Suspended on wires and gently lowered using ballast, the first 180m long element of the 3.2km undersea tunnel part of the Busan-Geoje Fixed Link project was lowered into the sometimes violent sea dividing the two parts in February 2008. The undersea section will be the deepest immersed road tunnel in the world.
Eighteen elements need to be immersed to complete the undersea tunnel. Considering each element weighs 48,000 tonnes and 180m long, 26.5m wide, and 10m high concrete box sections, it is no mean task.
Just towing an element from its casting site into position takes ten hours. The actual process of immersion takes a mammoth forty hours. "The immersion is all done remotely," says deputy project manager for COWI, Tommy Olsen. "Each element has cameras mounted on it that allow us to monitor the process." Olsen likens the immersion process to the way a submarine dives and surfaces: "Ballast tanks inside the element can be adjusted to control its descent. Pleasingly, the first element was lowered and set without a hitch."
All the elements will rest on a gravel bed in a dredged trench on the seabed and are joined by bulkheads. The elements are joined to conventional cut-and-cover tunnels where they exit the water.
The project is at the mercy of nature, with weather and currents capable of disrupting the process. "The weather is one of the big challenges for the immersion, the depth being another," says Olsen. "The contractor gets regular forecasts. The waves can become very high, while strong currents can also cause severe problems. We've made various hydraulic models in an effort to prepare for these effects."
As well as the undersea section, the Busan-Geoje link includes two cable-stayed bridges, each two kilometres long. The entire USD 5.8 billion project is expected to open in 2010.By Martyn GlanvillePublished: 04.03.2008