As a world first, an offshore immersed tunnel is being built in Korea at a depth of 48 metres. The construction is on delicate, clay soil in an area with extremely high waves and a risk of earthquakes.
Boilerplate solutions are a distinct "no-no" for a large team of international consultants currently designing and constructing the world’s first offshore immersed tunnel, which is being built in unusually deep waters. New methods have had to be taken into service here to allow for waves 11 metres higher than in the Øresund Sound, and with an ocean floor as soft as butter and the potential risk of the area being hit by powerful earthquakes. The job has made great demands in terms of including factors like soil mechanics, hydraulics and tunnel design.
The work, which will be completed in four years, will make it possible to drive from Busan to Geoje, an island off the southern tip of Korea in the South Sea. But even now, preparations are being made to cast the segments, which will be lowered to a depth of 48 metres to be assembled to make a tunnel four kilometres long. At the same time, the foundations are in the process of being cast for two cable-stayed bridges, which are part of the same motorway link. The bridges are designed to withstand gusts of wind up to typhoon force and built by sailing large prefabricated segments out to sea.
A number of challenges are posed by constructing the immersed tunnel in a design that was actually invented for shallower depths. One of them is the extremely soft ocean bed of clay. The soft subsoil means that there is a risk of major settlement and hence large movements in the segmental joints, which simultaneously have to withstand the water pressure at more than 50 m. It has therefore been necessary to refine the familiar joint designs and ground reinforcement methods. This involves strengthening the foundation soil in order to avoid problems with load capacity, settlement and stability. In addition, sophisticated earthquake analyses have been conducted in order to work out the movements of the joints and the stability of the tunnel segments. By Christina Tækker, cht@cowi.com Published: 24.04.2006