HS2 has released impressive new timelapse footage showcasing engineers completing the heaviest and longest single-stage viaduct deck slide on the project to date. This four-day operation, involved moving a massive 4,500-tonne deck across a road and railway line near Wendover in Buckinghamshire.
The Small Dean viaduct, stretching 345 meters, is one of 50 major viaducts on the HS2 project. Once finished, this high-speed railway will enhance travel between the UK’s two largest cities and alleviate congestion on the southern end of the existing West Coast Main Line.
The viaduct’s concrete and steel deck was fully assembled before the slide began, with most precast concrete deck slabs installed in advance. This increased the slide's weight but will save time in later project stages. The A413 was reopened around 8:30 PM on Friday, nine hours ahead of schedule.
During the slide, special PTFE-covered pads minimised friction between the deck and temporary steel bearings on top of each pier. A cable system moved the deck forward at speeds of up to 10 meters per hour. The team spent four years preparing the site, including diverting utilities, realigning the road, building abutments and six piers, and assembling the deck.
With the deck now in place, engineers will begin the challenging task of lowering it by 60 cm onto the permanent bearings, supporting the viaduct's full weight. This two-month operation will lower the deck by 20 cm at a time, pier-by-pier, until it reaches 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 just 6 meters above the road, supported by large Y-shaped piers. Like the nearby Wendover Dean viaduct, Small Dean’s deck is made from weathering steel, which naturally ages to a dark russet finish, blending with the surrounding landscape.
Both viaducts feature innovative ‘double composite’ structures, with reinforced concrete layers on the top and bottom, creating a more structurally efficient box-like span that reduces embedded carbon. The Small Dean beam's side includes prominent deck stiffeners, reminiscent of Victorian railway bridges, to manage structural forces efficiently.
The viaduct is being delivered by HS2 Ltd’s main works contractor, EKFB, a team comprising Eiffage, Kier, Ferrovial Construction, and BAM Nuttall, in collaboration with design partner ASC (a joint venture between Arcadis Setec and COWI). They worked closely with Network Rail to plan and execute work near the railway.
In addition to constructing the viaduct, HS2 will realign Small Dean Lane and the A413 under the viaduct, with landscaping featuring subtle terraces to manage height changes and the interplay between road, paths, railway, and viaduct. A shared foot and cycle path will also be created alongside the main road under the viaduct, forming part of the Misbourne Greenway project linking Wendover with Great Missenden.
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.”
Martin Gamble, EKFB’s Senior Project Manager 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.”
Significant progress has also been made on other major HS2 viaducts, including key structures forming the Delta Junction in North Warwickshire and the viaducts leading into Birmingham Curzon Street.
Image & video credits: Network Rail
