SOUTH KOREA'S £450M Youngjong Grand Bridge, due for completion late next year, will link the capital Seoul on the mainland with Inchon International Airport being built on an island 40km to the west.
The suspension bridge section will be the world's first two-storey, self-anchored bridge carrying both road and railway. It crosses a major shipping channel and rises to 35m above sea level to allow ships to pass beneath.
The 4.42km long double deck bridge and will carry six lanes of traffic on the 41m wide upper level plus two lanes either side of twin railway tracks on the lower level.
Foundations for the main 500m long suspension bridge section presented a challenge for main contractor Samsung, is responsible for the two bridge towers and three pier supports for its approach structures.
The original plan was to support each tower on 35, 3m diameter reverse circulation drilled piles installed from within a cofferdam. But the idea had to be abandoned when driving steel sheet piles for the cofferdam walls became too difficult.
'In the end we chose to use an unmanned pneumatic caisson which we had seen operating in Japan,' says site director Man Geun Yoon. Two 47m by 18m precast concrete jackets were towed out to sea by a floating crane and lowered on to the sea bed.
Once in position, compressed air at a higher pressure than groundwater was forced into the working chamber at the bottom of the caisson. Remote- controlled mechanical shovels then dug down 20m through 7m of silty clay and 6m of weathered rock to reach load bearing strata.
Pier foundations were put down inside cofferdams in the channel, with additional bracing to provide support against tidal pressure. Between 50 and 60 1.5m diameter reverse circulation bored piles were installed to an average of 27m depth for each. These were filled with 30MPa concrete and capped with a 37.5m wide, 18.75m deep and 5m thick slab, also cast from 30MPa concrete.
Client for the bridge is the New Seoul International Airport Freeway Company. As well as Samsung, contractors include Hanjin, Kumho, Kolon and Dong Ah. Design is by Japan's Chodai and Yooshin Corporation of Korea. The UK's Tony Gee & Partners is working for Hanjin on aspects such as the erection scheme and temporary works.