Engineers working on HS2 have completed sliding the deck of a 1,300-tonne viaduct into position near Thorpe Mandeville in Northamptonshire.

The deck is 220 metres long and constructed of steel and concrete. It was assembled on one side of the viaduct before being slid into position in just three days, completing the slide at the north abutment last Friday, 20th June.

To facilitate the slide, special PTFE-covered pads were used to minimise friction between the deck and temporary steel bearings on top of each of the five concrete piers, the same technique used on four other viaducts on the central section of HS2 by HS2’s contractor EKFB.

The next stage is for engineers to lower the deck 60cm onto permanent bearings, before starting work on the concrete deck and parapets.

All five viaducts are being constructed using similar installation techniques.

They have a novel ‘double composite’ structure that uses significantly less carbon-intensive concrete and steel than traditional designs.

Construction of the other four viaducts, at Wendover Wean, Small Dean, Westbury and Turweston, is more advanced, with engineers constructing Westbury working on the concrete deck on top of the beam that will carry the track and parapets.

A cantilever formwork traveller is being used to quickly and efficiently pour the concrete. This is effectively a mould that allows the concrete for each of the eight spans to be poured individually, with winches pulling the traveller forward to the next section as each pour is completed, reducing the number of pours and eliminating the need for cranes. It is also more efficient and cost-effective, and improves safety by providing better access for operatives.

The techniques used for these viaducts enable longer spans and shallower beams, with a hollow structure based around steel beams along the side of the viaduct and layers of reinforced concrete laid on the top and bottom to create a stronger and more efficient span. This has the benefit of a carbon footprint between 39% and 59% lower than traditional construction using pre-stressed concrete beams lifted into place with a crane.

“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 50-mile 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.”

Janice McKenna, EKFB’s Technical Director