Marmaray Bosphorous Rail Crossing

The Marmaray Project is one of the most challenging infrastructure projects in Turkey. The project involved upgrading the commuter rail system in Istanbul, which connects cities on the European side of Turkey with the Asian side, with an uninterrupted, modern, high-capacity, commuter rail system running under the Bosphorus Strait. It’s the world’s deepest undersea immersed tube tunnel. The name Marmaray (Marmara Rail) comes from combining the name of the Sea of Marmara, which lies just south of the project site, with ray, the Turkish word for rail.

Project challenges included working in one of the most famous historical cities in the world, and the selected project alignment that passed beneath the historical and cultural heritages of Istanbul, as well as under ancient and densely inhabited areas of the city. Additionally, the connection between the immersed and bored tunnels was made directly and totally underground, without the usual intermediate shafts, and beneath the deep waters of the Bosphorus Strait. This operation needed the utmost control of the tunnel excavation face to ensure its stability and to minimize water ingress.

The project was constructed as a design-build and was delivered in separate contracts. CALTROP was a subconsultant to the design-build joint venture and responsible for tunnels and stations in the Bosphorus Crossing Contract.

LOCATION: Istanbul, Turkey
CLIENT: Turkish Ministry of Transport
PROJECT VALUE: $1 billion


  • Design-Build Joint Venture
  • Undersea Tunneling
  • Commuter Rail System
  • Historic Site


  • Tunneling and Stations