The Bosphorus road tunnel project which broke through Aug. 22 in Istanbul, Turkey, is a major achievement on one of the world’s biggest and most challenging tunnel drives. At 13.66 m in diameter and 11 bar of water pressure in fractured rock and soft ground, the 3.34-km long bore is unprecedented. It has needed skillful engineering on site and extended TBM technology to do it.
Making the job more difficult for both tunnelers and tunnel machine is the combination of challenges. Many tunnels face high water pressure, though not quite so high as this, and many tunnels face blocky, fractured, abrasive and difficult rock. Many tunnels have to go through soft ground and some have to make a transition between hard and soft. Some tunnels have very big diameters. For the Bosphorus all these factors came together at once. Additionally, there was a high seismic risk from the major Marmara fault line just to the south of Istanbul. Special yielding joint technology has been applied at two critical points, the joint segments produced and tested in Japan.
The tunnel runs across the narrow Bosphorus seaway connecting the Black Sea to the small Sea of Marmara and the Mediterranean beyond it. The sea channel famously separates not just two sides of the city but two continents, Europe and Asia. The new route sits at the southeastern end of the Bosphorus, on the populous urban coast of the Marmara Sea, running between the historic Golden Horn peninsula on the west with its Blue Mosque and Hagia Sophia, and the port districts on the Asian side. There has been no connection directly between these important areas.
Currently ferries link the bustling conurbation at various points and there are two major road bridges at the mid-point of the Bosphorus. But both are clogged with traffic at rush hours. Now the new tunnel section, and the overall 14.6-km long highway it is part of, will cut journey times from a 100 minutes round trip to a 15-minute direct journey. The link will also relieve traffic loads on the bridges but without intruding on the famous skyline. But to do it, the alignment must go 106 m deep under the Bosphorus.
Meaning “throat” in Turkish, the great channel was carved through the local Trakya formation, a complex mixture of sandstones and mudstones. In the glacial period it was partly filled with soft alluvial deposits but the water is still 62 m deep where the tunnel crosses, with bedrock 40 m below that. The tunnel goes through the permeable sediment and highly fractured rock, each saturated with the seawater above.
“That meant using a TBM that could handle hard and soft ground and at high pressure.” says Gert Wittneben, TBM construction manager for the YMSK JV building the tunnel. YMSK JV is a combination of Turkish contractor Yapı Merkezi Construction (lead) and Korean firm SK Engineering & Construction. “It had to be a slurry machine, a Mixshield type.”
The impermeable reinforced concrete segments used in the subsea tunnel have an average compressive strength of 70 megapascal and were produced by the Yapı Merkezi Prefabrication division.
The TBM from Herrenknecht has a face provided with hard rock discs cutters, mounted on six radial arms, scrapers and buckets, and openings between for the slurry support and spoil removal. The TBM was designed with special features to tackle the conditions. Chiefly, it had to resist the very high pressure, up to possibly 12 bar. To the best extent possible, the TBM had to allow working conditions inside at atmospheric pressure and a key element in this was to be able to change disc cutters.
“We have used a sophisticated system to allow the disk changing directly from within the cutter arms at atmospheric pressure,” says Werner Burger, head of engineering at Herrenknecht. He says the design approach was first used on a machine in Hamburg but not at this size and depth. Discs, which are 19-in. double edge types, are removed from the face along with their housing and a new cutter-housing unit is installed, the operation taking about two to three hours. Overall, approximately 500 excavation tools were changed during the drive – all under atmospheric pressure from within the cutter arms.
Inside the control cabin of the TBM, a special monitoring system from VMT was used for the first time. It monitors the performance of the disc cutter system, measuring key values like rotation and temperature. Results are displayed on a screen where sound units are shown in green and abnormal discs show a yellow or red alert.
“Normally a disc should be revolving and if it slows or stops that is an indication of wear or even possible breakage,” says Wittneben. “It is important to know if a unit is jammed or broken, particularly to prevent metal debris from getting into the rest of the face or spoil crusher system, where secondary damage can occur that is difficult to cope with.”
He says that after a few teething problems the system has proved excellent and beneficial for the disc cutter performance. It works in conjunction with a wear detector pin system, which is a hollow projection alongside the disc with pressurized hydraulic fluid inside. If the disc wears too far, the top of the pin is worn off and fluid released which a pressure detector picks up, again alerting the operator.
To change the heavy disc cutters and housing units, weighing up to 1 tonne, the TBM is fitted with an internal railway with a carrying trolley and lifting equipment. The same rail is also used to transport a diving pressure transfer chamber in case of interventions needed into the pressure chamber behind the TBM cutting wheel. The pressure compartment fits onto a special airlock at the top of the bulkhead, lifted up by a scissor platform at the end of the rail. Divers use the chamber to return to surface and enter a decompression unit where they have to stay up to three weeks after a dive.
Airlocks are part of a specification on the machine allowing for hyperbaric interventions, a feature to allow for possible maintenance or repair needs that would otherwise have been extremely difficult if not impossible.
Başar Arıoğlu, Chairman of Yapı Merkezi, leaves no doubt that the tunnel under the Bosphorus is an engineering masterpiece. “It was a difficult path from the outset, but finally we made it despite the great challenges,” he said at the ceremony in Istanbul on Aug. 22, 2015. “The fact that we have now successfully completed the breakthrough is like a dream come true for me.”