Seabirds are the reason why a bridge nearly 1.2km long is being built at Kincardine, east Scotland. The town is located 32km inland at a point where the Firth of Forth estuary is relatively narrow. There is an 500m long bridge at Kincardine, but this is often overloaded with traffic so a second structure is under construction.
Nesting and migration grounds on the shoreline salt flats mean that the new bridge must follow an oblique 45° alignment to the south shore, making it almost twice as long as its neighbour.
Skewing it across the estuary enables the structure to avoid the most sensitive marshy areas and to make landfall in the coal yard of a disused waterside power station.
'Shutdown of the power station freed a slightly less important, semi-browneld, part of the shore, ' explains Simon Young, project engineer at Jacobs, which did the scheme's outline design.
'The rest of the shoreline consists of Sites of Special Scientic Interest, protection areas designated under the European Habitat Directive and special protection zones, making this a very ecologically driven project.' The new Kincardine bridge forms part of a new 6km long road for Transport Scotland. It is being built to supplement the existing crossing, and to serve as a relief diversion for lorries that are unable to use the Forth Road Bridge further downstream during high winds.
To reduce environmental impact further, the client and Jacobs selected an incrementally launched precast deck construction method, using a single casting and launch yard. The launch will be 1,190m in total, just short of 1,000m across open water with an additional 200m to clear marshland on the southern shore. 'This way we avoid using an embanked causeway, which would have churned up the habitat, ' says Young.
Bridge aesthetics are tightly controlled. It will have a low prole with a slender 17.5m wide deck. An examination of other projects, especially the Broadmeadow launched motorway bridge north of Dublin, led the client to opt for a curving soft to soften the deck's silhouette. The Irish bridge has a square box section.
Single column piers will support the deck. 'With the angle of the bridge, multiple columns would appear as a forest of concrete, ' says Transport Scotland project director David Mustard. There were fears that as well as being visually clumsy, multiple columns could affect birds.
All of this adds up to a signicant challenge for design and build contractor Morgan Vinci which, with its specialist consultant Benaim, began work on the £42M crossing in April this year.
Both have previous experience of launch schemes. Benaim worked on Broadmeadow, and Morgan Vinci worked on the Channel Tunnel Rail Link's Thurrock viaduct and Medway Bridges. But the concrete bridge deck here at Kincardine is exceptional and will be second in length only to a Bavarian viaduct scheme.
Total construction time allowed is 29 months. Disrupting the birds for more than two winter seasons could cause permanent behavioural changes, Transport Scotland's specialist environmental advisers warn.
The key to the job is in refining the design, and then achieving a tight sequence for bridge deck construction and launching, maintaining a fairly rigorous cycle time, says Morgan Vinci project manager Pierre Villard.
'We especially need a high level of accuracy in setting out the launching yard, ' he adds.
'We learned from Thurrock that reducing friction is very important, so we are setting exceptional tolerance for casting the skid beams [on the underside of the deck] and xing the steel top plates [on the piers].' No more than 0.5mm variation is allowed.
Benaim project director Simon Bourne explains that as the push advances, friction resulting from variations in geometry will accumulate with each successive pier top crossed. With 32,500t of deck to move, and pushing forces of up to 2,000t, the smoother the surfaces the better.
An important early factor has been piling for the casting yard, 'which sits on 20m of toothpaste-like clays in the old coal yard', says Bourne.
Some 130 continuous flight auger (CFA) piles are being installed down to sandstone 20m below; these are each 750mm in diameter and heavily reinforced, says Bourne. The piles give rigidity for precision casting, but must also take the transverse loads as the jacks thrust the cast sections forwards. Hydraulic jacks used at Thurrock are being redeployed at Kincardine.
But the yard piles are nothing compared to those for the bridge piers themselves. These will be giants, 3m to 4m in diameter and up to 35m long to key into the bedrock. Rock sockets will vary from 3m to 12m depending on whether the piles are sitting on hard sandstone or softer mudstone.
Choice of such big piles has been made to fit the bridge aesthetics and the marine conditions, says Bourne.
Installing a conventional pile cluster at each of the 25 piers would have been time consuming and awkward in the open estuary, and would have required a pile cap. This would have meant creating a protected space inside a coffer dam, which would have taken time, caused disruption and carried the risk of causing environmental damage.
The single piles work well with the 2.5m diameter single piers for most of the 45m span sections. Larger 3m diameter piers are used for a central 60m span and the two adjacent 53m spans, all designed for navigation. These will have bigger 3.85m diameter piles, with substantial reinforcement to resist ship impact.
'A single pile is actually more efficient in use, ' says Bourne.
A cluster would have to take bending loads during the deck launch process, imposing extra compression or tension in the piles which would then need to be made stronger.
'The single pile and column above is like a big stick in the ground, ' says Bourne. 'The bending load is taken by the pile and the ground around it.' Even the soft clays higher up in the ground have a considerable spring action, he says, 'though you do need some fairly careful calculation'.
Being sure of the ground has been a major task, because it is exceptionally variable. Though there is hard sandstone at about 20m down, the level goes up and down like a mountain range.
There is also a complex mix of alluvial clays, gravel layers and mudstone above. Cores were taken at each pier location for the client's studies and another dozen were made by investigation specialist Fugro over the summer.
The ground prole is particularly important for main piling contractor, Seacore, which has brought a large jack-up barge to site with a big reverse circulation rig on board, a little like an inverted raise bore drill.
'Choosing this method meant being sure we could find a contractor that could do it, ' says Bourne, 'and there are not many worldwide.' Once selected there were ongoing discussions about the details of design.
Seacore's challenges included getting the rig to the site in the first place - the old Kincardine bridge downstream has a swing section at the centre but this 'solidied' more than 20 years ago and the barge had to remove its jack-up legs completely to get through the navigation span, re-tting them afterwards.
'We had to fit the barge crane with a new, longer jib to let it do that, ' says Seacore contract manager Julian Cockett.
Piles for the piers have a 25mm thick tubular steel casing. They are driven through the softer ground before the rock socket is formed. The subcontractor's job on each pile is completed when it lifts a reinforcement cage into the pile casing. Morgan Vinci make cages up on shore.
Concreting is the next challenge. Each pile takes about 300m 3 in a single pour lasting as long as 12 hours. The team looked at various options for achieving this, including supplying concrete using fleets of barges, along jetties and using cranes.
'Eventually we worked out a method with a circular steel platform supported on the pile casing, ' says Villard. A long gangway connects the piles to the shore to allow safe access and this also carries a concrete pump line.
Villard plans to use the empty upper part of the pile casings as mini-cofferdams, enabling the bridge columns to be built directly onto the piles. Once these are at full height they will be ready for the advancing bridge deck, and the casings will be cut off.
That work begins in earnest in February, when the piles and piers are well advanced. So far only a half dozen have been completed, although a test push of the first deck section will be carried out shortly.