High Speed 2’s (HS2) Thame Valley Viaduct could be a game-changer for the industry, proving that pre-engineered structures can be attractive too.
Last month, HS2 Ltd announced that the 880m long viaduct will be completely prefabricated before being assembled on site.
HS2 Ltd’s Head of Civil Structures, Tomas Garcia, said NCE that its visual qualities show that a balance can be found between functionality and appearance.
“This viaduct will potentially change the outlook for the industry,” he said. “Traditionally, when you talk about off-the-shelf or off-site solutions, most people tend to think of this horrible functional solution.
“While it’s a very good example that something functional and engineered can look great with the right design input. I hope this will change the perspective of many stakeholders.
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.
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.
HS2 Ltd’s Chief Discipline Engineer, David Smith, explained that having architectural input was essential from the outset.
“It is a success of the strategy provided by HS2 and the requirement to ensure that our engineering supply chains also include architectural input,” he said. “It is probably the first project certainly in Britain that will have had an architect involved in each of the bridges. It is not just the big iconic bridges, but even the smaller ones will not have been forgotten.”
The 35 concrete piers that support the viaduct will be entirely built off site before being placed on their foundations. Traditionally, viaduct girders are secured above each of the piers with a cast-in-place concrete diaphragm. However, 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.
This improves durability and reliability, saves time, reduces costs and improves safety by reducing the need for people to work at height.
EKFB Technical Director Janice McKenna explained: “There is a huge safety benefit to removing the number of hours on site and removing the temporary work that we would have needed to complete the solution. ‘origin.
“There is also a reduced risk in terms of accidental spillage because we don’t do a lot of concrete in situ. We have advantages in terms of improved quality control because we make so many pre-made parts and it’s in the factory.
In addition, the team opted for two large “box girder” beams per span instead of eight smaller beams, in order 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.
“By being more efficient and delivering fewer items to site, we are also improving our own productivity,” McKenna added.
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 viaducts elsewhere on the route.
“We are actively considering using this solution for some of our other viaducts on the C2/3 part of the contract and will look to bring this into play if we can because of all the benefits,” McKenna said. “From an overall solution perspective, to be able to improve safety, improve carbon, all those things, it’s really worth it.”
The solution and design philosophy can be “tailored to a range of different scenarios”.
McKenna explained, “Overpasses don’t have to be identical. Our spans on this viaduct are around 20m to 25m but this solution could be used for larger or smaller spans.
“For each situation, all of the phase one contractors tried to ensure that we matched the design solution to the site we were working on.”
Going forward, Garcia said he wants this solution to be “the new benchmark for all phases of HS2.”
He added: “Design is usually a compromise between competing requirements. In my opinion, this is an excellent example of the right balance from the customer’s point of view. It proves that a beautiful design doesn’t have to be something spectacular.
“It’s a very simple design but very elegant and efficient. In terms of durability, for me as a customer this is very important as it reduces the need for maintenance.”
Do you like what you read? To receive daily and weekly New Civil Engineer newsletters, click here.