between 3lane and 1lane sections, the initial lining acts as a two directional slab and bending moments are mobilized in both transverse and longitudinal directions and both directions have to be designed against the mobilized forces. KEYWORDS: Tunnel, bifurcation, NATM, rib, numerical modelling I NTRODUCTI ON In recent years, by growing demand for transportation in metropolises, the application of tunnels has become increasingly common, particularly to provide sufficient space to solving traffic problems. Therefore, Tunnels have important or perhaps key role in the redevelopment of urban areas. On the other hand, tunnel construction in urban area also have problems that the main concerns of tunnel construction in populous regions are the excavation instability and - 1457 -