The High Speed 2 (HS2) project team behind the construction of the Small Dean viaduct have revealed the challenges behind sliding the 4,500t, 345m long structure over a road and a railway in one continuous stage over four days.

Constructing the viaduct marks the heaviest and longest single stage viaduct deck slide of the whole HS2 project so far, culminating four years of work that has involved preparing the site, diverting utilities and realigning the road.

The four day operation was completed yesterday, 2 February, and involved sliding the deck across the A413 and Chilterns Railways line near Wendover in Buckinghamshire in one go.

This viaduct, as well as nearby Wendover dean viaduct, have innovative “double composite” structures, with layers of reinforced concrete on top and bottom of the steel structure. The side of the Small Dean beam also features prominent deck stiffeners which help to manage the structural forces while allowing for a more structurally efficient span.

The viaduct is being delivered by HS2’s main works contractor, EKFB, made up of Eiffage, Kier, Ferrovial Construction and Bam Nuttall, working with their design partner, ASC (a joint venture between Arcadis Setec and Cowi).

An aerial view of the viaduct in its final position

Speaking on site in the midst of the four-day slide, EKFB senior project manager Martin Gamble told NCE: “It’s the only HS2 viaduct that has a road and a railway involved and believe me that has been a challenge at times over the last few years.

“The complexity of the interfaces with the road and railway have provided a real challenge which, with the assistance of Network Rail and Buckinghamshire Council, we have risen to and overcome.”

The viaduct’s concrete and steel deck was assembled to its full 345m length before the slide began. Most of the precast concrete deck slabs were also installed in advance of the launch. These decisions increased the weight of the slide but allowed the team to save time during the later stages of the project.

HS2 Ltd project manager Jason Bicknell told NCE that “we tend to have to launch viaducts over three or four stages, this one we were able to do it all in one go”.

One reason for the team deciding to do the slide continuously concerned the topography, according to Bicknell. Having a large and long embankment allowed them to build the whole viaduct before beginning to move it.

“The second reason we’ve done it this way is because it means we only have to close the A413 for one day, which reduces disruption to locals,” he added.

He further explained that, while doing it in one operation presented challenges, the situation also provided an opportunity.

“This is a really unique situation, it was a big challenge at the design stage to come up with something that would allow is to cross and dissect the railway all at the same time,” he said.

“We’ve worked continuously throughout the whole thing next to the railway [in preparation], building the six piers we’ve got between the two large abutments and they are piled quite deep; they’re about 45m deep piles.”

Small Dean Viaduct deck in position above the railway, February 2025

One of the hardest elements of construction the viaduct involved the lifting of the structure’s hammerhead piers to hold the deck.

EKFB agent Elliot Bright explained that they “are made offsite and brought in before we lift them in. They’re 45t, which is quite heavy, we’re able to install one in about half an hour.

“[After lifting the piers into position], the next bit is lifting in the prefabricated reinforcement cage. This is probably the most difficult element we had with the piers because you have to fit them inside where there’s only 30/40mm gap all the way around.

“They don’t fit very well, so you have to get them in and wobble it around, look into the cage and see where the conflicts are.

“The first one took us about eight hours to do but the last one took about an hour and a half so we got a bit better by the end.”

The nose of Small Dean viaduct deck over the A413

With the deck now in position, engineers can begin the challenging job of lowering it by 600mm onto the permanent bearings, which will support the full weight of the viaduct. The two-month long operation will see it carefully lowered by 200mm at a time, pier-by pier, until the whole deck settles into its final position.

The viaduct is one of only two places in the Chilterns where the HS2 route will be above ground level, with the underside of the viaduct just 6m above the road, supported by the huge Y-shaped piers.

The piers to support the viaduct deck were completed in April last year.

The video below includes a timelapse of the Small Dean viaduct deck slide operation.

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