Concreting huge tunnel elements for the Bosporus Crossing

An important part of the Marmaray Project in Istanbul is the 1.4km tunnel segment which lies on the floor of the Marmara Sea at the Southern end of the Bosphorus. This segment consists of enormous prefabricated concrete elements which are lowered to a depth of 58m, sealed and interconnected. The tunnel links the European suburb of Halkali with Gebze on the Asian side of Istanbul. The tunnel will be connected with the existing rail and metro network via 9.8km of driven tunnel sections, three new underground stations and 37 stations located above ground which are currently being modernised. Concreting of the eleven 135m-long, 15.3m wide and 8.6m high tunnel segments is being conducted in a dry dock on the Asian side of Istanbul in the suburb of Uskudar. Approximately 5000m3 of concrete in varying strength classes (up to C40) is required for each tunnel segment. The 30,000 tonne reinforced concrete constructions are towed into position above the excavation in the seabed and are lowered into position. Large rubber seals are used to provide a watertight connection between the segments. The top of the tunnel element is subsequently protected against damage by a 4.5m sediment layer. Earthquake safety measures include special protection for the connection point of each individual segment under water. where the Marmaray Tunnel merges with the land tunnel, particularly powerful, flexible and elastic rubber seals are used which will not break in the event of an earthquake. A shield tunnelling process with tubing support is used for the land tunnels where there are soft ground conditions and in the other sections the inner formwork is concreted using stationary Putzmeister concrete pumps. Other sections a built using an open construction method.