Successful breakthrough of the 13.5m diameter Mixshield TBM S-762 at the end of its 3.34km drive under the Bosphorus Strait in Turkey records an achievement of industry significance, reports Shani Wallis from Istanbul.
He Eurasia TBM highway connection under the Bosphorus in Istanbul had the TBM subjected to constant hydrostatic pressure and to a maximum 12 bar pressure at the deepest point and through complex geological materials (Fig 1). Working from the Asian side to the European side, the grey Trakya Formation bedrock is hard, highly crushed and fractured, with intrusions of volcanic diabase and andesite of up to 125MPa in UCS. The sedimentary deposits at the bottom of the Strait are soft sandstone and mudstone creating two interfaces between the very hard and very soft deposits.
To complete the technically demanding project, the TBM was equipped with a significant number of innovative features including an accessible cutterhead in free air that allowed for the change of all 35 x 19in double ring monoblock disc cutters on the 13.5m o.d. cutterhead as well as many of the 194 cutting knifes. A disc cutter rotation monitoring system (DCRM) provided early notice of the need to change disc cutters and wear detector bars on the cutterhead, while a significant hard facing protected the body of the shield and the cutterhead structure.
“We could not have completed the drive without these innovations,” said Basar Arioglu, Chairman of the Board of the Yapi Merkezi company and of the BOOT (build-own-operate-transfer) company with equity and construction JV partner SK Engineering & Construction of Korea. “The cutterhead looked in reasonable shape when it emerged at the breakthrough.”
The landmark achievement was concluded after 16 months (420 days) of below-the-radar operation and progress by the Herrenknecht Mixshield and the crews of the YM SK construction partnership. Following launch in April 2014, the machine made average daily progress of about 7m over two 12hr production shifts per day, erecting in its wake a precast segmental lining of eight double-gasketed and bolted 600mm thick segments, plus key, in each 2m wide ring. The best shift achieved 12m (six rings) in 12 hours.
“We had our adventures along the way,” said Gert Wittneben, JV Construction Manager for the TBM drive. “The 5% slope of the alignment was a particular challenge with a vertical TBM face on a 75cm overhang on the way down and an equal look-up on the way towards the breakthrough on the European side.” VMT guidance and monitoring systems kept the TBM on course and able to warn of potential trouble ahead.
“We changed 400 discs in the free air-accessible spokes of the cutterhead but also had to set up for four hyperbaric interventions with the divers,” said Wittneben. Two or three divers for each intervention lived in the accommodations lock on the surface for up to 15 days to complete the work. They were transported in a compressed air shuttle or pod to dock with the TBM’s bulkhead, and decompressed through about seven days from the high working pressures of up to 12 bar.
“We needed the saturation divers,” explained Wittneben, “because the Trakya Formation was often so crushed it was like a field of big stones. On occasions these big stones broke off a knife pick tool on the openings on the cutterhead and these caused damage on three occasions to the slurry circuit extraction grill in the excavation chamber, allowing big rocks to be drawn into the slurry circuit. Divers had to be in the excavation chamber under high pressure to complete the repairs. During the work, the divers also had to wear fairly bulky hoods or helmets to be fed helium gas during the interventions. A very tough job.”
As well as a challenging TBM drive, the final construction of the double deck highway tunnel within a highly seismic zone also includes particular features to ensure its 100-year design life. The road decks on their corbels, for example, are free-standing to ensure that each element will move independently during an earthquake. “The project is designed to withstand a 7.5 magnitude earthquake,” explained Bahriye Yaman, the method statement engineer for the YM SK JV. “To further protect the structure of the tunnel there are two seismic joints in the lining at the transition from hard rock to the soft sedimentary deposits. These joints, which are installed in the lining like a tunnel ring, allow movement of 75mm in contraction, 75mm in expansion and 50mm shear. We had to modify the machine’s vacuum segment erector to a mechanical erector to install the steel segments of the seismic joint.”
The 3.34km long TBM drive is the critical underwater part of the total 14.5km long highway project. On the Asian side a set of twin 1km long NATM tunnels take the undersea highway lanes to the surface and on the European site the transition to the surface is via cut and cover and open U-sections.
The project is a world leading undertaking in its set-up and construction, and TunnelTalk will publish a second article to explain the contractual system of the total US$1.248 billion investment, the financial make-up and other elements of the 14.5km long highway crossing of the Bosphorus.