HS2 Ltd has announced that its 880m-long Thame Valley Viaduct will be entirely pre-fabricated before being assembled on site.
Crossing the flood plain of the River Thame, just outside Aylesbury, the ambitious modular design has been worked up 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) and architects Moxon.
HS2 Ltd claims that prefabricating every major element of the viaduct will cut its carbon footprint by two-thirds.
Set low into the landscape with a simple and consistent profile, the underside of the viaduct will be just 3m above the ground, with 36 25m-long spans crossing the river and surrounding wetlands.
The 35 concrete piers that support the viaduct will also be entirely made off site before being placed on their foundations. Traditionally, viaduct beams are secured together above each of the piers with a concrete diaphragm which is cast in situ. The larger pre-cast beams that will be used at Thame Valley can secured directly to one another, removing the need for the diaphragm, improving durability and reliability, saving time, cutting cost and improving safety by reducing the need for people to work at height.
HS2 Ltd head of civil structures Tomas Garcia said: “HS2 trains and stations will be zero carbon from day one, providing a cleaner, greener way to travel and helping the fight against climate change.
“But we’re also serious about reducing the amount of carbon we use during construction, and Thame Valley is a great example of how our contractors are using the latest engineering techniques to do just that. Prefabrication and off site manufacturing offer huge benefits in terms of efficiency and this design will help us deliver a more efficient, durable and elegant structure with less concrete and steel."
Applying lessons from recent high speed rail projects in Spain, the design team cut the amount of embedded carbon by simplifying the structure of the viaduct so that every major element can be made off site.
In a major step forward for viaduct design in the UK, the team opted for two wide ‘box girder’ beams per span instead of eight smaller beams - to simplify and speed up assembly. The new lighter-weight structure is expected to save 19,000t of embedded carbon in comparison to the previous design.
As well as cutting embedded carbon in terms of materials, this approach also requires less lorries to deliver material to site, cuts waste and will reduce disruption for the community during construction.
EKFBs technical director Janice McKenna added: “Cutting carbon during construction is a priority for EKFB as the team commences the building works of some of the main structures along its 80km section of HS2, and this process starts right at the initial design stages.
“Working alongside our design partners, we have tackled the carbon challenge from two angles. The structurally efficient solution means we minimise the embedded carbon in the viaduct materials; and we have also been able to reduce emissions during construction by maximising off-site pre-fabrication to achieve an efficient build, as well as reducing the number of HGVs on local roads. The techniques used in the Thame Valley Viaduct are also being used in other structures along our line of route.”
Preparatory works have already begun on site near Aylesbury, with the design team also looking at whether a similar modular approach to construction can be applied to other, smaller viaducts elsewhere on the route.