Golden Horn Unkapanı Highway Tube Tunnel – Istanbul: Underwater bridge design in seismic environment

Abstract

The Golden Horn in Istanbul is being crossed by several bridges, one of them to be replaced by a tunnel. The river has a steep profile with densely populated banks. Because of the planned connections to the boulevards, traditional solutions (immersed tunnel in a trench, TBM tunnel) proved not feasible. Commissioned by the Istanbul Metropolitan Municipality (IBB), Arcadis designed a shallow tunnel above the river bed. Because of the severe seismic conditions, the ability to respond to dynamic soil displacements without leakage or collapse was key in the design. The basic design philosophy was ‘limited flexibility’: flexible enough to follow the movements without build-up of detrimental forces, but limited in order to avoid detrimental joint openings. The reference design was supported by piers consisting of steel tubes founded in the bedrock, 30–50 m below the tunnel axis. The tunnel would consist of short immersion elements, connected to the transition structures with a flexible seismic joint. The contractor followed the same design philosophy, but chose for longer elements to reduce the number of immersion operations. Additionally, the contractor proposed a design alternative in the form of an SFT (submerge floating tunnel), which could have been the very first SFT in the world. Because of changing economic and political situation, the project was put on hold after contract close.The article describes the reference design, the bid design and the design alternative with emphasis on the seismic influence on general concept, details and construction methodology.