![The A413 open to traffic after the Small Dean viaduct deck slide [Images courtesy SAP Photographie for Eiffage Métal]](/assets/news_articles/2025/02/1738650830_the-a413-open-to-traffic-after-the-small-dean-viaduct-deck-slide-feb-2025.jpg)
The four-day operation – completed on Sunday – involved sliding a 4,500-tonne deck across a road and railway line near Wendover in Buckinghamshire.
Stretching for 345 metres, the Small Dean viaduct is one of 50 major viaducts on the HS2 line between London and Birmingham.
The viaduct’s concrete and steel deck was assembled to its full length before the slide began, with most of the precast concrete deck slabs installed in advance of the launch. This increased the weight of the slide but saves time later.
The whole operation was therefore the heaviest and longest slide of a single-stage viaduct deck on the HS2 project so far.
The nearby Wendover Dean viaduct, which was slid into place last year, is longer at 450 metres but lighter at 3,700 tonnes. It was also slid in three sections rather than all at once.
The Small Dean slide began on Thursday 30th January and the A413 was reopened at around 8.30pm on Friday, nine hours ahead of schedule. The whole operation completed on Sunday 2nd February.
During the slide, pads coated in polytetrafluoroethylene (PTFE – or Teflon) minimised friction between the deck and the temporary steel bearings on top of each of the piers. A cable system was used to slide the deck forward at speeds of up to 10 metres per hour.
HS2 Ltd project manager Jason Bicknell said: “Small Dean is the longest continuous deck slide on the project and an amazing engineering achievement. But it would not have been possible without the huge amount of preparatory work put in over the last few years and I’d like to thank everyone involved. Two years ago, we set a target to install the deck in summer 2025 and it’s great that we came in ahead of schedule.”
Before the slide could begin, the team spent four years preparing the site, diverting utilities, realigning the road, building the abutments and six piers that support the viaduct and assembling the deck.
With the deck now in position, engineers can begin the challenging job of lowering it by 600 mm onto the permanent bearings which will support the full weight of the viaduct. The two-month long operation will see it lowered by 200 mm 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 six metres above the road, supported by the Y-shaped piers.
Like the Wendover Dean viaduct, Small Dean’s deck is made from weathering steel, which is expected to age to a dark russet finish to blend in with the surrounding landscape.
Both viaducts have French-style ‘double composite’ structures, with layers of reinforced concrete on the top and bottom to create a more structurally efficient box-like span, which cuts the amount of material in the design. The side of the Small Dean beam has deck stiffeners that help to manage the structural forces while allowing for a more structurally efficient span.
These viaducts are being built by EKFB, a joint venture of Eiffage, Kier, Ferrovial and Bam Nuttall, HS2 Ltd’s main works contractor for the 80km section between the Chiltern Tunnel and Long Itchington Wood. EKFB’s design partner is ASC, a joint venture of Arcadis Setec and Cowi.
EKFB senior project manager Martin Gamble said: “The team at Small Dean Viaduct has worked tirelessly over a number of years to prepare for this momentous event. Through careful planning and by implementing time-saving measures, we’ve accelerated the bridge and support construction, bringing the launch date forward.
“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, while keeping disruption to the surrounding area to a minimum.”
The last few months has also seen progress at HS2’s other major viaducts, including the structures that will form the Delta Junction in north Warwickshire and the viaducts taking the railway into Birmingham Curzon Street station.
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