The long, long push launch for the new road crossing at Kincardine in Scotland is just passing through its most complicated stage after successfully reaching the halfway point.
Extra work was needed both at the river and at the casting yards to cope with bigger spans and two-stage pushes.
Since April, the 16.5m wide curved soffit bridge deck has been edging out in 45m increments along the oblique route across the Firth of Forth.
It slides onto a row of single-column piers currently being formed above 3m diameter monopile foundations that were installed by Seacore last winter (NCE 15 March).
More than a dozen spans are now complete and the crossing will eventually comprise 26 spans, a total length of 1.18km, making it the second longest continuous deck launch in the world.
The £42M Upper Forth Crossing, built for Transport Scotland – or the “Fourth Forth” as it has been nicknamed – is a total of 6km long including approach roads, and will take most Highlands-bound traffic from the existing right-angled crossing.
This, located at a relatively narrow point on the Firth, needs just a 500m reach to get across but for environmental reasons double that length is necessary for the new crossing. By opting for a 45° alignment it can avoid protected salt marsh areas and important bird migration grounds on either side of the Forth.
To help the enormous push go smoothly, it has been tightly engineered to reduce the weight as much as possible.
Contractor Morgan-Vinci has worked closely with its own consultant, bridge specialist Benaim Group, to remove nearly 6,000t from the initial tender design.
At 32,000t there is now not only less to move, meaning smaller jacks and less chance of things going wrong, but considerable savings in reinforcement and concrete.
"Essentially, we are making best use of the conventional reinforcement in the bridge during the launch as well as for the dead load," says Benaim's Simon Bourne.
The trick, he says, is to use external prestressing supplemented with some additional straight reinforcement rather than relying on full prestress for the launch.
That in turn is possible by using external rather than internal stressing, explains Benaim engineer Paul Mullins who was on site during the summer period.
"It means you can use different parts of the code, allowing some crack development in the concrete."
Durability codes allow no cracking at all in bridge decks if internal stressing is used. This means the concrete must not go into tension at any point. But passing the deck over multiple pierheads into the sag of a span centre, means there will be considerable flexing. Stressing must be high – and the concrete stronger.
With external prestressing small 0.25mm cracks are allowed, which means some tension is possible. In fact, only partial prestress is needed during the launch because the reinforcement needed for the dead load on the complete bridge, also comes into play during the launch. The full length of the bridge will be post-tensioned to its full capacity afterwards.
But there is a lot more to ensuring a successful launch. Just how the casting operation is set up and the jacking carried out is critical too says Morgan Vinci project manager Pierre Villard from the French side of the venture.
He has brought experience not only from France but also the High Speed 1 Medway crossing and the Thurrock viaduct.
The main conclusion from these projects was that it was worth spending time to set up the casting yard just so, and thus ensure the greatest possible accuracy in the casting bed.
For that reason the casting area was heavily piled with some 130 CFA piles and the sliding beams for the deck section very carefully laid out above. These are made in concrete with a steel top and were set to an accuracy of 0.4mm or less says Villard: "That has reduced friction to 15% or less compared to the 30% we achieved at Thurrock."
There is a carefully choreographed sequence of casting, done in two sections; 33m long for the bulk of the deck section length and a separate 12m in another casting bed.
"This is the pier head section which has cable anchorages and more complex reinforcement," says John Osborne, the Morgan Vinci project director for the bridge and approach roads.
By the time the pier head section is ready, most of the three-part pours of the longer deck sections are finished and the two are jacked together for a final deck top and stitching pour.
All this allows as much standardisation and reuse of forms as possible, says Villard. Reinforcement cages are standardised, too and made up in jigs around the site.
A rail-mounted Potain crane has the reach and capacity to lift and place these.
It was originally bought to allow the 26t hydraulic jacks to be moved after the pierhead sections were jacked along, but has been the focus for the yard layout.
Two initial castings for the deck were slow, says Villard. "But that is a normal learning curve issue" he says "and we are now reaping the benefit of the careful planning."
With the first pour taking three weeks, some time was lost. To Osborne's relief that has been caught up and overtaken to give a 20-day edge on the schedule. The casting yard is achieving a nine-day cycle for each section to be made and pushed forwards.
But now at the centre of the Firth there are three extra long navigation spans to place, a central one of 65m and two of 53m located either side.
Passing the launch nose over these requires additional support with temporary piers.
Because the bridge is effectively symmetrical with another 12 spans to go on the far side, this is also the point at which the longer span lengths have to fit into the deck casting sequence to eventually arrive in position as the launch completes. So, the casting yard also has extra work.
The 12m and 20m lengths which, added to the standard sections, will make up these longer spans, are being cast using carpenter-made formwork.
"At sea" meanwhile, work continues on the formation of the piers, one by one. These are cast in a single 12m-high section working from inside a section of the existing pile casing which projects above water and acts as a cofferdam. The two-section cylindrical steel forms are handled by a crane working from the contractor's own spud leg barge.
The piers are fitted with temporary steel bearings over which the deck passes during the launch, riding on PTFE pads fed through as it goes.
A steel trough restrains a welded steel rail on the underside of the deck to keep it on course; this was felt to be the best way because of the curving underside of the bridge.
The temporary piers also rise from monopile casings.
"We had additional piles installed for this" explains Osborne.
These are steel frame structures with a precast concrete pierhead platform at the top.
Once the deck has reached its final point these will be removed.