PREVIEW

Engineers working on the HS2 rail line have pulled off an extraordinary feat of construction - sliding the enormous 4,500-tonne deck of the Small Dean Viaduct into place in just four days.
The operation, completed Sunday, saw the Viaduct slid into place across a road and railway line near Wendover in Buckinghamshire.

Stretching 345 metres, Small Dean is one of 50 major viaducts on the HS2 route, part of the high-speed railway designed to improve journeys between the UK’s two largest cities while easing congestion on the southern end of the existing West Coast Main Line.

The viaduct’s deck was fully assembled before the slide, with most of the precast concrete slabs installed in advance, increasing the weight but reducing time needed for later construction stages.

To facilitate the move, engineers used special pads coated in PTFE – a Teflon-like material used in non-stick frying pans – to minimise friction between the deck and temporary steel bearings on the piers. A cable system then pulled the structure forward at speeds of up to 10 metres per hour.

Jason Bicknell, HS2 Ltd’s Project Manager, hailed the achievement, saying: “Small Dean is the longest continuous deck slide on the project and an amazing engineering feat. It wouldn’t have been possible without the years of preparatory work, and I’d like to thank everyone involved. Two years ago, we set a target to install the deck in summer 2025, so it’s fantastic to have completed this ahead of schedule.”

The operation followed four years of preparation, including utility diversions, road realignments, and the construction of six supporting piers. With the deck now in place, engineers will begin lowering it by 60 centimetres onto its permanent bearings, a process expected to take two months. The deck will be gradually lowered in 20cm increments, pier by pier, until it reaches its final position.

Small Dean is one of only two locations in the Chilterns where HS2 will run above ground level. The viaduct, supported by large Y-shaped piers, stands six metres above the road. Like the nearby Wendover Dean Viaduct, it is constructed from weathering steel, which will develop a dark russet finish to blend into the landscape.
The viaduct also features an innovative ‘double composite’ design, with reinforced concrete layers on the top and bottom, creating a structurally efficient box-like span that reduces embedded carbon.

Elsewhere, HS2 has reported significant progress on other key viaducts, including those forming the Delta Junction in North Warwickshire and the approach into Birmingham Curzon Street. The project’s largest bridge, the Colne Valley Viaduct, became the UK’s longest railway bridge when its deck installation was completed last year.