The cable stayed section of the Busan-Geoje Island crossing is an impressive feat. A two pylon cable stayed bridge spans between Jungjuk island and Jeo island, with a main span of 475m and a shipping clearance of 52m. And a three pylon cable stayed bridge spans between Jeo island and Geoje island,with two main spans of 230m and a shipping clearance of 36m.
What the bridge has in common with the tunnel is the difficulty of building across the open sea. "There’s nowhere for a big construction yard, so construction is about prefabbing as many elements as you can, as big as you can and transporting them," explains Halcrow technical advisor Don Fraser.
There are five pylons in total for the two cable stayed bridges which have to be built in deep sea conditions. There are also numerous approach span piers also in the deep sea waters.
Caisson foundations have been used for the piers and pylons as they eliminate the need for water excluding temporary works like cofferdams. The caissons are 33m high precast concrete cellular structures and once in place take loads from the piers and pylons to the sea bed. "With 30m of water, it would have been a significant cofferdam," says Fraser.
The caisson foundations were fabricated at a casting yard on the mainland. The heaviest weighs 9,600t but the natural buoyancy of the open-celled structures was utilised when they were taken out to sea. By floating them, the weight could be shared between the water and the crane so a smaller 3,000t floating crane could be used.
The ground on which the caisson sits was prepared by dredging the alluvial layer so that the caisson sits on the weak rock layer below. A video inspection of the formation layer was done before caissons were positioned. Three precast landing pads were placed on the formation layer ready to receive the caisson. The caissons were sunk using water ballast which was later replaced by rock ballast once they were in place.
Once a caisson had been positioned on its three landing pads, grouting was carried out to fill the gap between its underside and the seabed, so that when the bridge is complete, pier and pylon loads can be transferred evenly to the ground. Grouting is critical to the stability of the piers, and the team has taken extra precautions to make sure it all goes to plan.
During the grouting process on one of the Øresund bridge piers, cement particles were washed out from under the pier. This led to a loss of foundation strength. "Mistakes have been made on similar projects which we’ve learnt from," says Fraser. "Grouting went wrong at Øresund and took a year to fix. Øresund initially did not have the means of preventing the ocean current from washing out the cement particles from under the base."
To stop cement washout, a trench flap was used. The trench flap is a geotextile membrane secured around the perimeter of the caisson and held down with a steel bar. When the caisson was being transported, the geotextile member was rolled up around the rebar and then once positioned, the membrane was unrolled by divers. "It’s enough to stop current and debris and makes for 'still water' conditions [around the caisson]," says Fraser.
The grout must fill all voids between the underside of the caisson and the sea bed to be effective. Venting holes with thermal sensors inside them are closely spaced over the plan area of the base. When grout reaches a sensor a change in temperature is registered indicating displacement of sea water by the arrival of warmer grout. "When the warm grout reached the vent pipe, we knew it was in contact with the caisson under side,” says Fraser. Two large scale tests were done to ensure that caisson grouting would work, as failure would have been costly.
A 3,000t floating crane was used to place the pier shafts onto the caisson tops where they were connected via an insitu reinforced concrete joint. The smaller approach pier shafts came with their cross beams attached, but the bigger ones had their cross beams attached insitu with couplers and post tensioning bars as the combined weight would have overloaded the crane.
The main pylons are up to 156m tall and are cast insitu. From their caissons, two legs splay outwards until they are level with the road deck, at which point they incline towards each other. The lower sections of the pylons are filled with rock ballast up to a height of 16m to protect against ship impact. "It’s a nice shape, but it’s a challenge for the guys to build," says Fraser. "The setting out is difficult – requiring very strict camber control and the use of temporary intermediate props between the upper legs."
The pylons are made from insitu concrete distributed by a 180m3 capacity floating concrete batching plant. They are being constructed in 4m high lifts using climbing formwork and concrete pumps. All five pylons are scheduled for completion by July 2009.
The bridge deck consists of a steel framework which supports a concrete slab. The sections are prefabricated offsite in a casting yard in Obi. Sections of steelwork are fabricated off site and brought to the yard where they are welded together before the 300mm-thick concrete deck is cast on falsework in one seven hour sitting. Once it has cured, the completed deck sections are "finished" as much as possible, to the extent they include drainage runs and cable trays. They are thens moved to the waterside to be lifted by a floating crane and transported to the bridge site.
The depth of the steel girders for the cable stayed bridge deck is only 2m compared to the 3.6m deep girders needed for the approach spans. The deck structure for the cable-stayed spans is slightly lighter as it is supported by stay cables along its length. The concrete deck for this section is also lighter with a thickness of 260mm. The cable-stayed deck sections will be erected in a balanced cantilever sequence. The sections are taken out to site, lifted into place, starting at the pylon and working outwards, and the cables are connected. Both sides of the pylon are placed consecutively to balance the loads.
Superstructure erection will start in mid March this year, with the last segment being lifted in April 2010.