Heavy lifts for bridge and tunnel

Floating multi-winch pontoons, 3,000t shear leg barge cranes, a floating drydock lifting 9,000t units and assorted tower cranes are some of the most important tools being used the complex and difficult Busan-Geoje fixed link project currently under construction in South Korea. It is the most challenging project to date for Korean contractor Daewoo, leading a six firm consortium on the $2.2bn scheme.

The pontoons are critical for the moving and placing of huge 48,000t precast tunnel units which will make up nearly half the length of the crossing. The concrete boxes will form an immersed tube tunnel 3.7km long and, for a while anyway, will be the deepest road tunnel placed this way in the world, at up to 48m depth.

The cranes have been moving giant concrete sections for two bridges and their approaches.

The new link crosses three main channels, at the entrance to a large sheltered bay at Busan, Korea’s second largest city. The tunnel links to two cable stay bridges and their approaches pass over the remaining two channels. Short viaducts and rock tunnels on two small intermediate islands tie the crossings into a single 8.2km long, dual two-lane road link.

When it opens in 2010 the motorway connection will have a major impact on the southern coastal region of South Korea. This is the location of the world leading Korean shipbuilding industry, which takes advantage of the deep water and wide bays on the coastline, and a constant flow of ships to sea for trials and delivery, and to Busan’s new container port.

Geoje is Korea’s biggest island and home to nearly a quarter of a million people, with major shipyards including Daewoo and Samsung. It is also highly scenic, attracting tens of thousands of tourists each year.

Access has been difficult with only a one hour ferry link from Busan city or a road trip around the west side taking three hours. The new route takes just 40 minutes.

An immersed tunnel, formed from sections lowered to the seabed, was chosen because the steeply sloping coastline would have meant a very deep alignment for a bored tunnel. Extremely long approaches would be needed.

For the tube, huge 180m long 26m wide units are made five at a time in a casting yard 30km away along the coast. Meanwhile a seabed trench is prepared by dredging at depths of up to 48m and an accurate flat gravel foundation layer is placed with a jack up barge.

When the units are ready they are sealed with steel bulkheads at each end and the precast yard is flooded, so they float, mostly beneath the surface like icebergs. Winches and tugs pull them out to a nearby mooring until the weather is good for placing.

Two special winch pontoons are connected around the floating tunnel element ends before it is hauled to site by tugs. The pontoons were designed by Daewoo in conjunction with specialist immersed tube consultant Strukton of the Netherlands. Daewoo specified that they should fitted with nine winches each, five of 60t capacity and four of 30t, and there is a computer control system for them all.

Only two of the 60t winches each end are connected directly to the element while the rest are linked to a series of anchors on longitudinal and lateral cables, which finely control the pontoon position.

“Actual ‘lifting’ of the mass is primarily done by the buoyancy” explains Bong-Hyun Cho, Daewoo deputy general manager for planning and engineering. Six 1,000 cu m water tanks sit inside the hollow elements with interconnecting pumps which tune the buoyancy. These are slowly filled with additional ballast water to start the element’s descent.

The pontoon winches provide fine adjustment as the element slowly moves downwards to settle in the trench. Millimetre accuracy is required for the placing despite the huge size of the tunnel elements, otherwise special rubber Gina seals at the end will not fit correctly to ensure the tunnel is watertight.

Once in position more water is added to the tanks to settle section tight against the previously placed unit, and the trench is backfilled. Extra concrete later adds more weight, the tanks are removed and the bulkheads dismantled. The interior becomes part of the tunnel length which is eventually fitted out.

The argument for using large scale block construction for the three cable stay bridges was unassailable. Firstly, it allows much of the construction work to be done away from the site, which is exposed to open sea conditions. Devastating typhoons lash the coast in the summer months with winds strong enough to lift cars, wreck boats and leave dockside cranes in twisted heaps.

Secondly. the equipment and technology to use precast was easily at hand; a precast facility was already needed for the immersed tube tunnel. Where better to find high capacity floating cranes, and build floating transport docks, than in the centre of a major shipbuilding area?

“The bridges are being built just like giant Lego” says Don Fraser, from consultant Halcrow, which is construction advisor to Daewoo Engineering & Construction. “Most of the approach piers, the decks and parts of the main pylons are made in sections onshore and then taken to site and assembled in position.”

That is appropriate enough, since the large scale precast technique was brought to maturity on the long Storebælt and Oresund crossings in Denmark, home of the children’s plastic block assembly toy. Lessons learned on those projects are now applied to this design by Danish consultant Cowi, and to construction by Halcrow’s advice.

The scale is a little greater than Lego: the main two pylon twin-plane cable stay for the central channel, has 475m central span and a total length of with approaches of 919m, and the smaller three pylon cable stay, has two central spans of 230m. Precast caisson bases for the main pylons on the larger bridge are 33m high and weigh as much as 9,570t.

Even the pier shafts for 22 approach spans on the two bridges are substantial, each between 1,300t and 1,800t.

For moving and positioning these Daewoo has hired local freelance shearleg vessels which are constantly in use for the shipyards.

“Mostly we have used a unit from Heung Woo Industrial company. It is a shearleg made by HanJing Heavy Industry” says Daewoo’s technical support manager for the bridges Dr Sang Kyoon Jeong. The crane has a maximum 3,000t lift and is powered by a 1,600Kw diesel electric system. It operates in two modes, lifting 1.25m/minute for heavy work and 2.5m/min for lighter loads.

The shearlegs are moved by up to three tugs. On station a system of anchor winches control their position. Normally they can only operate in relatively good conditions with less than 1.5m wave height and under 8m/sec wind speed.

To move the biggest caissons a 13,200t floating drydock was specially designed by Daewoo and built for the project in a local yard, owned by Daewoo itself. It is flooded to ballast it down into the water and then manoeuvred underneath the bigger elements, once they have been partially lifted out of the water by a floating crane. It is floated up and tugs pull it to site 35km away.

Many of the approach columns are made as one unit, though larger ones have the “coping”, or crosshead, made separately to ease transport weights. The big multicelled caisson units which form the lower part of the piers and the main pylons, are made in two sections, with a handleable size lower portion made first and lifted by 3,000t shear leg into a wetdock area just offshore where the top section can concreted and finished.

A secondary casting yard has also been used for some of the work at a location on Geoje island itself, at Obi Bay. Here a production line has been set up to produce the composite steel and concrete deck span sections for the approaches, each 90m long. Steel is brought in from a fabrication plant near Seoul and the concrete deck formed by road pavement plant, again in controlled conditions under shelters.

Accuracy of the placing work has been critical. Onto prepared rock surfaces up to 31m below water level, three rectangular precast concrete pads are made to within +/-10mm and the crane has to position units on these. They are grouted down firmly.

For the bigger sections “the crane cannot lift the full weight, so the process relies on the buoyancy they have from their air filled cells,” explains Fraser.

Units arriving on the floating dock are partially supported by the crane as the dock is submerged and pulled back out of the way. In position the caissons are filled with water ballast and eventually this is replaced with rockfill to resist ship impact.

With caissons in place much work this year has been on positioning the precast approach piers, which are up to 35m high. These are formed with a “belled” out base which slots over the caissons, “just like Lego”. A three point flat jack system at the bottom is used to make fine adjustments to level before a concreted in-situ stitch finishes the job.

At the end of 2008 most of the piers were in place with half a dozen to go on the smaller bridge approaches. Many of the 90m deck sections were also installed, the units brought by floating crane from the Obi Bay yard, which has a deep water wharf from which the big shear leg cranes can simply pluck them in one go. The deck units have lifting eyes pre-stressed through temporary holes in the top flange of the steel side beams to allow for this.

The main piers of the bridges are now rising to the full 158m height on the larger cable stay and 110m on the smaller ones. Because they are so large they must be done by traditional in situ casting methods. Five 290HC12 Liebherr towers cranes assist with formwork and general lifting, one on each tower, and on the smaller bridge also skip in concrete.