First Concrete Element to be Placed on World's First Offshore Immersed Tunnel in KoreaIn 2005, construction commenced on two cable-stayed bridges, with main spans of 475 m and 230 m, and a 3.2 km immersed tunnel, which eventually will become part of a new 8.2 km motorway connection between Busan and Geoje Island at the southern tip of the Korean peninsula. Once completed, the four-lane combined bridge and immersed tunnel motorway will link Busan, South Korea's second largest city, with Geoje Island, the second largest island and a place renowned for beaches, forests and mountains and industries such as shipbuilding, heavy manufacturing and aquaculture.Already one of South Korea's first summer tourist destinations, the Busan-Geoje fixed link will allow visitors easier access to the region. The new fixed link will reduce driving time to just under an hour, fostering the development of a thriving tourism industry and allowing the region's aquaculturalists to transport their produce cost effectively by road to mainland markets. At present, the journey from the mainland is via ferry, or a circuitous string of roads and bridges. This project is due to be completed in December 2010.COWI of Denmark is the lead consultant on the tunnel and bridge project, which is being delivered in two contracts. For the tunnel, COWI teamed up with Daewoo Engineering. The client, Daewoo Construction and Engineering (E&C), has brought together the bulk of its organisation and more than 200 consultants to Busan, where they are working on the large-scale logistic project, reminiscent of a smaller version of the project to build a fixed link across the Oresund between Denmark and Sweden. Click kr/22. Visit www.dwconst.co.kr and www.cowi.dkHalcrow and Tunnel Engineering Consultants (TEC) of the Netherlands are technical advisor to Daewoo Construction, who leads the consortium that won the USD1.1 billion contract to design, build and operate the Busan-Geoje fixed link. Mott MacDonald was appointed to assist Yooshin Engineering Corporation in supervising the construction of the link. Visit www.halcrow.com, www.tec-tunnel.com and www.mottmac.com. The IDC organization, consisting of Arcadis, Ingerop and Seo Yeong, worked during the whole design process and during construction as independent design checker to make sure that the tunnel will indeed be able to withstand the forces of nature. Visit www.arcadis.nl and www.ingerop.com Challenging design and construction in soft clay seabedThe tunnel, comprising 18 180 m-long prefabricated concrete elements with each element consisting of eight segments measuring 22.5 metres in length, is positioned in a dredged trench at a depth of 48 metres and then covered.For the team of international consultants entrusted with the design and construction management of the world's first offshore immersed tunnel, which is being built in unusually deep waters, new methods have had to be taken into service to allow for waves of 11 metres higher than the Oresund Strait, 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.Casting of the rectangular concrete caissons is under way. The project is progressing well and the first batch of four immersed tunnel segments, each of 180 metres in length, is to be floated out of the dry dock in September. Immersion is planned for later in the year of this first batch of elements. There are several short rock tunnels that form part of the project.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.Constructing the motorway link offshore calls for completely different working conditions than ashore. In order to find out how often the team can actually operate at sea, detailed calculations have been carried out, showing that tools have to be downed whenever the waves are any higher than 40 to 60 centimetres. Challenging immersed tunnel on soft clay seabed (illustration courtesy of Mediafarm) 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 soft clay, which has not been hardened during the ice age, like in Denmark. 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 metres. 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 elements.Finally, scale model studies have been performed to determine how the extremely high waves and strong current will affect the tunnel both during construction and following completion.Watertightness in deep watersDaewoo Engineering and Construction has been tasked with overcoming the twin challenges of an unforgiving environment and demanding technical specifications to complete the project. The challenges associated with such a project are immense. One of the most important by far has been ensuring that the immersed tunnel's joints are watertight.To get the job done, Daewoo E&C invited Trelleborg Bakker to supply Omega seals and injectable waterstops, one of the biggest assignments in the history of the company.Although Trelleborg's injectable waterstop makes the seals between concrete segments watertight, by forcing injection fluid through pipes placed every four metres, the rubber sponge at the ends of the seals are compressed, creating a channel filled by injection fluid that seals fissures or gravel spots, thereby ensuring impermeability. As the tunnel is in a seismic zone, some of the injectable waterstops are also secured using a secondary Omega seal. Trelleborg Bakker is supplying 1.2 kilometres of Omega seals, 1.87 kilometres of secondary Omega seals and nine kilometres of injectable waterstops for the project. Trelleborg's long experience with tunnel seals meant it could quickly come up with a custom solution for the project that guarantees long-term success. Earlier tunnels Trelleborg worked on were 25 metres in depth. This one is nearly 50 metres, making it necessary for the company to qualify their Omega seals and injectable waterstops through new tests to meet the specifications. The immersed tunnel section of the Busan-Geoje fixed link represents therefore an outstanding opportunity for the company to further enhance its reputation as a world leader in engineered polymer solutions for civil construction projects. Click here to view illustrations. Visit www.trelleborg.com 37/07. Tunnelbuilder thanks COWI for its article published in COWI feature magazine (April 2006) and Daewoo E&C and Mediafarm for the illustrations of this article.