The first of 72 huge beams for an 880m long viaduct that will carry HS2 across the flood plain of the River Thame near Aylesbury have been lifted into position.

Engineers used two massive cranes to lift the beams, each 25 metres long and weighing 90 tonnes into position on top of the viaduct piers as if they were giant Lego blocks.

Instead of using multiple smaller beams for each span, a simple structural solution has been adopted using just two larger hollow U-shaped beams for each span. That makes for simpler construction and results in a lighter structure.

With a lighter-weight design, that was inspired by viaducts on the Spanish high-speed rail network, the amount of carbon-intensive concrete and steel used in the structure has been reduced, which cuts its carbon footprint by about a third.

The viaduct beams are being cast off-site at PACADAR UK’s factory on the Isle of Grain, in Kent. In normal construction, they would be secured together above each of the piers with a concrete diaphragm that is cast in situ.

On the Thame Valley viaduct, as well as another at Edgcote, the larger pre-cast beams do not need a diaphragm as they are secured directly to each other. The viaduct’s 68 giant concrete piers weighing 42 tonnes each are also being cast off-site at PACADAR UK’s factory, and the first 36 of these were lifted into position earlier this summer.

The deck on top of the beams that will eventually carry the track will also be constructed from pre-cast concrete plank, meaning that all parts of the viaduct, including the piers, beams, deck planks, and parapets can be pre-fabricated off-site and slotted together.

Coinciding with the lifting into position of the first beam, the nearby Princes Risborough to Aylesbury line, has been reopened after being closed for ten weeks while a bridge was installed over the HS2 mainline just south of Aylesbury.

Jennifer Lennon, HS2 Ltd’s Senior Project Manager said: “HS2 will offer zero-carbon journeys from day one, but we’re also working hard to reduce the amount of carbon used in construction. That’s why we’re applying the latest approaches from across Europe to help us cut the amount of embedded carbon in our major structures and speed up construction.

“The post-tensioned double-beam approach used here has enabled the major elements of the viaduct to be manufactured off-site – dramatically improving efficiency, safety and quality while delivering outstanding performance and durability.”

The viaduct was designed by HS2 Ltd’s main works contractor, EKFB, a team comprising Eiffage, Kier, Ferrovial Construction, and BAM Nuttall, who are working with their specialist on-site construction partner, FC Civils Solutions.

FC Civils Solutions’ Head of Operations, Tiago Palas, said: “With four beams lifted into position and 36 piers already installed, we’re thrilled with progress made so far. The installation of both the piers and beams has gone smoothly and we’re looking forward to seeing this viaduct coming together. And, despite some adverse weather recently, we’re delivering as planned, and this is really down to the expertise of the team and benefits associated with the construction method we’re using.”