The second, 180m long element of the Busan-Geoje immersed tube road tunnel in South Korea was due to be towed into position late March following contractor Daewoo Engineering & Construction’s successful placement of the first 180m long section in mid- February.
In total, 18 elements each weighing up 48,000 tonnes will be cast in the Anjeong drydock, towed 36km and during a 40-hour long immersion process be placed on a prepared seabed trench in about 15m-50m depth of water. There are 16 ordinary elements comprised of eight 22.5m long segments (1m thick walls; cross section of approximately 26.5m wide by 10m high) and two special elements.
The 3.2km long tube will be the deepest immersed road tunnel in the world, said Danish consultant Cowi which has designed the permanent works and some temporary works for the tube as part of joint venture work with Daewoo Engineering Co.
The contractor is being supported further by a joint venture of Tunnel Engineering Consultants (TEC) of the Netherlands, which is advising, and Halcrow.
The crossing is part of a 8.2km long link between the mainland port city of Busan and the island of Geoje. Design work on the US$5.8bn fixed link project began in 2003 and the project is scheduled for completion in 2010.
Float out of elements will only happen in winter due to sea swells, and towing takes 10 hours. The offshore conditions are harsh and never been seen before for an immersed tube, said the design JV. A further key challenge has been preparing the dredged trench foundation by improving the stiffness of the soft marine clays, which have low unit weight in comparison to the load from the tunnel and rock cover for scour protection.
Following the immersion of the second element, Cowi anticipates that another one or two elements will be placed this winter season. The remaining elements are to be placed over two more winter seasons – 2008/9 and 2009/10.
Four elements were cast in the first batch in the drydock in Jinhae Bay for placing this season, and a further four elements are to be completed in drydock in June. Afterwards, there will be two more batches each of five elements.
The immersion is executed by Dutch subcontractor Strukton Afzinktechnieken, operating as Mergor Underwater Engineering.
Float-out of the first element of the Busan-Geoje immersed tube The first of 18 precast elements is towed-out to the prepared seabed trench
