Medway Valley Crossing has been re-engineered from a balanced cantilever into something more unusual: a post-tensioned structure.
Project: Medway Valley Crossing (part of the Peters Village infrastructure works)
Contract value: £33m
Main contractor: Bam Nuttall
Structural engineer: WSP
Temporary works supplier: RMD Kwikform
Start date: July 2014
Completion date: August 2016
New post-tensioned bridges are not common these days.
The structures have a history of problems, caused primarily by corrosion of the tendons where the steel was not sealed properly with concrete. It was this failing which led to the emergency closure of the Hammersmith Flyover in 2011, with repairs only completing last August.
So it is surprising to discover that Kent’s new Medway Valley Crossing is being built with post-tensioned concrete.
Main contractor Bam Nuttall sees obvious benefits from using this approach on the £16.5m project, with considerable help from temporary works supplier RMD Kwikform, which is providing some 300 tonnes of equipment after winning a £450,000 work package.
“The original design was a balanced cantilever,” explains Bam project lead Charlie Erven. “At tender stage, we proposed changing the design to a post-tensioned structure built in situ. We reckoned it would save about six to eight weeks on the programme.
“The planning authority was concerned about the aesthetics and thought we were proposing a chunky concrete box girder bridge. But actually, it will look exactly the same.”
The bridge’s design is elegant, with a curved deck, oval-shaped piers and sweeping soffits that chime with the rolling landscape and meandering river. The light concrete echoes the chalk escarpments of its backdrop.
Medway Valley Crossing Bam Nuttall RS21337 DSC 0386
It is 152.5 m long, 13.7 m wide and will carry a two-lane carriageway. The depth of the bridge superstructure ranges from 3.6 m over the piers, to 1.4 m at its shallowest, with the top slab adding another 550 mm.
At the apex, the distance from the soffit to the water below is 6.9 m minimum, though often it will be much greater because of a significant tidal range.
Caught in the crash
The Medway Valley Crossing is another project that was delayed by the 2008 financial crash.
It was originally conceived in 2007 by developer Trenport as part of a package of infrastructure works for a planned 1,000-unit residential development called Peters Village on the east bank of the river. But it was shelved during the recession and not revived until 2014.
Bam’s £33m contract with Trenport includes the new road bridge, which links the A442 to Peters Village and represents about half of the contract value, as well as a rail crossing over the Medway Valley Line, a 700 m stretch of access road on the west bank, and a further 4 km of road on the other side.
“The air then comes out through bleed pipes – it’s rather like bleeding a radiator”
Charlie Erven, Bam Nuttall
Work started in July 2014, and the infrastructure improvements on the east bank are now mostly finished.
Before beginning work on the road bridge, Bam had to demonstrate the integrity of the post-tensioned design. This involved constructing a trial panel witnessed by the local authority and structural engineer WSP.
“We cast the panel, post-tensioned the cables and grouted around them to prevent water ingress,” Mr Erven explains. “The air then comes out through bleed pipes – it’s rather like bleeding a radiator. Then we cut through a section of the panel to prove no air had got in.”
The ground conditions are not ideal for bridge building, with the river banks being soft, alluvial clay peat. “We have had to carry out extensive surcharging and ground improvements to the permanent road embankments, installing vertical band drains and a drainage layer,” Mr Erven says. “That started in September 2014 and the settlement is continuing, which we are monitoring.”
For the 2.7 m diameter piers, site workers built an 18 m-deep sheet-piled coffer dam, some 15 m by 19 m in area. The pier forms were created by curved metal shuttering supplied by another temporary works firm, Special Formwork.
The rest of the temporary works are effectively split into a lower and upper section.
The lower section, which is Bam’s responsibility, comprises 150 tonnes of steel tubular piles extending out from each river bank by 60 m, driven to depths of 18 m. These support 165 tonnes of steel beam grillage. Above this is the upper section of the temporary works: RMD’s vast and complex network of falsework.
“The design teams from both Bam and RMD had to work closely together at our Camberley office to ensure loads from the concrete bridge were carried by the formwork and shoring, and that these loads together could be carried by the steel beams and piles below,” Mr Erven says.
“The design period for the temporary works was six to eight months – and ran concurrently with the permanent design – which is exceptionally long, and the installation didn’t actually start until July 2015.”
Medway Valley Crossing Bam Nuttall RS21972 DSC 0522
Once Bam’s piles and the steel grillage were in place, 135 tonnes of RMD’s Rapidshor falsework – a galvanised steel, modular shoring system – was erected.
“The legs of the falsework were designed to sit straight on to the steel beams, connected by upside-down u-head attachments,” explains RMD project manager Russell Stanley.
“The leg loads are high so the bracing is crucial. But we designed the falsework into bays and, as a result, we have managed to retain a clear access corridor through the bird cage across the whole structure.”
Soldiers bear the load
Above the Rapidshor, super-slim soldier beams – RMD’s most-used product, though more commonly deployed vertically – have been arranged horizontally for the primary level of falsework.
On top of these, the secondary level comprises aluminium beams clamped on to the ‘lips’ of the soldiers, again using upside-down u-head attachments. Finally, above the aluminium beams is the plywood form for the bridge deck.
“The loads generated come through the trusses, into the megashor towers, down through the steel beams, and into the piles”
Russell Stanley, RMD
“One big advantage of the soldiers is they can be easily bolted together,” Mr Stanley says. “Also, by adjusting the connections between the soldiers slightly, we can open up each joint fractionally so the beams follow the curve of the soffit.”
A requirement of the construction project was that a 30 m-wide navigable channel be retained in the middle of the Medway, which is still a working river.
For this reason, RMD has used seven 30 m-long R700 trusses for the centre of the bridge, supported on either side by ultra-heavy-duty propping towers built from its Megashor system, which sit on the steel beam grillage.
“We have used five triple trusses in the middle – where the concrete is deepest – and two double trusses nearest the banks,” Mr Stanley explains. The triples weigh in at 21 tonnes and the doubles at 14 tonnes, each one having a depth of 2.1 m.
“The loads generated come through the trusses, into the megashor towers, down through the steel beams and into the piles,” Mr Stanley adds.
Medway Valley Crossing Bam Nuttall RS21890 DSC 0440
Lifting the R700s required two 110 tonne-capacity crawler cranes, sited on temporary jetties which extend out from the river bank and are also supported by 18 m-deep tubular piles. “The poor ground ground conditions on either side of the river would not have supported the cranes,” Mr Erven points out.
The trusses were loaded onto what Mr Stanley describes as “a sled and pulley mechanism”, one at either end of the structure, and then slid along the steel beams and into position in the centre, over the navigable channel.
Above the R700s, because of the extra height in the centre of the bridge curvature, there are more soldier beams and shoring, followed by the primary level of super slim soldiers, the secondary level of aluminium beams, and then the plywood form of the bridge deck itself.
Although it took months to conceive and install, when viewed from a distance the logic of RMD’s temporary works structure is plain.
“Where the bridge is deepest [above the piers], the falsework is closest together to support the load,” Mr Stanley says. “The maximum load capacity designed for this area is 80 kN – the highest anywhere on the whole temporary works structure. In actuality, the loads generated were 73 kN.
“It is a complicated design, but it is all built from ‘off-the-shelf’ kit provided by RMD. The beauty of the system is its simplicity.”
The concrete pouring started in August 2015, once the first phase of temporary works had finished, and progressed from the banks outwards. The jetties used for the crawler cranes were also used for the concrete pumping equipment.
By the time Construction News visited last month, the casting of the bridge deck was complete. It has involved some 2,300 cu m of concrete and 600 tonnes of rebar.
“All the tendons are in and we have started the tensioning,” Mr Erven says. “We have begun to remove the wing sections of the temporary works, but the centre section will remain until the deck is struck.
“We expect the last of the temporary works structure will be taken out in May. Then all that’s left to build are the parapets, curbs, ducts and blacktop.”
And with that, this newest example of an increasingly rare structure will be complete.