A tunnel between the continents

The route of the "Istanbul Strait Road Tube Crossing Project" runs around 100 meters below sea level at its deepest point. There the water pressure is about 11 bar: eleven times atmospheric pressure. Never before has such a large, high-performance tunnel been built underground under such complex and extreme conditions. The interior diameter of the tunnel is 12 meters so that two lanes in each direction can be accommodated. They extend one above the other on two levels. To avoid obstructing ship traffic the tunnel was dug with a specially designed tunnel boring machine (TBM). 3.34 kilometers of the first road tunnel between the Asian and European parts of Istanbul, with a total length of 5.4 kilometers, were created by a Herrenknecht Mixshield.

Relief for city traffic

The sheer size of the tube did not make construction easier. But only with this diameter can the Eurasia Tunnel fulfill its task: accommodating several ten thousand cars a day. A figure that has far-reaching consequences for traffic in the city. Commuters from the south in particular, who had to take the chronically congested First Bosporus Bridge in the center so far, can cross to the other side directly now. In rush hour that can shorten the travel time of many from up to 100 minutes down to 15.

The more northerly Fatih Sultan Mehmet Bridge, the second traffic bottleneck between the continents, is also relieved. According to plan the tube helps to get the slow-moving traffic in the 18-million metropolis flowing again. A circumstance that has a further advantage in addition to a huge time saving for the three million Bosporus commuters: a significant reduction in CO2 emissions in the city.

The all-rounder among tunnel boring machines

Tunnelling in the heterogeneous geology was conduced with a TBM type Mixshield. The Mixshield technology allows an advance of 8 to 10 meters in 24 hours even in the extremely challenging environment under the Bosporus. The cutting wheel was equipped with different tools for all eventualities: large cutting knives scratched softer subsoil from the tunnel face, from where – combined with a water-clay mixture – it was flushed through a hydraulic slurry circuit to the separation plant in the launch shaft.

When the machine encountered harder solid rock, the 35 twin disc cutters did most of the work. The 19-inch discs, weighing 1 tonne each including the cutterbox, were pressed against the tunnel face by the displacement cylinders of the cutting wheel drive and rolled off it. Individual pieces, so-called chips, broke out of the rock. Smaller pieces were sucked through an intake screen and removed by the flushing cycle. The rest was crushed by a jaw crusher in front of the screen which – like a powerful hydraulic nutcracker – crumbles even large boulders effortlessly. At the same time, the hydraulic circuit was used for building up and regulating the so-called face support pressure against the water and ground pressure of the Bosporus. A principle developed further by Herrenknecht that has proven itself worldwide countless times.

High-tech TBM for the first Bosporus road tunnel

To detect strong material wear early and to tackle necessary maintenance accesses in a targeted manner, wear detectors were integrated into the excavation tools as well as in the steel construction of the cutting wheel. Moreover, the disc cutters were equipped with the DCRM system (Disc Cutter Rotation Monitoring) which was developed by Herrenknecht. It provides data about the rotational movement and temperature of the disc cutters in real time to the machine operator in the control container. Thus, conclusions can be drawn regarding the condition of the tools and change intervals can be better planned. 

True teamwork and high tech unite Asia and Europe

The tunnelling for the "epoch-making project" was launched on the Asian shore of the strait in April 2014. With pioneering technology and an ideal cooperation of all project partners the TBM accomplished best performances of up to 92 meters per week under the Bosporus. After only 16 months the Mixshield pierced the target shaft wall on the European side exactly to plan on August 22, 2015. 

"State-of-the-art technology is one of many puzzle pieces. More decisive, however, was our unshakeable willpower and mutual trust in each other. Only through these have we jointly achieved the breakthrough in this exceptional project," Martin Herrenknecht emphasized. "Being here today is like a dream come true for me," added Yapı Merkezi Chairman Başar Arıoğlu proudly at the breakthrough ceremony. 

The successfully completed drive marks new feasibility standards in tunnel construction. The new tunnel infrastructure is the first direct link between the historic Golden Horn peninsula on the European side and the port area on the Asian side. In Istanbul the new road tunnel provides major relief of traffic congestion on the strait crossing. Since late 2016 several ten thousand cars a day are crossing between the continents in the Eurasia Tunnel.

Eurasia Tunnel