HS2 today revealed new timelapse footage showing engineers carefully sliding a 1,300-tonne viaduct deck into position near the Northants village of Thorpe Mandeville.
The steel and concrete deck of the Lower Thorpe viaduct – which stretches for 220m – was assembled to one side before being slid into position in just three days, reaching the north abutment on Friday 20th June.
Set into a valley, the viaduct will carry high speed trains across Banbury Lane and help manage flood risk by ensuring that rainwater can continue to run off into existing watercourses. Made of a distinctive russet-coloured weathering steel, it has been designed to help match the natural tones of the surrounding countryside.
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 five concrete piers.
It is the last of five viaducts being built using this technique by HS2’s main works contractor responsible for the central section of the new railway, EKFB - a team made up of Eiffage, Kier, Ferrovial Construction and BAM Nuttall – working with specialists at Eiffage Metal.
With the steelwork now in position, engineers can begin the next stage of the operation, lowering the deck 60cm onto the permanent bearings before work on the concrete deck and parapets can begin.
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.”
HS2 Ltd’s CEO, Mark Wild, is resetting the project to make sure it can be delivered to the lowest reasonable cost. While this work in ongoing, HS2 and its contractors continue to make progress on the civil engineering, such as viaducts, earthworks, stations and tunnels.
EKFB are delivering 15 major viaducts for HS2 between the Chilterns and South Warwickshire. Lower Thorpe is the last of five that are being slid into position.
As well as similar installation techniques, the five viaducts also share a novel ‘double composite’ structure which uses significantly less carbon-intensive concrete and steel than a more traditional design.
Instead of using solid pre-stressed concrete beams to form the spans between the viaduct piers, the hollow structure is based around steel beams along the side, with layers of reinforced concrete on the top and bottom to create a stronger and more efficient span.
This helps cut the carbon footprint of the viaducts by between 39% and 59% in comparison to a traditional approach using pre-stressed concrete beams lifted into place with a crane. It also allows for longer spans and shallower beams.
EKFB’s 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 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.”
Once complete, the viaduct will carry high speed trains between London and the West Midlands, improving journeys between the UK’s two largest cities and freeing up space on the existing mainline for more freight and local services.
The other four double composite viaducts – Wendover Wean, Small Dean, Westbury and Turweston – are all at a later stage of construction. The most advanced is at Westbury, near Brackley, where engineers are now working on the concrete deck on top of the beam which will carry the track and parapets.
This is being created using a cantilever formwork traveller. This is effectively a mould that allows engineers to quickly and efficiently pour each of the eight spans individually. Once a concrete pour is completed, winches pull the traveller forward to complete the next section.
This reduces the number of pours, eliminates the need for cranes and makes the process more efficient and cost-effective. The lighter, easier-to-install traveller also improves safety by providing better access for operatives, reducing the risk of accidents.
