Only on the boat from the Songdo casting yard do you get a sense of the scale of work now under way on South Korea's £800M, 18.2km long Incheon bridge.
The bridge is half complete and piers and pylons which will support its six-lane road deck jut above the horizon as far as the eye can see.
As the boat draws nearer, it is clear that the precast deck for the 5.95km of viaduct leading from the crossing's western landfall is rapidly taking shape. 'We're on the 65th girder - the rst went up on 16 June last year, ' says Amec project investments vice president Mike Doran.
Meanwhile, the two inverted Y-shaped pylons, which will carry the 800m span cable stay section across the shipping lane to Korea's second largest port, have passed road deck level.
They are reaching towards each other as climbing formwork takes them skywards to their full 230.5m height.
Either side, huge precast concrete segments are sprouting from the tops of massive insitu concrete piers. These will eventually form the 145m span, balanced cantilever approach linking the western viaduct with the cable stay section.
The privately financed toll bridge is being built as part of a 21.3km expressway connecting Seoul's Incheon International Airport on Incheon island with a new free trade zone, known as Songdo, within the mainland city of Incheon.
The project includes the 5.95km western approach viaduct, the 2.45km eastern approach, each made up of pairs of 50m long precast girders supported by piers. The longer western viaduct has a total of 238 precast girders, while the east viaduct has 152. These approaches lead up to two 889m balanced cantilever viaducts comprising 12 pairs of balanced cantilever concrete box girders resting on piers 145m apart.
These are on either side of the 1.48km cable stay structure, which spans the busy shipping lane serving South Korea's second largest port. When complete in 2009, the crossing will connect with a network of roads, financed publicly, running though Songdo and into the mainland city of Incheon, which in turn borders Seoul.
British construction services company Amec is a 23% shareholder in Koda Development, the private sector company which won the concession to design, build, finance, operate and maintain the project in May 2005. Other shareholders are investment banks Macquarie of Australia, IBK and Kookmin Bank of South Korea. The banks are also supplying the debt finance, which is government guaranteed.
Amec is also Koda's project manager, overseeing the construction contract which is being undertaken by a group of construction firms led by Samsung and known as Samsung jv.
With the exception of the toll plaza at the airport end, the bridge is being built over water, although at low tide, large areas of sea disappear, exposing mud flats which hamper work. 'We've got a 9.3m tidal range, ' says Doran.
This created a challenge for Samsung, as the aim was to work regardless of the tidal conditions so the project could keep to its tight, four-year timetable.
The solution was to use an army of barges to support piling operations on the outer sections of the bridge. In all, 68 barges - including 20 jack up barges and seven marine batching plants - have been supporting work on the pylons and piers. 'During the piling phase we had 24 rock drills and a dozen vibro hammers, ' says Doran.
At the western end a temporary 2.05km jetty was built to give access to the piers nearest the shore, as the water was too shallow for contractors to float jack up barges in for the piling work and pier construction.
Construction of the approach viaduct piers finished in June last year. Samsung is now installing the 50m long 1,400t precast, prestressed concrete girders, working from the deep water end of the western viaduct. It is using one of the project's two giant floating cranes to lift them off barges that bring them from the casting yard at Songdo.
The floating crane lifts the girders onto a transporter vehicle, which runs on hundreds of pneumatic tyres. This then transports them slowly along the viaduct to a gantry. The gantry in turn raises and positions them (see diagram). When the viaduct is complete, it will be posttensioned using cables running through the girders.
The girders and piers have to be especially robust to withstand the loads imposed on them during construction. They are prestressed longitudinally along both webs and transversely across the top flange.
'The highest loads on the viaduct are actually during the construction phase, ' says Amec Project Investments deputy project director Alan Platt.
The added complication of the shipping lane meant that during the detailed design phase in 2005, engineers were forced to increase the span of the central cable stay section from 700m to 800m. This alteration was needed to accommodate the largest ships as they turned on their way out of the port. The cable stay deck must also rise 74m above the shipping lane to allow clearance for 1M dwt container ships.
If working around the tides and shipping lanes presented a challenge, so did the weather.
Winter in South Korea is extremely cold and work at sea comes to a halt in February and March, when temperatures plunge as low as -20 oC. Work on many of the pylons and piers was carried out simultaneously and installation of bored piles for these finished last December. Now that most of the bridge structure is out of the water, construction has speeded up, fed by the massive production line at Songdo which produces precast segments for the balanced cantilevers and the huge girders for the approach viaducts. 'We are trying to push out one girder every two days, ' says Platt.
At Songdo, the reinforced girders are cast into steel formwork which is set up in a huge indoor factory. When the formwork and reinforcement are ready, concrete is poured and steam-cured at 60 oC in bespoke curing tents to ensuring consistent concrete quality.
At present the girder installation is concentrated on the longer western approach viaduct. When the western end is complete, the contractor will switch its gantry and transporter vehicle to the eastern approach .
The central cable stay deck will be an orthotropic steel box girder made up of 107, 15m segments. Of these, 28 are being manufactured in Nantong China by Zhenhua Port Machinery with the rest made in Korea by Hanjing Heavy Industries.