HS2’s Thame Valley Viaduct will be completely precast

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HS2 Ltd has announced that its 880m Thames Valley Viaduct will be fully pre-fabricated before being assembled on site.

Crossing the Thames floodplain just outside Aylesbury, the ambitious modular design was developed by HS2 Ltd’s main works contractor, EKFB – a team comprising Eiffage, Kier, Ferrovial Construction and Bam Nuttall – in collaboration with their design partner ASC (a joint venture between Arcadis Setec and Cowi) and Moxon architects.

HS2 Ltd claims that prefabricating every major element of the viaduct will reduce its carbon footprint by two-thirds.

Embedded in the landscape with a simple and coherent profile, the underside of the viaduct will be just 3m above the ground, with 36 spans of 25m in length crossing the river and the surrounding wetlands.

The 35 concrete piers that support the viaduct will also be entirely built off site before being placed on their foundations. Traditionally, the viaduct girders are secured above each of the piers with a cast-in-place concrete diaphragm. The larger pre-cast beams that will be used at Thame Valley can be attached directly to each other, eliminating the need for a diaphragm, improving durability and reliability, saving time, reducing costs and improving safety reducing the need for people to work at height. .

HS2 Ltd Civil Structures Manager Tomas Garcia said: “HS2 trains and stations will be carbon-free from day one, providing a cleaner and greener form of transport and helping to combat climate change.

“But we also take seriously 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 achieve this. Pre-fabrication and off-site fabrication offer huge efficiency benefits and this design will help us deliver a more efficient, durable and beautiful structure with less concrete and steel.”

Applying lessons learned from recent high-speed rail projects in Spain, the design team reduced the amount of embedded carbon by simplifying the structure of the viaduct so that every major element could be fabricated off-site.

In a major breakthrough for UK viaduct design, the team opted for two wide ‘box’ girders per span instead of eight smaller girders – to simplify and speed up assembly. The new, lighter structure is expected to save 19,000 tonnes of embedded carbon compared to the previous design.

In addition to reducing embedded carbon in terms of materials, this approach also requires fewer trucks to deliver materials to site, reduces waste, and will reduce disruption to the community during construction.

EKFB Technical Director Janice McKenna added: “Reducing carbon during construction is a priority for EKFB as the team begins construction work on some of the major structures along its 80km section of HS2, and this process begins in the early design stages.

“Working with our design partners, we tackled the carbon challenge from two angles. The structurally efficient solution means that we minimize the carbon embedded in the viaduct materials; and we were also able to reduce emissions during construction by maximizing off-site prefabrication to achieve efficient construction, as well as reducing the number of heavy goods vehicles on local roads. The techniques used in the Thames Valley Viaduct are also used in other structures along our route. »

Preparatory work has already started at the site near Aylesbury, with the design team also examining whether a similar modular approach to construction can be applied to other smaller viaducts elsewhere on the route.

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