Massive piles are being installed for what will be the world's second longest incrementally launched bridge. Adrian Greeman reports from east Scotland.
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 a 500m long bridge at Kincardine, but this is often overloaded with traffi c so a second structure is under construction.
Nesting and migration grounds on the shoreline salt fl ats mean that the new bridge must follow an oblique 45infinity 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.
the design and build contractor is Morgan Vinci which, with its specialist consultant Benaim, began work on the £42M crossing in April last year.
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 Benaim project director, Simon Bourne.
Some 130 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.
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 13m depending on whether the piles are sitting on hard sandstone or softer mudstone.
Such big piles were chosen 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 will sit beneath 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 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 pro e is particularly important for main piling contractor, Fugro 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 nd a contractor that could do it, ' says Bourne, 'and there are not many worldwide.'
Once selected there were ongoing discussions about design details.
Seacore, using its Teredo T40 reverse circulation drill working from the eight legged jack-up platform Excalibur, has to drill rock sockets up to 13m deep. It has been installing the 16, 3m diameter and the four central almost 4m diameter ship impact pier piles in a straight line diagonally across the river. On completion of its foundations contract, Morgan Vinci will follow on, building reinforced concrete 'Y' shaped columns on top of each pier pile to support the concrete bridge deck, which will be incrementally launched and jacked across from the casting yard on the river's north shore.
Seacore's challenges included getting the rig to the site in the rst place - the old Kincardine bridge downstream has a swing section at the centre but this 'solidi ed' more than 20 years ago and the barge had to remove its jackup legs completely to get through the navigation span, re-fitting them afterwards.
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.
'Keeping the pile casings plumb in over 20m of overburden and to the contract's ± 75mm pile top positional tolerance, has been a real challenge, ' says Seacore project manager, Phil Wilkinson. 'But we have a lot of experience with this type of work and with the use of our adjustable leader system and surveying techniques we have worked to keep something in reserve and have achieved a tolerance of ±50mm.'
Drilling then continues to form the socket below the casing toe into the 80MPa strength sandstone. On reaching full depth the bottom of the socket is cleaned to produce a uniform bearing surface for the concrete, by slowly rotating the bit while recirculating the water in the pile to remove all the cuttings.
'Working our normal 24 hour, round the clock operation, we've been averaging about a four day cycle on each of the 3m piles, ' says Wilkinson. 'These, which we started fi rst from both shores, and the fi rst of the 4m ship impact piles that are in the centre section of the bridge, are all at 45m centres.
'The distance between the two outer 4m piles is 53m, leaving a 65m main navigation channel. After fi nishing all the 3m piles first we'll then move onto the 4m ones and expect to fi nish them all on schedule by about the middle of March. We'll then have to dismantle Excalibur again to take her back under the swing bridge at an appropriate low tide and downstream to Rosyth for reassembly ready for towing to her next job.'
Concreting is a 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 fl eets of barges, along jetties and using cranes.
'Eventually we worked out a method with a circular steel platform supported on the pile casing, ' says Morgan Vinci project manager, Pierre Villard. A long gangway connects the piles to the shore to allow safe access and this also carries a concrete pump line.
The concrete is batched on site and pumped out along supply lines on the walkway to a pair of receiving hoppers and tremie pipes.
As the concrete rises, pushing the water out of the casing, the diluted concrete at the interface with the rising concrete and water eventually spills over and is collected into a blanked tube in the bull's eye at the concrete cut off level near the top of the reinforcing cage. This acts like a sump and the sloppy material is pumped out until consistent concrete reaches the desired level.
The largest pour in the 3m piles was over 200m 3 and overall took about 10 hours. The pours in the 4m diameter piles are expected to be completed in a similar timeframe by introducing additional concrete pumping capacity.
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 get to full height they will be ready for the advancing bridge deck, and the casings can be cut off. This work was due to begin in earnest in February as GE went to press.
The £42M bridge will form part of a new 6km, £100M road scheme and is due for completion in 2008.
Together with the recently opened Kincardine Eastern Link Road, it will allow traffi c crossing the Forth to bypass Kincardine and provide much-needed relief to local roads.
The bridge 'Shutdown of the power station freed up a slightly less important, semi-brownfield part of the shore, ' explains Simon Young, project engineer at Jacobs, responsible for the scheme's outline design.
'The rest of the shoreline consists of Sites of Special Scientific 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 south 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 profile 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 soffit 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 significant challenge for design and build contractor Morgan Vinci which, with its specialist consultant Benaim, began work on the £42M crossing in April last year.