When workers broke through the Eurasia Tunnel in August 2015, it marked not only the first time Europe and Asia were connected by a road tunnel, but also represented the crowning achievement for one of the world’s most challenging tunnelling projects.
What’s more, it is also due to be completed and opened ahead of schedule.
The tunnel runs across the south-eastern end of the Bosphorus strait, linking Kazlicesme on the European side of Istanbul with Goztepe in Asia. Boring the tunnel involved working at nearly 12 bar of water pressure in fractured rock and soft ground.
Located about 1.5km south of the Marmaray subsea rail tunnel, which opened in 2013, the 5.4km-long twin-deck Eurasia Tunnel will reduce the journey time between the two continents from 100 minutes to just 15 minutes and ease the pressure on the two existing bridge crossings further north.
It will have two lanes on each deck, a toll plaza and an administrative building on the western side and ventilation shafts at each end. The tunnel is part of a 14.6km route which will carry an average of around 100,000 cars and light vehicles a day, providing an additional 20 per cent capacity across the Bosphorus and aiming to reduce Istanbul’s notorious congestion. In this year’s Tom Tom Traffic Index Istanbul topped the congestion list of 146 countries, with a 30-minute commute taking around an hour, adding an extra 110 hours a year to the time people spend in their cars. The survey found the congestion level on highways was 79 per cent, and 50 per cent on non-highways.
Boring the tunnel
The tunnel-boring machine (TBM) named Yildirim (meaning lightning in English) began boring the 3.4km subsea section on the Asian side of the waterway and at its deepest it was 106m below sea level and never less than 26m. The maximum depth of the seabed is 62m along the route of the tunnel.
The excavation diameter was 13.71m, allowing an inner diameter of 12m with a 600mm-thick lining – making it the sixth largest diameter tunnel in the world.
In addition to building the tunnel, the project has involved widening the approach roads on both sides, constructing bridging intersections, an overpass and a total of 10 pedestrian crossings.
Despite the complexities of the tunnel crossing, Basar Arioglu, chairman of Avrasya Tüneli Isletme Insaat (ATAS), the joint venture between Turkey’s Yapi Merkezi (leader) and Korea’s SK Engineering & Construction, says a bridge across the strait at this point was never an option.
“A bridge would have obstructed the view of the historical city when entering Istanbul by boat. No-one wanted to build a bridge there,” he says.
Going under, rather than over, the stretch of water that connects the Black Sea to the small Sea of Marmara and the Mediterranean, has entailed dealing with the extremes of fractured rock – the Trakya Formation – on either side of the channel and soft, marine sediments in the centre section.
“It was very complex geology. About every 50m we hit rocks with volcanic intrusions essentially shooting up from below,” says Arioglu.
To cope with these two extremes of hard rock and soft sediment, and the high pressure, Herrenknecht supplied a mixshield (slurry) TBM.
The face had both hard rock disc cutters, mounted on six radial arms, scrapers and buckets and openings between for supply support and spoil removal.
The TBM, launched in April last year, made average daily progress of 7m, installing 600mm-thick lining segments behind it. The five per cent incline of the alignment was another challenge, especially considering the heaviest of the segments weighed 15 tonnes, but Arioglu says the operation went without incident.
Despite the challenges the project has faced – the complex geology, the high atmospheric pressure, the hyperbaric interventions, and the difficulties involved in building infrastructure in a large, densely populated, working city – the TBM broke through two weeks ahead of the 72-weeks programmed.
The tunnel is expected to open in December 2016, three months earlier than the original opening date of March 2017.
The ability to change the discs inside the cutterhead and the success of the hyperbaric interventions helped operations but Arioglu also attributes much of the smooth-running of the project to the planning afforded by the long lead-in time.
“The joint venture was awarded the project in 2008 and the finances were agreed in March 2013 so we had more than four years to prepare,” says Arioglu. “Four months after the finances were agreed I was in Germany taking over the TBM, which had been ordered even before the financial closure. We took some risks but they, and all the preparation, paid off.”
* This is a version of an article that first appeared in Tunnels & Tunnelling: www.tunnelsonline.info