Not only did it mark the moment that the two continents of Europe and Asia were joined by a road tunnel for the first time, it was also the crowning achievement for one of the world’s most challenging tunnelling projects.
Boring the tunnel, which runs across the south-eastern end of the Bosphorus strait, linking Kazlicesme on the European side of Istanbul with Goztepe in Asia, involved working at nearly 12 bar of water pressure in fractured rock and soft ground.
And making it even more remarkable is that it will be completed and opened ahead of schedule.
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.
The 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.
NATM and cut and cover were used along 1km of the connection tunnel sections on each side of the sea crossing.
The Asian side transition box, allowing for the parallel traffic to be stacked in double-deck configuration to enter the TBM tunnel, provided the launch shaft for the 120m-long TBM, while the longer cut cover section on the European side provided the machine’s retrieval.
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.
Atmospheric pressure was maintained for the working conditions inside the TBM, and a key element of this was the changing of the disc cutters through accessible cutterhead technology.
"We used a sophisticated system to allow the disc changing directly from within the cutter arms at atmospheric pressure," says Werner Burger, head of engineering at Herrenknecht.
The design approach was first used on a machine in Hamburg but not one of this size or at this depth. The 19in double-edge discs were removed from the face, along with their housing, and a new cutter-housing unit was installed, all without losing pressure. The operation took about two to three hours – a huge time-saving compared with the days it would have taken for divers to complete the same task.
Arioglu estimates that during the TBM’s operation the discs were changed more than 400 times and the scrapers more than 200. Arioglu believes this technology will become a ‘must-have’ on big TBMs in the future.
Carrying out hyperbaric intervention at this pressure is also problematic and fortunately only three were required, the shortest lasting one day and the longest 10 days.
"Because of the high pressure it’s hard to do any intervention at the front of the machine but we did it three times, one at 10.8 bar pressure," says Arioglu.
"Our team and our machine were well prepared for this kind of operation and as a result the interventions were very quick and very safe."
For these operations, the railway that transports the carrying trolley and lifting equipment for the disc cutters was also used to move a diving pressurized transfer shuttle into place. The pressurised shuttle fitted onto an airlock at the top of the bulkhead, lifted up by a scissor platform at the end of the rail. Prior to the intervention the saturation divers entered the pressurised living chamber on the surface where they compressed to 9 bar pressure before entering the pressurised shuttle and transferring to the TBM.
After they finished their daily work inside the slurry chamber of the TBM, the divers returned to the pressurised living chamber and rested until the next working day. This cycle had to be repeated for every working day between the surface and TBM.
Once the intervention was completed, the divers entered a decompression phase in the living chamber where they had to remain for up to 10 days after a dive. Sometimes two teams of divers were used, each working around seven hours at a time.
Another first for the project was a disc cutter monitoring system in the TBM’s control cabin. Designed by VMT, the system measured key values such as rotation and temperature. The results were displayed on a screen where healthy units were shown in green and abnormal discs as a yellow or red alert. Arioglu says this feature also proved its worth. "When we saw the cutterhead coming out after the breakthrough, it was almost intact and worn uniformly," he says.
Another feature on the TBM was extra ‘chisel’ grill bars across the slurry openings in the face. These allowed for the heavily fractured quality of the Trakya Formation which meant large blocks coming away from the face. The large blocks were kept in front of the cutting wheel until the TBM action reduced their size sufficiently to allow them through the spoil grill into the jaw crusher and then the slurry line to the surface.
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.
"The lining segments were carried into the tunnel on an MSV – fully loaded carrying 130 tonnes. The machine carried very heavy segments safely and without any accidents," he says. While the varying geology was a challenge, another was the tunnel’s proximity to the North Anatolian fault line. The tunnel is designed to withstand a 7.5 (moment) magnitude earthquake and to mitigate movement two rubber and steel seismic joints, manufactured by Japanese company SEIBU, have been installed in the lining at the transition from hard rock to the soft sedimentary deposits on either side of the tunnel. The joints were installed by the segment erector and temporary supports were required to ensure that the huge forces pushed by the TBM did not compress the joint. The seismic connections allow movement of 75mm in contraction, 75mm in expansion and 50mm shear.
"The joints ensure water tightness but also give the lining the flexibility in the transverse and longitudinal directions," says Arioglu. Fireproofing will be applied on the tunnel segments. Emergency stairs are located every 200m along the tunnel and they are accessible through fireproof emergency rooms, equipped with CCTV monitoring for those unable to use stairs. Safety lanes are positioned at 600m intervals for breakdowns in the tunnel. The upper deck of the tunnel was the first to go in, cast in situ behind the TBM and advancing around 12m a day, while the lower deck is being manufactured offsite and will be installed at a rate of around 100m a day.
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."
