Preliminary design and analysis for immersed tube tunnel across the Izmir Bay

Abstract

In this study, a preliminary design and analysis of an immersed tube tunnel is presented. The tube tunnel will connect the two coasts of the İzmir Bay and whereby will ease the transportation of the city. The reason to suggest an immersed tube tunnel is due to the shallow water depth (<25 m) and that the soil profile of the İzmir Bay is made up of silty-sand. Hence, the Bay is appropriate for an immersed tube tunnel.First, a possible alignment was assigned for the tunnel. The technical, geometric properties of the tubes were determined, and the detailed drawings of them were made.The allowable bearing capacity of the seabed was calculated and it was determined that the soil has not enough capacity to withstand the design load. The liquefaction risk of the soil was investigated as well, and it was shown that the soil has high liquefaction potential.A static analysis of the tunnel was made in Calculix, a finite element program.The vertical displacement of the tube unit under static loads was calculated to be above the permissible settlement value. Afterwards, the seismic analysis was made to investigate stresses developed due to both racking and axial deformation of the tunnel during an earthquake. It was found that, the max stress due to the racking effect is less than the compressive strength of the concrete, and max stress due to the axial deformation is larger than compressive strength of the concrete. The high in the tube occur, because of the tubes high stiffness. This problem was solved by releasing the rigid connections in between two tube units. If these connections are made by using same form of elastomer joints, the deformation will occur in these joints, releasing the tubes internal stresses.Considering these drawbacks, ground improvement was recommended for the seabed and an increased value of the standard penetration of the soil was estimated.Then, the analyses were repeated and it was found that all drawbacks were eliminated.As a conclusion, it was decided that if suggested improvements are made in the seabed soil, the immersed tube tunnel can be constructed across the İzmir Bay.