When the new link from the Korean mainland to the island of Geoje opens in December 2010, the journey time between them will be slashed by two hours. The 8.2km-long transport connection will provide a four-lane highway link from Busan, South Korea’s second largest city, to the island of Geoje, home to two of the world’s largest shipyards. Two cable-stayed bridges and an immersed tunnel will take vehicles on an island-hopping route across water depths of up to 40m, at a site exposed to severe open sea conditions from the Pacific Ocean (Bd&e issue no 43). The US$2.5 billion fixed link is being built by GK Corporation, a seven-contractor consortium led by Daewoo Engineering & Construction. A joint-venture of Halcrow and TEC is providing technical consulting services to Daewoo E&C, in a design team led by Cowi.

This privately-financed BOT project is divided into three tranches of work, with six years planned for construction and a 40-year concession period. The first tranche is a 1.65km-long three-tower cable-stayed bridge with span lengths of 230m, 36m navigational clearance and 102m-high pylons; the second is a 1.87km-long two-tower cable-stayed bridge with a main span of 475m, 52m navigational clearance and 156m high pylons; the third tranche is a 3.24km-long immersed tube tunnel passing below the busy shipping channel that leads to Busan’s container ports and the South Korean naval base at Jinhae.

The bridge works are 50% complete and are phased to tie in with critical-path immersed tunnel works, which in February 2008 passed a major milestone with immersion of the first 45,000t, 180m-long tunnel element. The second element has since been immersed and the third one was due to be placed by the end of April as Bd&e went to press. Immersion works will then be suspended until October, when the typhoon season is over. At 48m below mean water level it will be the world’s deepest concrete immersed roadway tunnel and is the project’s biggest challenge.

The cable-stayed bridges are also significant structures with their own special challenges for the contractors and the design team. The exposed locations with limited working space led to a decision to fabricate the bridge in large sections at off-site construction yards, transport them by sea and lift them into place using floating shear leg cranes.

The designers' experience of these techniques on the Oresund Crossing between Denmark and Sweden is notable, particularly in the substructure’s precast concrete caissons, approach bridge pier shafts, and copings and cross-beams for the two-tower bridge. Only the tower legs, abutments and one easily-accessible approach bridge pier are being fully built in situ.

Two purpose-built yards some 35km from the bridge sites are producing prefabricated parts for the two bridges and the immersed tunnel. At Anjeong the 286,000m² facility produces all the bridge precast substructure sections as well as 18 immersed tunnel elements.

A smaller yard at Obi Bay is producign precast cross-beams for the two-tower bridge, complete approach bridge spans, precast deck units and many thousands of concrete blocks for sea protection works.

All but five of the bridge foundations are concrete caissons on weak to strong granodorite up to 31m below water level. For the deep water piers, founding level is reached by grab excavation of the overlying alluvial clay/sand followed by airlift cleaning. After placing three precast concrete landing pads on the approved formation the caissons are placed by floating shear-leg crane, water ballasted, under-base grouted, and then have stone ballast placed inside them before being backfilled externally with selected rock fill topped with rock and concrete block scour protection. A capping concrete completes the caisson works to 2m above water level. All in situ grouting and concreting works are supplied by a floating concrete batch plant which has a capacity of 180m³ per hour.

To reduce the risk of problems with the under-base grouting, two large-scale grouting trials were carried out to develop techniques and test the material before use in the works. To ensure that the grout has the required contact with the caisson underside, venting holes with thermal sensors inside them are closely spaced over the plan area of the base slab. When grout reaches a sensor a change in temperature is registered indicating displacement of sea water by the arrival of warmer grout. Use of trench flaps between the caisson and excavation sides help create still water conditions during the grouting works.

The 23 caissons for the deep water piers and tower are first cast on land and then moved by floating crane to temporary foundations on the seaward side of the precast yard where they are completed. Pylon caissons are up to almost 33m high with dry weight of up to 9,573t, which exceeds the capacity of the largest available floating shear leg crane. To overcome this they are partly submerged, then taken by floating crane and placed onto a submerged floating dry dock. The floating dock transports them to the bridge site where the dock re-submerges. The floating crane follows, lifts the partly-submerged caisson off the dock and positions it onto its landing pads.

The lighter caissons are lifted from their precast position on land and carried by floating crane directly to the bridge site. Despite the potential for problems carrying out heavy marine works in deep water, the contractor has now installed 21 caissons without incident.

At two of the piers, the amount of overburden is excessive so piled foundations of up to 53m deep are used. These piles extend above water level and are currently are being prepared to receive precast housings that will provide permanent formwork for the in situ cast pile caps. The first one was expected to be placed as Bd&e went to press.

Twenty-two precast concrete pier shafts, some with the pier copes attached, weighing from 1,318t to 1,814t and up to 35m high, are placed onto the caissons and vertically aligned using temporary hydraulically-linked flat jacks. Caisson and pier shaft vertical rebar is coupled, after which an internal in situ concrete stitch completes the structural connection. As Bd&e went to press, five shafts had been placed.

One of the approach bridges on the second parcel of work has a climbing lane over part of its length, requiring a wider deck on copes that are up to 34m long and weigh 2,079t. If they were cast in one piece with the pier shafts, their weight would exceed the floating crane capacity, so six of the largest copes are precast separately, as are the copes for the two anchor piers, for the same reason. Their 5.5m wide by 4.8m-deep cross-sectional dimensions require the use of cast-in cooling pipes to control temperature differentials and hence avoid early age thermal cracking. These copes are then post-tensioned, transported to site by floating crane, placed onto the pier shaft and aligned using flat jacks. The gap between the shaft and the cope is grouted, and vertical post-tensioned bars complete the structural connection between the two.

With foundation works nearing completion, tower construction is the main focus for the bridge construction teams this year. The shape of the tower legs provides the biggest challenge for contractors - they splay outwards from the caisson top to a lower cross-beam which is just below superstructure level. For aesthetic reasons, they then curve upwards and remain separated apart from connections to upper cross-beams. This design gives improved aerodynamics over traditional H-shaped towers, and a smaller foundation footprint but it is more difficult in terms of requiring very strict camber control and the use of temporary intermediate props between the upper legs.

Peri ACS jump-forms are being used to build the legs in 4m lifts, with concrete being pumped into place through pipelines inside the tower legs. Graded rock ballast is placed inside the tower legs up to 15m above water level, from this level upwards the towers are open-celled.

Progress on the bridge with two towers sees 16 of its 41 segments concreted; its substructure is due for completion early in 2009. The three-tower bridge which is due for substructure completion in May 2009 has five of the 20 segments completed on the most advanced of the legs.

The bridge team achieved a significant milestone at the end of last year with successful placing of the precast lower cross-beams on the two-tower bridge. At 806t it was not their weight that posed the challenge but their stiffness. Maintaining an equal load distribution while lowering the cross-beams from the floating crane on to four temporary steel supports on the tower legs required careful planning, calm sea conditions and finely controlled placing. All the other cross-beams will be built in situ on falsework.

All 29 superstructure spans for the approach bridges are being fabricated at the Obi Bay yard in span lengths of up to 90m, which weigh from 1,601t to 2,427t; the first five have now been completed. Sections of steelwork are fabricated off site and then welded together at the yard into 90m lengths made from 3m by 3.6m-deep longitudinal steel plate girders with bolt-on cross girders. Once the steelwork has been set to the right camber, the 300mm-thick concrete deck is cast on falsework, and once it has cured, the completed spans are skidded to the waterside to be lifted by floating crane and transported to the bridge site. The first span was due to be erected on the two-tower bridge approach at the end of April, as Bd&e went to press.

Cable-stayed spans for both bridges will consist of twin 2m-deep outer steel plate girders, cross-girders at 4m centres and a 260mm-thick concrete deck. The 'floating' superstructure will have no vertical bearings at the towers and be supported only by stay cables along its length, enabling a more slender and economical design. Lateral and longitudinal restraints are provided at all towers.

The cable-stayed superstructures will be built using traditional balanced-cantilever techniques, using prefabricated 12m long, 96t steel segments that will be brought by barge to the bridge site and lifted into place by derrick cranes. Once they have been bolted to the previous segment, a set of stay cables will be attached, and precast concrete deck slab panels will be connected to the steelwork using in situ castings.

Stability of the superstructure under typhoon conditions will be a major consideration, since there may be up to eight cantilevers under way at one time. Engineers are planning to install a combination of guy cables which will be connected to anchors on the sea bed, and tuned mass dampers.

Fabrication of the superstructure steelwork has started, the cable-stay contract for the two-tower bridge has been awarded and it is planned that superstructure erection will start early next year, with closure of the last key segment in the middle of 2010.

On the Jeo Island connection between the two main bridges, work continues apace. Two bored tunnels are substantially complete and two of the three viaducts have steel girders erected awaiting arrival of precast deck panels.

Don Fraser is Halcrow's advisor for bridge and immersed tunnel structural works

Government authority: Busan City, Gyung Sang Nam Do
Concessionnaire: GK Fixed Link Corporation
Main contractor: Daewoo Engineering & Construction
Bridge designer: Cowi

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