On South Korea's southern coast, an innovative highway sea crossing is providing many engineering challengesThe new Busan-Geoje crosses from South Korea's second city to its biggest island and is slightly shorter than the 12km of the country's famous Incheon project. In addition the main cable stay bridge for the Busan-Geoje project has a 475m span rather than the 800m of the Incheon central span. However the 8.2km Busan-Geoje project faces perhaps greater technical challenges and also includes a second bridge crossing, with not just one but a pair of cable stayed spans of 230m each. Much more significantly, it has an immersed tube tunnel section 3.7km in length which will briefly be the deepest seabed road tunnel in the world. It is also the first to be built in South Korea.
Strategically the crossing is at least as important as the Incheon, because it will link the 3.6 million people in Busan, to the industrially and scenically important Geoje island. Plunging hillsides, strange rock shapes and indented bays make it a famous tourist spot with the southern half of Geoje designated a national park. At the north end live 250,000 people and several major shipbuilding yards that have made Korean industry world famous.
Access to these areas has only been possible by ferry; an hour's journey from the city plus road time. Recently a cable stay bridge has been added on the island's west side but the journey round the bay takes nearly three hours, particularly for trucks delivering to the yards.
Linking the city more directly, with a 40 minute journey, has high economic value, saving US$300 million in traffic costs each year and generating economic output up to US$42 billion over the project's life span.
That is 40 years, the concession period granted by the Korean government to the winning consortium to build the project. The BOT scheme was awarded in 2004, to the GK Fixed Link Corporation made up from a seven strong group of contractors,
The companies have put up part of the $2.2 billion investment, borrowed $1 billion and are receiving nearly $500 million from government. This last is recognition of the project's strategic purpose as it links two north-south motorway corridors in the country into a single U-shaped route.
Daewoo is the lead contractor for the construction work itself, tackling one of its most ambitious schemes ever. The deep tunnel, equalled only by a narrower rail tube under the Bosporus in Turkey, passes up to 48m below the surface of the main shipping channel into Busan, between a small island just off the coast and an even smaller outcrop towards Geoje.
Another small island separates the remaining distance to Geoje into two more channels, one to be crossed by the big 475m span cable stay bridge and the other the twin 230m span bridges.
The larger bridge will have a relatively high 52m clearance for the big new ships that constantly leave the yards for ocean trials or delivery, and the cargo ships going into Busan, which is currently carrying out major expansion of its container port. But that is only for emergencies and most of the ships, and certainly the largest, will mainly use the eastern channel which therefore needed the tunnel.
An immersed tube was chosen because of the steeply plunging hillsides which continue undersea to create deep channels; a bored tunnel would need to run very deep meaning very long approaches. Tube elements will sit just in the seabed, in a dredged trench that is backfilled around the hollow boxes.
To tackle the job has required some innovation and skilled engineering from Daewoo. A pair of remote-controlled underwater jacking frames, an exceptionally deep level jack-up barge with pinpoint GPS aggregate placing, and a mini-submarine marine access vehicle, are among the tools it has devised.
Casting of the tunnel elements is also complex as there are difficult undersea conditions while the water pressure increases considerably at the greater depths bringing dangers of leakage with it. A new level of precast placing and positioning techniques for the cable stay bridges has to be developed as well, taking advantage of lessons learned from the building of the Storebælt and Oresund crossings from Denmark in Northern Europe.
All this has to be done at a site on the edge of open sea, with high waves and vulnerable to typhoon condition storms throughout the summer. Most years see three or four such onslaughts with howling winds that can lift and smash dockside harbour cranes, rip the roofs off houses and throw cars along the highway. Despite that, work on the bridge sections is well underway and currently the whole six year long project is 65% complete, with a target competition for the end of 2010.
The immersed tube is perhaps the most challenging part of the project and has taken much of the attention of the contractor, designers and technical advisers. After a slow start it is now going well however. The first of eighteen huge tunnel elements was placed early this year and in late January construction crews were readying the seventh huge 180m long element for sinking. The undersea crossing is now one third complete.
Daewoo studied a number of existing schemes, including the Oresund project as well as others in Hong Kong, Japan, and Europe and has used the Oresund bridge designer
A total 18 tunnel elements are being used for the undersea section, certainly the longest and, at 48,000tonnes apiece, among the most massive such units made anywhere. Each is 10m high and 26.46m wide unit has for a two lane road in either direction separated by a central service passageway. These are made in a drydock facility created for the project at Anjeong on the coast near Geoje, alongside other ship yards.
Getting the casting work right is the key to the immersed tunnel integrity says Hyun Chil Lim, Daewoo's General Manager responsible for all pre-casting works. Don Fraser, Halcrow advisor explains further. "This is massive civil engineering project with large amounts of concrete to be poured, with all the problems that can cause such as heat cracking and shrinkage. But it is also the detail that matters very much; joints, waterproofing of form tie holes, accuracy of embedded anchor plates."
Elements are poured in eight separate segments along the length of each element. The jointed structure gives some flexibility to each to cope with future seismic vibration and shrinkage of the concrete. Movement is limited by locking the joints with big shear keys. Robust waterstops are required at the joints between the segments, embedded in the concrete, a double layer for the first time on any immersed tunnel. Five units are made at a time sealed at either end with steel bulkheads. When the dock is flooded they float up and are taken by tugs for mooring to the seabed nearby.
When there is calm weather elements are hauled by tug to site one by one and sunk into the prepared trench on an accurately laid gravel bed. A pair of computer controlled pontoons control the descent with winches, as tanks inside the elements are flooded with water. A jacking frame adjusts the position, lining the unit up with the existing section, before pumping out the connection and compressing the special Gina seal, a rubber ring around the element end which contacts a 'female' ring plate on the previously placed section. The element is backfilled to fix it in the trench and cover it with protective rockfill, and then the bulkheads are removed to join the tunnel into one.
Construction of the road can then be carried out with the asphalt surface being laid, followed by fitting out.
Meanwhile the casting yard location has also been used to prepare huge precast units for the bridges, which are installed on the seabed by floating cranes. Luckily the shipbuilding area has plenty of these huge shearleg cranes available for hire.
In the same way that oil rigs are built ashore, the foundation caissons for the bridge piers, weighing as much as 9,000tonnes, though more usually around 1,000tonnes, are settled on the seabed and then grouted and ballasted into place. Giant precast concrete sections are fitted on above "just like Lego" said Fraser, to form above the piers and lower pylons.
The main bridge pylons, rising 158m, are poured by in situ means as they are too large even for the 3,000tonne cranes and a floating drydock. But the deck units, which will start to be installed shortly, are pre-made ashore as composite steel and concrete frames lifted in as one piece on the approaches. On the main bridges the steel frame is fitted first, attached to the bridge cables as they are installed from the towers. Precast panels then fill the steel frame to make the deck.
The crews have their work cut out next year to achieve all this. But Daewoo is confident it will hit the December 2010 completion date for this innovative crossing.