HS2 Wendover Dean Viaduct: how to launch a deck in a tight space

To minimise disruption in a designated area of outstanding natural beauty, EKFB is using a nifty winching system to fix the decks onto the Wendover Dean Viaduct

Each year, more than half a million people visit the Millau Viaduct in southern France every year. There are many different ways to experience the world’s tallest motorway bridge, which opened in 2004. Take a scenic car drive, a kayak among the Norman Foster-designed piers, or maybe a detour through the visitor centre and gift shop.

“We wanted the piers to look like they hold the bridge up without much effort”

Tim Murray, Moxon Architects

During construction, the piers were too tall and the ground too uneven to crane in the bridge deck, so contractor Eiffage had to come up with an innovative solution. Each of the two decks were pushed from opposite ends using a hydraulic jack before meeting in the middle, 268 metres above the Tarn valley.

Twenty years on and 1,000km north, Eiffage is applying its experience of the deck launch to a project of a very different scale. The Wendover Dean Viaduct, bridging a valley in the Chiltern Hills, will rise at its highest point to a more humble 20 metres.

But the technical challenge is no less daunting. Located in the middle of private farmland in a designated area of outstanding natural beauty, EKFB (a joint venture between Eiffage, Kier, Ferrovial and Bam Nuttall) is under pressure to minimise disruption.

What’s more, the team doesn’t have much space to work with. A planning fracas left the project with only a thin strip of land, nowhere near enough to host the kind of crane that could lift a viaduct deck.

EKFB rejected the idea of an overhead launching girder, which would have been prohibitively expensive. It opted instead for a nifty winching system. The technique involves pushing the deck from one side of the valley to the other, two pier-spans at a time, as new sections are welded on from the back following each of five pushes.

Pier pressure

Before it could launch the deck, EKFB had to install the first two piers for the deck to slide onto.

There’s something anthropomorphic about the hammerhead piers – they almost look like cupped hands. “We wanted the piers to look like they hold the bridge up without much effort,” said Tim Murray, director of Moxon Architects, which designed the viaduct.

Each 14-metre pier (there will eventually be nine) sits on concrete foundations supported by four concrete piles ranging from 38 to 45 metres deep.

Once the foundation is installed, EKFB builds a steel reinforcement column before slotting a precast concrete shell around it. A 450-tonne crane then lifts a 50-tonne hammerhead shell on top, a reinforcement cage is placed inside and concrete is poured in.

All hands on deck

With the piers in place, the next step is to prepare the deck itself. The 25-metre-long steel beams forming the chassis are manufactured in Eiffage Metal’s Lauterbourg factory in France, where the firm also produced structural steelwork for the Louvre Pyramid. When a beam arrives on site, it is welded to another on the platform behind the north abutment. Once a 90-metre section is assembled, it is ready to push.

Or rather, pull. Cables attached to the front and back of the deck wind around a pivot on the edge of the north abutment and circle back to the far end of the 180-metre-long platform. As generators winch the cable backwards, the steelwork cantilevers out. Teflon pads on top of the stainless-steel temporary bearings minimise friction, helping the deck glide more smoothly.

The first 90-metre deck section was pushed out in January. It went over two spans at a glacial 9 metres per hour. The process took around 10 hours.

Further piers will be built in the valley as the next set of steel beams are welded together and then to the back of the previously-moved section, ready for the next push. Each of the five deck-pushes will involve more heft than the last. The final structure, weighing 3,700 tonnes, will be pushed across the ninth and final pier onto the other side of the valley at the end of the year.

Once the steelwork is in its final position, the temporary supports will be removed one by one and replaced with the real bearings, before the deck is lowered by 700mm to sit on top. “It’s a bit like changing a tyre,” said EKFB sub-agent Euan Hobbs.

EKFB has yet to appoint a subcontractor to carry out the bearing replacement – it’s a highly specialised job that not many companies have carried out before.

The deck will be lowered into place incrementally, 200 millimetres at a time, to account for a slight uphill slant of 1.8 metres between the north and south abutments.

Like all HS2 structures, the viaduct is designed to last for 120 years. The bearings, however, have a design life of half that. because they take so much stress.

Double trouble

Millau is not the only French viaduct that provided inspiration for Wendover Dean. The deck’s double-composite structure drew on Eiffage’s experience building the 337-metre-long Vicoin Viaduct in north-west France.

Instead of building the deck solely out of pre-stressed concrete, concrete and steel are layered like a Victoria sponge cake. A thin layer of weathering steel deck girders is supported by a series of precast concrete planks and topped with a reinforced concrete deck.

The result is a structure that’s lightweight but sturdy, strong enough to withstand the force of a live train performing an emergency stop.

Another benefit of the layering is that it requires less material overall. As well as evading the worst of steel inflation, the designers hope to save nearly 13,000 tonnes of embodied carbon – about the same amount the US state of Delaware emits in a year.

Wendover Dean Viaduct may lack a gift shop, but sustainable and durable rail infrastructure is surely a gift in itself.

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