The London and Scotland-based practice has built its reputation on bridge design. But it has never before worked at this scale and level of scrutiny. Daniel Gayne found out how they tackled the job.
You do not have to travel far in Great Britain to find a great bridge. From the grand Victorian viaducts that litter the north of England to iconic structures like Bristol’s Clifton Suspension bridge, the UK has shown a certain pride in its civic infrastructure. Even smaller structures such as Durham’s Kingsgate footbridge, Arup Ove’s last hurrah, are as marvellous a spectacle as they are feats of engineering.
But, in today’s world, where it is difficult enough to get new homes built without nimby-ish complaint let alone enormous infrastructure projects, designers of such structures face a different challenge – how to make a thousand-tonne mass of concrete and metal fit effortlessly into the rolling green pastures of the British countryside.
One man familiar with this challenge is Tim Murray, director at bridge specialist Moxon Architects. His practice is one of a few picked to design key infrastructure along the HS2 route. Even in its newly truncated form, the project involves the construction of 50 new rail viaducts, with a total of 500 bridge structures to be built once you include footbridges and road bridges associated with the scheme.
The work is divided geographically, with each of the large joint ventures responsible for delivering portions of the line picking their own design team. Moxon works under the umbrella of EKFB (Eiffage, Kier, Ferrovial Construction and BAM Nuttall), which is delivering an 80km section of the route between the Chiltern Tunnel and Long Itchington Wood, slap bang in the middle of an area of outstanding natural beauty.
Building Design meets Murray on a bitterly cold January morning to watch the fruits of his labour begin to take shape. The brief for Wendover Dean viaduct was “all about light touch,” says Murray.
“The AONB review and the LPA were very clear that they didn’t want this to be an iconic structure, but at the same time it needs to be beautiful. It needed to be, essentially, crafted, but at the same time always recognising the fact that it is a huge piece of infrastructure in a very rural location.”
This need for a subtle and self-effacing piece of infrastructure – along with the constrained site given to the JV by HS2 legislation – were behind the decision to use a gradual launch as the construction method.
Ben Sebastian-Green, senior project manager at HS2, says these constraints “forced [HS2] to do some really clever, innovative engineering” to build the structure.
Rather than using cranes to move components of the bridge into place, the deck of the bridge will instead be assembled in three stages and pushed out from the north abutment before the next section is attached behind it. This painstaking approach, inspired by French techniques used on the TGV network, involves the use of a “double composite” deck structure.
“The space that we have here plays into it, but it is kind of a function of all of these factors – geometry of the land, the scale of the viaduct, commitments that have been given to local stakeholders,” says Sebastian-Green, explaining the choice of construction method.
“This is obviously in an area of outstanding natural beauty, Moxon architects have worked really hard to create a structure that responds to that.”
The beams are made of “weathering steel”, which naturally fades to a dark brown colour as a surface layer of corrosion begins to form, which is meant to match the natural tone of the surrounding countryside and will remove the need for regular painting.
HS2 Ltd claims the double composite approach will cut the carbon footprint of the structure by 56%.
Murray says the construction method also helps to keep the profile of the bridge low, compared with other typologies such as suspension or cable stay, where much of the structure is above deck level.
This low-slung approach has been taken across the suite of HS2 viaducts, but at Wendover it was particularly important, being relatively high above the ground in an area of outstanding natural beauty. “It is quite unusual because it is bookended by the two cuttings,” says Murray.
“The majority of the line throughout the AONB is either in a tunnel below ground or within cutting, and here it breaks out – it’s quite unique and obviously exceptionally prominent. So, there was a huge amount of focus on the design and how it was going to look.
“It is little things – [such as how] the shaping of the piers is done; essentially we craft them so that, when light hits them, you will get that play between light and shade.”
According to Murray, his practice has been “quite fortunate to have significant input in the aesthetics” of the viaducts, and while the designs are relatively simple and unassuming in form, they have tried to introduce subtle elements of style and differentiation.
“There is a ribbed profile that gives texture to the piers; that form, as well having aesthetic benefits, also helps with the actual drainage, so water runs down those ribs,” he says. “The intention is to help break it down and make it look crafted.”
Murray also credits the local planning authority and the Chilterns AONB review group as being “intrinsic” in the development of their viaducts and that engagement with these bodies had resulted in “a few tweaks” to further this crafted look.
“For example, in some of the stiffener detailing – the elements within the phalanges of the main beam – that varies from viaduct to viaduct,” Murray says. “Also, on the parapet, there is an opportunity to introduce some texture. It is purely aesthetic, but it does help to create just a little bit of uniqueness.”
We have started a year-long, half-kilometre viaduct deck slide! 🤩👇.— HS2 Ltd (@HS2ltd) January 11, 2024
The first of three giant sections of the Wendover Dean Viaduct deck has been slid into position.
The 450m-long viaduct will be the first major railway bridge in the UK to be built with a ‘double composite’… pic.twitter.com/JwoSmykHhP
There are 15 rail viaducts in total being delivered by EKFB and Moxon, including Wendover’s sister scheme slightly further north at Small Dean, which shares its weathered steel and concrete design.
Further north are the Edgcote and Thames Valley viaducts, which, though further apart than Wendover and Small Dean, are themselves siblings of a sort. “They are practically identical in their form and geometry but they are different in terms of height,” says Murray.
“It is much higher at Edgcote, but the formwork for the piers and the beams and the parapet is all the same.
Engineering work for this pair has also taken inspiration from abroad, in this case fully pre-cast concrete construction that EKFB engineers say has proven effective for Spanish high-speed rail jobs.
But it is not just bridges that Moxon is working on. Murray’s eyes really light up when he mentions a unique little scheme near Calvert.
The Sheephouse Wood Bat Protection Structure is being designed to shield the flight path of the rare – and legally protected – Bechstein’s bat over the HS2 line without being killed by passing trains.
The 1km long structure is not technically a tunnel, but a kind of cutting with a protective layer over the top designed around a series of arches. “It is trying to be a light touch structure, but permeable, both visually and literally,” Murray explains.
Building Design asks if the firm has worked on anything similar and he replies: “Oh my God, no. We are used to working on unique structures and complicated structures, but in terms of the brief, to everyone’s knowledge, no one knows of any structure like that in the world.”
While Moxon does not solely work on bridges, it is this area in which it has built its reputation, with tasteful designs for a timber cycling bridge in Germany, a footbridge beneath another grade II listed bridge in Chiswick, and a new river crossing in the Cairngorms National Park. But, until HS2 came along, it had never taken on a job of such complexity and scale.
Murray, though, seems to have taken the project in his stride. “It’s a huge infrastructure project and we have been exceptionally lucky, but it has certainly not fazed us,” he says. “We have got years of experience in bridge design and it is what we have become known for.”
Given the events of the past year, it is an open question whether infrastructure of this scale will be attempted again in the UK in the coming years. But if it is, there is no doubt that Moxon is ready.