A big Potain MD600 H40 tower crane, which has already seen service on the Rion-Antirion bridge project in Greece, is now the central tool on a major bridge pushing operation in Scotland. The scheme is another high profile one since it is the second longest bridge push in the world, with a 1,190m (3,904ft) long concrete deck being jacked steadily across Scotland’s Firth of Forth.
The bridge will be the fourth firth of Forth crossing. Three others across the river estuary, or ‘firth’, which also has the famous railway crossing downstream, and alongside it a long suspension bridge for road traffic heading to the highlands from Scottish capital Edinburgh. A final bridge sits upstream at Kincardine where the Firth narrows considerably, allowing the crossing here to be just 500m long carried on multiple piers and with a swinging section in the middle for navigation.
Once the only road connection to the highlands and east Scotland from the capital Edinburgh, the Kincardine bridge is still important for some routes and a critical reserve route for heavy lorries when winds close the suspension bridge. But the bridge is old and overloaded so the government agency Transport Scotland is building the new £42m (EUR104m) bridge alongside, to increase capacity and take traffic away from the little town.
The new bridge must be much longer. The firth is significant environmentally: at this point, where each side is lined with salt marshes and tidal wading grounds, it is critically important for birds migrating to the Arctic. Much of it is protected under European Union rules or is designated a Site of Special Scientific Interest. To avoid these areas the bridge crosses the firth at 45°, running from the south end of the existing bridge to the coal yard of a decommissioned power station.
“The power station shutdown had freed up an old coal yard which is a slightly less important, semi-brownfield, part of the shore,” explains Simon Young from consultant Jacobs, designer of the 6km (2.7mi) long Transport Scotland road scheme that includes the bridge. Using the coal yard site, the work damages the environment less, he says.
To reduce impact further, the client asked for a construction method that would minimise disruption at the site. Jacobs suggested a launched deck method that could be performed from one side of the river only. That meant a deck launch of 1,190m in total: around 1km to cross the open river estuary and an additional 200m to clear important marshland on the southern shore.
That is a huge operation considering the deck is 16.5m wide (54ft) during the launch. In all it comprises three box section cells with walls running longitudinally beneath the top slab, with curved side and soffit. Even after some complex juggling with the reinforcement sequences by specialist bridge engineering consultant Benaim Associates, reducing the concrete needed, it will still weigh over 32,000t in total.
On top of that construction has to be done quickly, in just two and a half years, because ecological advice suggested two seasons of disruption to the birdlife was all that was sustainable. For Morgan-Vinci, the Scottish/French joint venture contractor building the bridge, that means a highly efficient site yard, carefully controlled construction methodology, and tight logistics are required.
The casting yard
The bridge is being constructed in 45m (148ft) lengths, each the full span between monopile supported concrete piers. Piles were installed in a row across the estuary last winter and the 2.5m piers are cast just ahead of the bridge push. There are three longer spans in the middle, two of 53m and one of 65m, which add complication.
Each new length becomes part of the existing deck, which is pushed forwards by huge jacks in the casting yard as it grows ever longer. A steel ‘nose’ at the front helps it jump the gap to the next pier. On each pier the deck rests on special low friction sliding bearings.
Once a new section is pushed forwards from the casting bay, work begins at the deck end to form the concrete for the next continuous section. When this is hard enough, the whole of the new longer deck is pushed forwards one length by the giant hydraulic jacks.
It is these giant jacks, capable of pushing 32,000t of concrete deck between them, which were the determining feature for the choice of crane. During the push they inch forwards along a sliding rail, gripping it each time for a 1.5m push and then moving themselves forwards for the next shove.
“They have to get back to the starting point at the end,” explains Pierre Villard, project manager for the scheme, “ready for the next launch in a few days time. But during the push we have already begun work on one portion of the new section, which is cast separately at the end of the casting bed. We do that to save time because it is the section sitting eventually over the pier head and it has much more complicated and heavy reinforcement to go in, and takes longer.”
The pierhead section prevents the jacks simply hauling themselves back along the sliding rail he explains. “And it would take some hours to do that anyway.” The 23t units must therefore be lifted out. The Potain crane was ideal, as it is capable of lifting such a load at 23m radius. In the configuration used on site the MD600 is working with a longer 60m jib, with a height under hook of 39m.
The fact that Vinci had one available from two used for the main piers at Rion-Antirion obviously helped the choice. On the Greek job they had been rigged with a shorter jib and lifted even bigger loads.
“Potain UK did a reconditioning maintenance on the crane as needed before assembling it on site,” says Villard. With the crane selected the rest of the operation could be built around it. The casting of each section is highly standardised to get as much speed and accuracy as possible into the operation, vital if the deadline is to be met, and the yard is laid out like a factory.
Formwork for making the deck sections is preassembled and lifted in and out from the casting bed as required. The steel shape of the bed forms the underside of the deck box. Interior web walls and the top need extra formwork, while a six-pour casting sequence is carried out. These pours create 33m of the deck length. For the final pour, the separate and complicated pier head section is jacked along and match cast into the whole, forming the complete 45m length.
The crane allows the assembly of large premade reinforcement cages, which are carried out away from the casting bed on special jigs. “These allows bars to be slotted in very accurately, and also speed up the work of the steel fixing teams,” says Villard, “because they can be worked systematically and repetitively, almost as in a factory.”
The cages, each weighing up to 12t, are lifted out complete and dropped into position on the casting bed by the Potain which can work with such loads at up to 47m. “The crane is rail mounted and can run up and down to cover the whole site,” says Villard.
Morgan-Vinci worked with consultant Benaim to optimise the design of the reinforcement cages to fit the site constraints, and allow the best placing of the various jigs and drop down points. It all helps give the operation a logistical edge, says Villard, which speeds the operations.
The crane is also available for unloading work and general site movement where needed. A small 70t crawler with a 38m long lattice boom, used for the marine operations, can supplement this work.
Despite some lost time on the first two or three segments, the yard has achieved a nine-day cycle for casting and launching. It has now caught up lost time and put the programme ahead slightly, much to the relief of project director John Osborne, who is overseeing both the bridge and associated approach roads and connections.
The launch operation has been underway since April and passed the halfway point in August, when the longer central sections were cast into the sequence. Once complete in February 2008, there is more to do with a major post-tensioning operation to be carried out along the length of the finished bridge, parapets to be added and the deck to be paved. But with the tight logistics of the casting working well, there should be time for all that.
Weldex has supplied a 70t Liebherr crawler for barge work on the project Weldex’s 70t barge-mounted Liebherr crawler The Potain tower lifts racks of special forming jigs that shape the bridge concrete The Potain lifts jig racks The casting of each concrete section is standardised for speed and accuracy. The tower crane forms the centre of the logistics operation Raising a jig The Weldex Liebherr installs pier steelwork The Weldex Liebherr installs pier steelwork The Weldex Liebherr has a 38m lattice boom The Weldex Liebherr has a 38m lattice boom
