HS2 completes 4,500-tonne concrete and steel deck slide
HS2 engineers have completed the heaviest and longest single-stage viaduct deck slide of the project so far. The 4,500-tonne deck of the Small Dean viaduct was moved into place over four days, crossing the A413 and a railway near Wendover, Buckinghamshire.
The viaduct is one of 50 on the HS2 route. Once complete, it will improve connections between London and Birmingham while freeing up space on the West Coast Main Line.
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. 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 slide was made possible by PTFE-coated pads that reduced friction, allowing the deck to move at speeds of up to 10m/hour. A cable system pulled the deck into position while engineers monitored progress closely.
Before the slide, the team spent four years preparing the site, which included diverting utilities, realigning the road, and constructing the supporting piers. The deck was built to its full 345-metre length before being moved, with most of the precast concrete slabs installed in advance to save time later in the project.
The bridge deck incorporates reinforced concrete elements to enhance durability and load-bearing capacity. Precast concrete slabs, which form the top surface of the deck, were manufactured off-site to ensure quality and efficiency before being installed. The deck also features a concrete bottom slab, contributing to the viaduct’s structural integrity through its 'double composite' design. High-performance concrete was used to provide resistance to weathering and wear, reducing long-term maintenance needs.
With the deck now in place, the next stage involves lowering it by 600mm onto its permanent bearings. This process will take around two months, with engineers lowering it by 200mm at a time.
The viaduct is one of only two above-ground sections of HS2 in the Chilterns. Its weathering steel deck will naturally develop a dark russet finish, blending with the landscape. The design uses a 'double composite' structure, with reinforced concrete layers on both top and bottom, reducing embedded carbon while maintaining strength.
Martin Gamble, senior project manager at EKFB, the main contractor, 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.”
HS2 will also realign Small Dean Lane and the A413 under the viaduct. A shared foot and cycle path will be built alongside the main road, eventually forming part of the Misbourne Greenway project linking Wendover and Great Missenden.
Progress continues at other major HS2 viaducts, including those forming the Delta Junction in North Warwickshire and the structures leading into Birmingham Curzon Street. The project includes hundreds of bridges, with the Colne Valley Viaduct recently becoming the UK’s longest railway bridge following deck installation last year.
Subscribe to the weekly newsletter
