HS2 has released remarkable timelapse footage showcasing the completion of the heaviest and longest single-stage viaduct deck slide on the project to date. The four-day operation, concluded yesterday, saw the massive 4,500-tonne deck of the Small Dean Viaduct slid across a road and railway line near Wendover, Buckinghamshire.
The 345-metre Small Dean Viaduct is one of 50 major viaducts on the HS2 project. It will improve journeys between the UK’s largest cities and free up capacity on the congested southern section of the West Coast Main Line.
The viaduct’s concrete and steel deck was fully assembled, with most precast concrete deck slabs already in place, before the slide began. This increased the weight but will accelerate later construction stages. The operation was the heaviest and longest single-stage viaduct deck slide on HS2 so far. The A413 was reopened on Friday evening, nine hours ahead of schedule.
During the slide, special PTFE pads (similar to the non-stick coating on frying pans) minimized friction between the deck and temporary steel bearings on the piers. A cable system propelled the deck forward at speeds 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.”
The team spent four years preparing the site, diverting utilities, realigning the road, building the abutments and six supporting piers, and assembling the deck.
With the deck now in place, engineers will begin the two-month process of lowering it 60cm onto its permanent bearings, in 20cm increments, pier by pier.
The viaduct is one of only two locations in the Chilterns where the HS2 route is above ground. Its underside will be just 6m above the road, supported by large Y-shaped piers.
Like the nearby Wendover Dean Viaduct, Small Dean’s deck is made of weathering steel, which will develop a dark russet finish, blending with the surrounding landscape. Both viaducts feature innovative “double composite” structures, with reinforced concrete layers to create a structurally efficient box-like span, reducing embedded carbon. The Small Dean beam also has prominent deck stiffeners, reminiscent of Victorian railway bridges.
The viaduct is being delivered by HS2 Ltd’s main works contractor, EKFB (Eiffage, Kier, Ferrovial Construction, and BAM Nuttall), working with design partner ASC (Arcadis Setec and COWI), and Network Rail.
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.”
HS2 will also realign Small Dean Lane and the A413 under the viaduct, with landscaping and a shared foot and cycle path, which will eventually form part of the Misbourne Greenway project.
HS2 has made recent progress on other major viaducts, including those at the Delta Junction in North Warwickshire and the viaducts into Birmingham Curzon Street. Overall, HS2 is building hundreds of bridges, including the record-breaking Colne Valley Viaduct.
