A bored 13.4 m diameter highway tunnel being constructed across the Istanbul Strait in Istanbul, Turkey under a maximum 12 bars of water pressure required a deep excavation on the Asian side of the waterway for launching the tunnel boring machine and initiating NATM tunnels on the opposite end of the excavation. The excavation is up to 38.2 m deep and 173.7 m long in very poor quality sedimentary rock consisting of siltstone/mudstone and sandstone. The selection of design rock parameters considered not only data from site explorations, but previous excavation experience in Istanbul. During initial excavation stages, unexpectedly high lateral movements of the anchored support walls were measured. Using the monitoring data, a back-analysis of the excavation support system was performed to determine revised design rock parameters. The support system was modified by adding additional levels of anchors, and the excavation was successfully completed December 2013.
The Eurasia Tunnel Project is part of a new 14.6-km road link between Kazlicesme on the European side of the Istanbul Strait and Goztepe on the Asian side (Figure 1). The project helps address Istanbul's increasing traffic problems and greatly reduces the travel time across the Istanbul Strait by avoiding the need to use either a ferry or one of the city’s perimeter bridges. The project includes a 3.4-km two-deck tunnel beneath the seabed at the southern end of the Istanbul Strait (also known as the Bosphorus) constructed with a tunnel boring machine (TBM), 2 km of NATM tunnels, and 1 km of cut-andcover tunnel structures. The project also provides roadway improvements for the approaches to the tunnel. The tunnel is designed for passenger vehicles and small trucks with a limited vertical clearance of 3.0 m. The east and west bound roadways are stacked within the 13.2-m outside diameter tunnel. The Republic of Turkey, Ministry of Transport, Maritime Affairs and Communications formulated the project technical specifications. Figure 2 presents a general plan and profile for the Eurasia Tunnel Project. The various underground elements of the project are described by Clark et al. (2015).