The four-day operation – which was completed on Sunday – involved sliding the 4,500 tonne deck across a road and railway line near Wendover in Buckinghamshire.

Stretching for 345m, the Small Dean viaduct is one of 50 major viaducts on the HS2 project.

Its 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 will save time during the later stages of the project.

The whole operation was the heaviest and longest slide of a signal-stage viaduct deck on the HS2 project so far. The A413 was reopened at around 8.30pm on Friday, nine hours ahead of schedule.

During the slide, special pads covered in PTFE – a Teflon-like material usually found on the surface of a non-stick frying pan – were used to minimise 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 60cm onto the permanent bearings which will support the full weight of the viaduct. The two-month long operation will see it carefully lowered by 20cm 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 6m above the road, supported by the huge Y-shaped piers.

Like the nearby Wendover Dean viaduct, Small Dean’s enormous deck is made from weathering steel, which naturally ages to a characteristically dark russet finish to echo the natural tones of the surrounding landscape.

Both viaducts have innovative ‘double composite’ structures, with layers of reinforced concrete on the top and bottom to create a much more structurally efficient box-like span which dramatically cuts the amount of embedded carbon in the design.

In a nod to the great railway bridges of the Victorian age, the side of the Small Dean beam features prominent deck stiffeners which help to manage the structural forces while allowing for a more structurally efficient span.

The viaduct is being delivered by HS2 Ltd’s main works contractor, EKFB – a team made up of Eiffage, Kier, Ferrovial Construction and BAM Nuttall – working with their design partner, ASC (a joint venture between Arcadis Setec and COWI). They worked closely with a team from Network Rail to plan and deliver work close to the railway.

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.”