It was a dream only sultans could dream: a tunnel under the Bosphorus, a link between the continents. In 1861 Abdülmecid I, ruler of the Ottoman Empire, had the daring idea of simply digging the missing piece of the Silk Road under the Istanbul Strait. Sultan Abdülhamid II commissioned feasibility studies in 1891. The result: not feasible. The rulers were ahead of their time.

Even 150 years after the sultan's dream it is a project of superlatives: 5.4 kilometers of tunnel are being built, 3.4 kilometers run directly under the Bosphorus. To avoid obstructing ship traffic the tunnel is being dug with a specially designed tunnel boring machine (TBM) from Herrenknecht which began its mission in April 2014 in a launch shaft on the Asian side. With a gradient of five percent the Mixshield bored its way through varying layers of rock and soft soil down to a depth of 106 meters below sea level, then slowly drove upwards again towards Europe.

At the deepest point of the alignment the water pressure is about 11 bar: eleven times atmospheric pressure. In combination with the huge diameter of the TBM – it measures 13.66 meters, about as much as a three-story house – this was absolutely new territory in technical terms. Never before has such a large tube been constructed under such high pressure with mechanized tunnelling technology.

“The fact that we have now successfully completed the breakthrough is like a dream come true for me,” says Başar Arıoğlu. The 48-year-old is chairman of the Turkish construction company Yapı Merkezi Construction and Industry Inc., which was awarded the contract for the tunnel in 2008 and is carrying out the ambitious construction project together with the South Korean company SK Engineering & Construction Co. Ltd. Arıoğlu has already managed highlight projects such as the subway construction of Izmir and Dubai. But the tunnel expert leaves no doubt that the tube under the Bosphorus will be his masterpiece. “It was a difficult path from the outset,” said Arıoğlu.

In 2008, after the contract was awarded, bureaucratic pitfalls slowed down the project, recalls Başar Arıoğlu. When they were out of the way, the search began for financial backers. That was in 2009 and the financial markets had just collapsed. The banks became overly cautious, demanding extremely meticulous project documentation and maximum safety standards during construction. “Really though, these obstacles were important for us, because they gave us the absolute belief that we really could handle this project and take it safely and reliably to completion.”

The decision in favor of a TBM from Herrenknecht was made in 2012. For Arıoğlu it was an important step: “With Herrenknecht we had a partner on board we can trust completely.” This was proven not least by an offer from Schwanau due to the technically unprecedented demands of the project. “That was a clear signal,” says Başar Arıoğlu. “Herrenknecht would accompany us on this challenging journey right to the end.”

200 meters before the breakthrough, Özgür Yurtaydin is sitting in his office in a stack of containers on the edge of the “Istanbul Strait Road Tube Crossing Project” jobsite. It is August. The outside thermometer shows 38 degrees in the shade. In Yurtaydin's office the air conditioning hums, 20 degrees. A good thing: the Herrenknecht project manager needs to keep a cool head until the tube for the gigantic Eurasia road tunnel is finished.

Jeans, lumberjack shirt, five-day beard: looking at him you would not necessarily recognize that Özgür Yurtaydin turns an important cog in the machine of the mega project. At just 31 the civil engineering graduate is Herrenknecht's project manager on the “Istanbul Strait Road Tube Crossing Project”. And anyone observing Yurtaydin, who h as worked for Herrenknecht for nine years already, will quickly understand what fits him for this role: he radiates calm when others are getting hectic. He makes decisions while others are still thinking about it. And he never avoids a task. “I'm just not the guy that says: that's not my problem.” An attitude that has caused him many 14-hour days on this project. “That's just the way it is,” he says. “From the first day on there is only one goal for us here: we have to do this job perfectly.”

The sheer size of the tube does not make construction easier. But only with this diameter can the Eurasia Tunnel fulfill its task: from late 2016 around 100,000 cars a day are expected to cross back and forth between the continents on two roadways one above the other. A figure that will have far-reaching consequences for traffic in the city. Commuters from the south in particular, who currently have to take the chronically congested First Bosphorus Bridge in the center, can then cross to the other side directly. 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, currently the second traffic bottleneck between the continents, will also be relieved. If everything goes according to plan the tube will help 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 Bosphorus commuters: a significant reduction in CO2 emissions in the city.

“No matter how much energy and effort you put into project planning – a certain residual risk always remains”, says Naim Isli, project manager at Yapı Merkezi. “But we are engineers: if problems occur unexpectedly we just have to solve them.” Like the matter of the fault zone. Just 17 kilometers from the tube is a geological fault zone where earthquakes can have a particularly strong effect – which is not uncommon due to Istanbul's location near two tectonic plates. The solution: two special tunnel rings that work like joints in the event of an earthquake. “They give the tube the necessary flexibility,” says Isli.

He is more concerned about the nature of the soil, says the passionate tunnel builder, and points to a technical drawing on his office wall. The sketch shows the subsoil below the waters of the Bosphorus, including the tunnel. The tube passes through a colorful patchwork quilt – each color represents a different type of ground the TBM needs to eat through. “We have to deal alternately with soft ground, sandstone, mudstone, volcanic rock and solid rock, always under a huge water pressure,” says Isli.

A subsoil that can only be overcome with a Mixshield – the all-rounder among TBMs. Its cutting wheel is equipped with different tools for all eventualities: large cutting knives scratch softer subsoil from the tunnel face, from where – combined with a water-clay mixture – it is flushed through a hydraulic slurry circuit to the separation plant in the launch shaft.

If the machine encounters harder solid rock, the 35 twin disc cutters do most of the work. The 19-inch discs, weighing 1 tonne each including the cutterbox, are pressed against the tunnel face by the displacement cylinders of the cutting wheel drive and roll off it. Individual pieces, so-called chips, break out of the rock. Smaller pieces are sucked through an intake screen and removed by the flushing cycle. The rest is 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 is used for building up and regulating the so-called face support pressure against the water and ground pressure of the Bosphorus. A principle developed further by Herrenknecht that has proven itself worldwide countless times.

The Mixshield technology allows an advance of 8 to 10 meters in 24 hours even in the extremely challenging environment under the Bosphorus. The harsh conditions and high mechanical loads, however, take their toll: even the best material does not get away without wear. Although the disc cutters are made of high alloy, ultra-hard steel, they have to be replaced regularly.

A task that presented Herrenknecht with a real challenge when designing the machine for the Eurasia Tunnel: even at the enormous pressure of 11 bar the cutting tools must be able to be changed quickly and easily. To achieve this the Herrenknecht design team reverted to the concept of accessible cutting wheel arms. This has already proven successful in previous projects in Hamburg or Shanghai – but under significantly lower pressure conditions. For the huge Mixshield under the Bosphorus the lock systems of the cutting tools were further developed especially ( Further information). The result: in Istanbul too the crews were able to safely replace the disc cutters and cutting knives without problems under atmospheric conditions.

Around 400 disc cutters have been replaced on the entire drive. Various detectors that measure the rotation and temperature of the discs indicate in good time when a change is pending. “At first it took seven or eight hours, but thanks to the routine we've developed, now we change a disc cutter in just two or three hours,” says Naim Isli.

If you want to take such an ambitious project in new technical territory to a success, you have to be prepared for all possible scenarios – however unlikely they may seem. The teams working with Naim Isli and Özgür Yurtaydin experienced this in Istanbul too. The huge flow rates of excavated material, the high pressure, large boulders that are thrown around inside the excavation chamber – all this constantly took its toll on the machine's components. About halfway through the advance, material wear on the intake screen and the jaw crusher was discovered. 

“It was immediately clear to everyone involved that we needed to repair the parts as quickly as possible,” says Isli. A Herculean task: screen and jaw crusher are located in the excavation chamber, which is under high pressure. They can only be accessed for maintenance and repair purposes by professional saturation divers. Equipped with breathing air equipment, via locks the specialists made their way into the heart of the machine. For three weeks the men toiled at approximately 9 bar pressure in areas where humans normally have no access. They spent the nights in special chambers outside the tunnel that are also under pressure. For safe transportation of the divers in and out of the tunnel the TBM was equipped with all necessary special equipment – including a pressure transfer shuttle. “With such pioneering projects you always need multiple technical fallback solutions to be prepared for all eventualities,” says Gert Wittneben. “The Herrenknecht TBM had everything necessary on board, the technology has proven itself – both back then in Hamburg and now again in Istanbul”.

Then the announcement of success: the components are repaired, the machine can be started again. “The men did a really great job in the short time,” says Özgür Yurtaydin and stresses: “Jointly solving such a tricky situation has brought us, the teams from Herrenknecht and Yapı Merkezi, even closer together. After this experience at the latest we are one big family here.”

In the afternoon Yurtaydin takes his helmet from the coat hook and makes his way to the TBM, which lies silently in the rock. It stands still because a transformer for a pump is being installed in the tunnel. The workers on the machine use the precious time. Protected by the shield skin they bolt the segments of a new concrete ring. And in the rear of the TBM, the back-up, they extend the two pipes of the hydraulic slurry circuit. Half an hour later everything is bolted and welded. The men start the TBM again. There are about 200 meters still to go. Another 25 days? Another 23? Özgür Yurtaydin grins. “In my office the bets have already started.” The goal is within reach: one of the most challenging tunnel missions ever approaches final completion. The sultan's dream is coming true.