High Speed 2 (HS2) engineers have successfully slid the 1,300t deck of the Lower Thorpe viaduct into place near the village of Thorpe Mandeville in Northamptonshire.

The 220m steel and concrete structure, which will carry high-speed trains across Banbury Lane, was moved into position over the course of three days.

Set in a valley, the viaduct is designed not only to support the new railway but also to help manage flood risk by allowing rainwater to continue draining into existing watercourses. Its distinctive russet-coloured weathering steel is intended to blend with the surrounding countryside.

The deck was slid sideways into place using special pads coated with polytetrafluoroethylene (PTFE), a low-friction material similar to that found on non-stick frying pans. These pads reduced friction between the deck and temporary steel bearings atop five concrete piers, allowing precise and efficient positioning.

This installation marks the completion of the last of five viaducts employing this sliding technique in the central section of HS2, constructed by contractor EKFB, a joint venture of Eiffage, Kier, Ferrovial Construction and Bam Nuttall, with specialist input from Eiffage Metal.

Aerial view of Lower Thorpe Viaduct looking along the deck during the slide, June 2025

With the steelwork now positioned, engineers will next lower the deck by 600mm onto permanent bearings before progressing to work on the concrete deck and parapets.

EKFB is responsible for delivering 15 major viaducts for HS2 between the Chilterns and South Warwickshire. The five sliding viaducts share a “double composite” design, which incorporates steel beams along the sides with reinforced concrete layers above and below, creating stronger yet more material-efficient spans than traditional pre-stressed concrete beams.

This design innovation is said to reduce the viaducts’ carbon footprint by between 39% and 59%, compared to conventional construction methods that typically involve solid concrete beams lifted into place by cranes. The hollow spans also allow for longer spans and shallower beams, offering engineering advantages.

The other four viaducts built with this method – Wendover Dean, Small Dean, Westbury and Turweston – are at more advanced stages of construction.

At Westbury, near Brackley, work is underway on the concrete deck using a cantilever formwork traveller, a mobile mould that enables the concrete to be poured in sections efficiently without cranes. This technique also enhances safety by improving access and reducing the risk of accidents.

Issues with the HS2 project came to a head last week when a pair of reports revealed the scale of slippage in the work on the high-speed railway so far. The government has now confirmed that the line will not open by 2033.

HS2 Ltd chief executive Mark Wild is currently reviewing and resetting the whole project and aims to set a new benchmark in 2026, a move aimed at delivering the overall project at the lowest reasonable cost.

Reacting to the successful deck slide on the Lower Thorpe viaduct, HS2 Ltd project manager Sam Arrowsmith said: “It’s great to see the viaduct deck in position and I’d like the thank everyone who’s helped get us to where we are today. The slide may only have taken three days, but it was the culmination of four years of work – developing the design, completing the groundworks and the piers and assembling the enormous steelwork.”

EKFB technical director Janice McKenna said: “The strategic design approach applied to these double composite structures has been a game-changer in how we’re building these viaducts. The double composite solution can be applied to multiple structures in different locations, and we have five across EKFB’s 80km route that are all well into construction.

“The philosophy was to design the viaducts with architectural input to 'blend' the structures into their respective landscapes and reduce the visual impact on the environment. We also required a solution that offered specific delivery benefits too, from saving embedded carbon in the materials we use, to enhancing productivity on site and reducing safety risk.”

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