In this brown, flat, landscape where the only feature is an enormous power station and water desalination plant some kilometres away, the jets create a lasting impression. But when Bd&e travelled to the bridge earlier this year, the site team had much more important things to think about. More than 1000t of new bridge superstructure, for example, which was being gingerly strand-jacked down the sloping deck to close the gaping hole which has been there for a decade.

The 2.3km long bridge crosses the Subiya Canal to Bubiyan Island, which is one of Kuwait's most strategic military outposts. The low-lying island is unimpressive, but its position, off the northernmost tip of the country, provides a good view of the short coastline of Iraq, and the estuaries of the Tigris and Euphrates rivers. The bridge to the island was completed in 1983 and allowed the army to access the land properly for the first time.

But the strategic importance of the structure was demonstrated in the Gulf War, ten years ago, when the bridge became a military target and was badly damaged. Two large holes each around 80m long were created where bombs hit the superstructure, and there was other damage from machine gun rounds and chunks of concrete displaced by the explosions. An intermediate contract was let by the Kuwaiti Ministry of Public Works in 1994 which involved stabilising the structure, removing the debris from the deck, replacing bearings which the blast had displaced, protecting prestressing cables and exposed reinforcement, and so on.

Money for the main repairs was not available, however, so after several years of waiting for the work to be done, the Ministry of Defence eventually took the project over and called for tenders to design and build repairs to the main spans.

The repair project involves replacing 80m of superstructure at the two gaps, along with additional strengthening measures and repairs at numerous locations. Under its original contract, the bridge was built by French contractor Bouygues and opened in 1983, and the superstructure design is for a concrete truss with top and bottom slabs. It is 18m wide and has 59 spans, all of which are just over 40m long except for the one over the shipping channel which is almost 53m long. The missing spans are between piers 8-10, where the bridge is level, and between 48-50 where there is a slope of almost three degrees.

A traditional method of carrying out this type of repair would have been to install precast concrete segments use a launching truss to bridge the gap. But contractor Freyssinet has come up with an unusual alternative which it claims has allowed cost savings on the traditional method. The solution uses a mixture of techniques - precast segmental and incremental launching - but with its own unique slant. According to structures & methods manager Fernand de Melo, Freyssinet opted to replace the concrete truss structure with two parallel box beams, which have bracing along the edges to mimic the appearance of a truss. Although the difference in superstructure will be apparent from side-on, from the land it will not be noticeable. The two box beams are installed separately, then an insitu concrete stitch is cast along the centreline of the deck.

In a nutshell, the construction method for each section of superstructure involves precasting sections of the box beam in 2.5m lengths, which are then brought to the site and assembled to the full 80m, on the deck on one side of the gap. The new section of deck has two launching noses, similar to those used for incremental launching, attached; one at the front and one at the back. The whole assembly is moved forward across the gap until the new deck is in the correct location, then the noses are removed and it is lowered down into position.

Of course, in reality the situation is much more complex. As de Melo explains, a great deal of preparation work is needed before the assembly of the beam can begin. The assembly bed must be constructed; this is essentially a pair of parallel skidding beams which are built into the deck of the existing structure. Because of the amount of work that was required to install the skidding beams, de Melo says that a decision was taken to use only one assembly bed for both halves of each new deck. Extensive temporary strengthening work had to be undertaken on the existing deck to ensure that it was capable of withstanding the load from the new spans as they were assembled.

But with only one set of rails, there obviously had to be some way of moving the new deck into the correct alignment ready for it to be launched into its final position. The rails were installed over truss node positions to ensure the most efficient transfer of load into the existing deck. Three sets of perpendicular skidding beams were also installed, each running almost the full width of the bridge over the pier positions where the existing deck truss diaphragms were located.

Before any precast segments could be brought to site, the front launching nose had to be put together on the assembly bed. Both noses are approximately 24m long, and are fabricated from two parallel I-beams with cross bracing the full length. They were designed by Freyssinet and made by a local subcontractor. Once the front nose was in position, the first precast segment could be attached to it and pushed forward along the skidding beams. Shear keys were incorporated into the joints between the precast segments; the faces of the joints were covered with epoxy resin and temporary prestressing of up to 2.5kg/cm2 was introduced to ensure that the resin was evenly spread across the whole face. Having assembled half the width and the full length of the new deck, the second nose was attached to the rear of the deck. Macalloy bars of 50mm diameter form a connection between each nose and the end of the new deck, and a cemetitious grout is also poured between them. Cables run below the new deck to connect the two noses.

Before the new deck unit can be shunted into the correct alignment for launching, it is jacked off the assembly beams and onto the three sliding beams which sit one over each pier. Once it is in the correct alignment, the 'launching' process is ready to begin.

Because access to Bubiyan without the bridge is very difficult, Freyssinet has worked from the mainland abutment, first repairing the gap between piers eight and ten. When Bd&e visited the site at the end of March, the team was starting to launch the first section for the second gap, between piers 48 and 50. As Freyssinet site manager Bruce Richards explains, this launch is much more tricky because of the gradient. "For the first two launches, we were moving the deck sections on the flat, which was very simple," Richards says. Once the initial friction had been overcome and the deck was moving, it was just a question of maintaining a steady tension in the strand-jacking equipment. This equipment consists of three

But the bridge rises to cross a navigation channel, and the second gap is on the island side of this hump. Consequently the site team is effectively launching downhill, and careful consideration has had to be given to the friction coefficient between the new deck and the sliding bearings. "We have a gradient of about 2.8% on this section," Richards says, "so we are monitoring the movement very carefully throughout the launch." Strand-jacks are also used on this launch, but the deck obviously moves to some extent under its own inertia. In order to control the speed at which it moves, the engineers had to first experiment with different pads for the sliding bearings. At first all of the Teflon pads were replaced with plywood, but this increased the coefficient of friction by too much. A combination of plywood and Teflon pads proved to be the most effective solution.

During the launch, de Melo explains, a number of options exist for slowing the deck if it starts to move too quickly. The simplest is to feed plywood pads in to the sliding bearings where Teflon pads are being used. As a second option, jacks at the rear of the deck can be brought into operation to slow it down. The final option in this 'belt and braces' approach is a hefty tow truck parked uphill from the launch site, with its steel cable attached to the rear of the deck.

Each launch for the first repair took just two days - on this repair de Melo expects it to take slightly longer, although progress on the first day was smooth and problem-free. When the launching nose reaches the central pier, jacks built into the nose will be used to correct the deflection so that the deck can run smoothly across the bearings and on to the far side of the gap. Damage to the bridge at this location included the destruction of the central pier, which has had to be rebuilt from the steel pile upwards as part of this contract. But because Freyssinet could not be certain of the capacity of the foundations at this location, temporary cables from the pier head to the existing bridge are being used to resist horizontal forces due to the launching procedure.

Once the deck has been fully launched, the engineers can begin preparing for the lowering stage. It takes about two weeks to install all the necessary equipment and temporary works for this operation. Lowering towers are installed on the existing deck at each side of the gap; those on the central pier were put into position before launching began. The towers are constructed using a stack of 600mm deep shims formed from tubular steel sections, filled with concrete and with steel plates top and bottom. Each tower also has a set of similar shims 100mm deep. At each of the tower locations - the two ends of the deck and the central pier - are two pairs of jacks. By jacking corresponding pairs at each location, shims can be removed from below the other pairs of jacks. These then take over the load of the deck while the other jacks are closed and shims removed from below them. The lowering process, which is carried out 100mm at a time, can take four days to complete.

With the deck fully lowered, continuity tendons are installed, in-situ concrete is poured at the connections, the tendons are stressed and the load is then transferred onto the existing piers.

As soon as the first section of this gap is complete, Richards expects the bridge to be busy with army personnel. Without the link, army bases on the island have had to be supplied by boat, and even facilities for landing boats at Bubiyan are very limited. This has proved restrictive for Freyssinet in the day to day operation of the site. Everything that is needed on the island side has had to be brought across by boat - early in the contract, Richards recalls, they hired a barge and brought across concrete mixer, forklift truck and so on; since then they have had to rely on the small speedboat that serves as personnel access, materials supply and so on. At times, says foreman Michel Joseph, they have not even been able to rely on the boat - sometimes the canal empties of water entirely, and all that is left is mud.

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