Episode Seven: Building Bridges
How to Build a Railway is a twelve-part podcast series exploring the story behind the construction of the UK’s new high speed rail line.
This episode of the HS2 podcast series goes back above ground to explore the design and construction of bridges, with a specific focus on HS2’s iconic Colne Valley Viaduct.
Good infrastructure will impact minimally on the surrounding environment. For some stretches of the HS2 route, the diverse landscape and ground conditions are not suitable for tunnels. Where elevated sections of the line are needed – like over lakes and waterways – it brings opportunity for the engineers and architects working on the project to construct a modern and complementary design for the railway, reflective of its natural surroundings.
Featuring
Episode seven features Billy Ahluwalia, HS2’s Senior Project Manager working on the UK’s longest railway bridge, the Colne Valley Viaduct. Taking us back to the Victorian era, Billy sets the scene exploring the evolution of bridge building.
Focusing on HS2’s iconic viaduct, we also hear from David Smith, Lead Civil Structures Engineer, and Laura Kidd, Lead Architect, about the design vision for the bridge and how it will stand the test of time.
And Ludovic Vergne, Align JV’s Construction Project Manager for the viaduct’s deck segment precast factory, tells us about the 160-metre long launching girder ‘Dominique’, the enormous bridge-building machine that’s lifting the giant concrete deck segments of the viaduct’s arches into position.
Episode Seven – Building Bridges (transcript)
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Episode transcript
This is a transcript of episode seven of HS2’s How to build a railway podcast, first published on 11 April 2023.
Fran Scott
Hello, I’m Fran Scott, and this is How to Build a Railway.
When an architect or an art restorer works on a dilapidated building or a damaged painting, they can take two approaches to authenticity.
They can use materials and techniques that match those of the project, but disguise their interventions. An art restorer can sculpt marble from the same quarry as the original, with tools contemporary to its creation. Guided by their knowledge of other depictions of the same saint or hero, they can invite the public to view the restored work as if unchanged from its first creation.
Or they can make use of modern tools, and make their changes clear. An architect can carefully assess the structural stability of a ruin, making interventions in steel and concrete that boldly contrast past and present.
In many cases, they will use a mix of these approaches. Where they can, they will hide their interventions and where they must, they will intervene boldly and visibly.
Railway engineers use both approaches when intervening in a landscape. They aim to be as unobtrusive as possible. Good infrastructure will impact minimally on the environment, and on our enjoyment of nature. This is why so much of the High Speed 2 route has been built in tunnels: they are the ultimate in unobtrusive engineering.
For some stretches of the route, hills or valleys are too steep for trains and the landscape or ground conditions are not suitable for tunnels. In these areas, where elevated sections of the line are needed, it is difficult for the infrastructure to go unnoticed.
But there is another option available to the engineers and architects working on High Speed 2: they can make their intervention bold, and of couse we are talking about the bridges of HS2.
2:24
Fran Scott
The Colne Valley Regional Park is a mosaic of farmland, woodland, and water, one of the first expanses of countryside to the west of London, with 200km of rivers, canals and over 60 lakes , and although it is called a valley, the impression of standing in the landscape is a low, flat stretch of nature, dotted with small villages and lakes.
This environment is sensitive but precludes tunnelling. So to preserve the landscape as much as possible while making their intervention both explicit and aesthetically appealing, HS2’s engineers and architects decided to make this the site of HS2’s largest viaduct.
The structure will stretch 3.4km, with some spans of the viaduct stretching as much as 80m, and the track will sit at a height in the region of 10 to 15 metres above ground. This offers an interesting challenge, specific to the Colne Valley environment.
3:38
Billy Ahluwalia
I’m Billy Ahluwalia, I’m HS2’s Senior Project Manager for the Colne Valley Viaduct.
So the bridge industry has evolved from being purely a functional provision of bridges and has started to consider the location and the site context of where these bridges need to be put in place. Clearly, bridges are now being designed and being constructed by bridge engineers who understand bridges.
4:13
Fran Scott
Victorian bridge engineers did not have all the tools and techniques we do today. Development in material technology and our understanding of structural behaviour allow for more efficient design. The massive brick arch viaducts that we see over Britain’s rail network are at once over-engineered, using more materials than needed, but they are also insufficient for modern rail. In many ways, they used the same techniques as the Romans.
4:44
Billy Ahluwalia
It’s a learning process that has happened and the industry has evolved. Functional, in the case of Colne Valley, we have teams that exist today just to determine what we shouldn’t have and what would look right. That’s been developed to the point where we have an integral and integrated design within the site context. and that includes the form.
5:13
Fran Scott
In design there is often a trade-off between form and function. Big and bulky is often strong but ugly. Sleek and minimalist can be weak and small, and the viaduct design team had a lot of obstacles to cross…
5:33
Billy Ahluwalia
Large, long length lakes, where the span lengths between these, structures have been opened up to provide wider-vista views. By opening up the spans the deck becomes deeper. But using the methodology of precast, segmental and post tensioned structures, we can minimise that as well.
5:55
Fran Scott
The design of the viaduct sits within a broader design vision for HS2.
This is David Smith, High Speed 2’s lead bridge and viaduct engineer.
6:07
David Smith
We were committed to good design, and good design being not just about function, but particularly form and context.
And the design vision means that every structure has to be thought about in its own context. So, whilst from an efficiency point of view, you want to try and standardise things and use the same sort of component. Just because something is standard, doesn’t mean that it’s bog-standard, it doesn’t mean that it’s low, low value. I mean, we’ve all got smartphones, and they’re all very, very, very similar and they’re quite standard. But they’re not poor quality are they? And it’s that sort of thing, we want to get the right quality in the right place.
One of the things we also did in HS2 was, and I think this is a first in the UK, is that we, in our tender requirements for the design and build main works contractors, said that no bridge designs can be done without an architect being in the team as well. So, we specifically asked our tendering organisations to include engineers and architects together.
7:15
Fran Scott
There are a lot of architectural regrets within the construction industry, and HS2 is not going to repeat mistakes of the past, instead it will leave a visual legacy.
7:28
David Smith
So not just designing something that is economic to build, and quick to put in place, but actually looks very, very functional for the rest of its life. Something that will stand the test of time. And something that will become like the Victorian brick arch viaducts that people look back on for 100 years, the last time we built sort of proper railways north of London. People look back very fondly with that and we should, we should be striving and we are striving to make people feel proud of the designs that we’re developing now in the 21st century.
8:10
Fran
Laura Kidd is High Speed 2’s Head of Architecture, and has had a lot of involvement in the stations and bridge structures.
8:25
Laura Kidd
Right at the beginning, we realised that this was going to be a major, probably the viaduct of the project.
8:34
Fran Scott
Knight Architects, supported by engineers from Atkins, was appointed to develop a design concept and vision for Colne Valley.
8:44
Laura Kidd
And they came up with a great idea. It took a while, but… The idea, because you’ve got the vegetation, then you’ve got the ground, and then you’ve got the vegetation and it’s like how the language would change, as you went through, what is actually kilometres of the route.
So, the idea was that of the skipping stone, the idea was that you’d be walking and planting your feet firmly on the ground. But when you got to the water, it would turn to the skipping stone or the arches as a stone skipped over the water. And then when it landed again you went more into the more solid viaduct structure.
9:27
Fran Scott
Some landscapes lend themselves to bold interventions. Norman Foster’s Millau viaduct floats in the clouds, hundreds of metres above the mountainous Tarn Valley. The rolling hills of southern England invited a different approach.
9:45
Laura Kidd
Our difficulty with HS2 is that most of our viaducts are quite low to the ground. And that is a real design challenge, to try and not make them look too stumpy.
So you know when you see a viaduct usually it’s got this huge elegance because it’s way up. You look at the Victorian viaducts certainly going from you know, hillside-to-hillside, these wonderful sweeps of arches. We don’t really get that opportunity. Because we’re sitting really close to the ground. We’re not high up in the air. So to actually design something that looks good in that way, architecturally good, is a really big challenge. And on the whole the project’s not done too badly at all.
10:33
Fran Scott
The cost of doing the job this way is more expensive up front, but it offers the chance to improve the landscape, rather than disrupting it.
10:44
David Smith
The design of the viaduct also recognises that as well, in that there are some elements – the bits over the water bodies with a triangular shape section – those are more complicated to design, they’re more complicated to build, they are inevitably more expensive, but they are right for that particular context.
We could have designed a viaduct that had those sorts of forms everywhere. But we chose to only use those forms of deck where they were essential over the water bodies, where there were going to be wide views, with not so much vegetation cover.
And away from those areas, we can we can change the design so that it is slightly more economic, but still been able to use the same technology, same construction technology the same, the same design.
11:32
Fran Scott
First and foremost, any viaduct must be robust. In Colne Valley, there are some very specific motives for designing it to require as little maintenance as possible.
11:45
David Smith
Part of my role in the chief engineer’s team is to write and look after the technical standards for bridges in my discipline.
11:52
Fran Scott
These standards are the rules the design team must follow when generating the design.
11:57
David Smith
There are a number of things that are different for high-speed rail, and there are a number of things that are different for HS2 in particular.
12:03
Fran Scott
HS2 has ambitious goals for train frequency.
12:07
David Smith
That means that your maintenance window, for doing routine maintenance, is reduced, on any of the infrastructure it’s very, very small. So we’ve developed and enhanced our technical standards for materials and durability to try and address that so that we can try and get lower maintenance structures.
12:28
Fran Scott
The goal is to keep maintenance requirements very small, smaller than a typical railway might have.
12:41
David Smith
All bridges are designed for 120 years. They will require some maintenance during that time, but we want to look at best details so that we can learn the lessons from things that have gone wrong or where we’ve spent a lot of time as, as the UK, repairing bridges. So we’re looking at new ways to develop them to develop the standards, to develop the specifications.
One particular aspect in the UK is that there has been a moratorium on the use of precast, segmental construction, which is used throughout the rest of the world. But there’s been a moratorium since almost when I started my career in the early 90s. And that stems from some failures that we found in the UK, between the joints in the joints between precast segments.
And one of the things I’m really passionate about for anything that I get out of HS2, is to move the UK forward to allow that form of construction. Again, that’s the form of construction that we’re using for Colne Valley. And we, I specifically, wrote some additional prescriptive requirements in our technical standards to address some of the shortcomings or the perceived shortcomings of that form of construction so that we can use it going forward.
13:53
Fran Scott
This moratorium has been in place for nearly 40 years. Aside from fulfilling the requirements of an efficient railway and its aesthetic duty – if the Colne Valley Viaduct can show that this type of construction can be robust – it has the chance to give the sector faith in a very effective methodology.
The viaduct is formed of 56 concrete piers, each weighing around 370t, that are cast in-situ with special formwork. They will support the deck segments. Each pier sits on concrete piles that penetrate into the ground 55m.
One thousand concrete deck segments will be cast on site, and then put in place by a 700t, 150m-long piece of equipment called a launching girder.
14:54
Ludovic Vergne
The launching girder had been designed in 2005 for one project in Hong Kong.
15:01
Fran Scott
For its first job in Hong Kong, on the Tuen Mun–Chek Lap Kok link project, the luanch girder had been nicknamed ‘Red Dragon”. It went to work on two other projects, in Hong Kong and Singapore, before being shipped to the UK and re-named ‘Dominique’. It is the only launcher of its kind in the UK.
15:20
Ludovic Vergne
It’s a big 150 metre horizontal crane.
15:25
Fran Scott
The launching girder lifts the loads using a trolley and hoist, mounted on a steel lattice beam, like that on a tower crane, which extends out from support points on the working pier. And as the girder installs span segments, it is then moved forward on to new supports on the next segment.
With the newly approved precast concrete segments in place, they then just need to survive the next 120 years.
16:01
David Smith
So I mean, it doesn’t mean that after 120 years, the structure will fall down. But what it means is it should be serviceable over that period of time. But serviceable doesn’t mean that you can build it and then forget about it for 120 years either. So you still do need to maintain it, you still need to inspect it regularly. And just make sure that it’s behaving and operating the way that you want it to.
16:24
Fran Scott
When the deck segments are placed, they are fixed to the piers and other deck segments. Concrete is strong in compression, which is why it is used in foundations. But it does lack tensile strength.
And this means when its stretched between points, like on the spans of a bridge, it must be tensioned. Steel rope is run through the concrete, and tightened at each end. This improves its tensile strength.
This can be done as the concrete is cast—pre-tensioning—or afterwards…
16:58
Ludovic Vergne
we are using what we call post tensioning cable.
17:02
Fran Scott
In post-tensioning, the steel cables are run through channels in the cast concrete, joined and tightened.
The cables are susceptible to corrosion, if water gets into these channels. That introduces a risk, which must be guarded against, if the bridge is to last for more than a century.
And when we think about risk, they consider two key measures: the severity of the risk, and its probability. Severity considers what will happen if a failure occurs: will someone stub their toe, or will they be killed? Will a structure need repair, or will it fail?
Probability of failure is measured in terms of performance levels. Performance levels are defined by the likelihood of a failure that may affect safety, per hour of operation.
Now, to ensure the bridge’s durability, HS2 chose to set a higher performance level than normal for the joins between tensioning cables, bridge segments, and piers.
18:08
Ludovic Vergne
For durability point of view we are using what you call PL3, Protection Level Three.
18:13
Fran Scott
This required VolkerWessels, one of the members of the Align JV building the viaduct, to adopt new techniques to achieve PL3 on these vital joins.
At the point where the spans meet the piers, understanding compression in concrete is again vital. But as David says, the engineers also considered aesthetics.
18:40
David Smith
Generally we have central piers, and then the deck is formed with balanced segments either side of it, so that we have one pier with a deck in-between the piers. Eventually, you need to get to, you need to create a fixed point, so that all of the thermal loads that the bridge will attract inevitably, can get transferred into the ground.
To do that, in one pier, that pier will be very, very big and probably quite ugly, or certainly harder to make it appear attractive. So, it’s more efficient ,and better- looking if you can transfer, distribute that force over maybe a couple of piers. So if you do it over two piers, you then need to connect those two piers together so they can share the force. And you do that in a portal arrangement, like a gantry over a motorway, that kind of structure, but for Colne Valley, we’ve sculpted those in such a way that they are a nice feature in themselves. And it can be seen as, being like a gateway to something that is uniquely HS2 and identifiably HS2, but also very in keeping with the local landscape.
19:58
Fran Scott
Creating a structure that flows, blends and celebrates the landscape has been a priority throughout the design process. But being beautiful is not enough. The team has also had to make sure that the project does not damage the sensitive local waterways in the Colne Valley.
20:16
Billy Ahluwalia
So you do need to consider the topography, you need to consider what obstacles we’re crossing. There are lakes, but these are manmade lakes over many, many years. They’re not natural formed lakes.
The lakes were formed through gravel extraction in the ground.
20:33
Fran Scott
The Colne Valley landscape has been extensively altered over the last 200 years. The area would originally have comprised floodplain meadows, but few remain. Sand and gravel were extensively extracted, mainly between 1920 and 1940. Following this, many of the sites have been restored as wetlands.
20:55
Billy Ahluwalia
The local ground conditions here include chalk, which is overlain by gravel, which has been extracted, that has created these particular lakes and the lakes are long in size. So, unlike say very wide rivers, the lakes here are, the longest one is about 400 metres, and it ranges between 400 metres to 300 to 200 metres. So, we need to place our supporting piers in the water and the intent really was that we consider minimising the intervention into the ground at that particular point.
We are in a water aquifer as well. Water is extracted from the aquifer. So, our intervention into the ground, and minimising that intervention, was essential for us to consider.
And it’s proven through our monitoring, that we’ve had very limited, or negligible impact of no concern, to the water quality at all.
21:56
Fran Scott
And on top of this, efforts have been taken in conjunction with High Speed 2’s Innovation Accelerator to reduce the carbon footprint of the structure.
22:06
Billy Ahluwalia
So what we have done here is we’ve used what is like concrete, ground granulated blast slag. So, that is a cement replacement. The mix volumes of that are regulated to ensure that we can meet our specifications.
22:23
Fran Scott
This will be the largest UK use of ground granulated blast-furnace slag. When it is ground to a cement fineness, the blast-furnace slag has good cementitious properties and can be used in combination with Portland cement to provide a strong concrete with a lower carbon footprint.
The whole viaduct is expected to complete in 2025, when the Align Joint Venture will hand over to the fit-out contractor for the installation of railway systems.
When building such a major structure in a beautiful environment, it is important to engage with the local community. This is always a challenge while a project is ongoing, as people see all the disruption, but have yet to receive the benefit.
23:16
Billy Ahluwalia
Obviously the local community don’t necessarily see the benefit, direct benefit to them. And we appreciate and understand the impact on that community in relation to direct benefit. But clearly HS2, in itself, is transportation infrastructure for the UK, it’s going to benefit the UK citizens. It connects communities, it brings people together.
So we had a wide ranging engagement with the local community. We took their views on board.
23:52
Fran Scott
The design of the viaduct, with its low profile, stone skipping flow, helps bring a structure that fits with and even enhances the countryside. But the construction process can be a blight on the local community and these projects take years to complete.
24:11
Billy Ahluwalia
The technology that we’ve used to construct the viaduct has minimised all of that. By engaging with the community, getting their viewpoints, being able to go back to that community and say, you said, you wanted this, this is what we’ve done in response to that. The community’s acceptance of what we’re doing has improved, to the point where they’re now comfortable with what we’re doing. And over the next few years, the impact on the community will still be there.
So I think in short, the community is grateful that we’ve listened to them, and that we’ve responded to their concerns. And we are going to create a piece of infrastructure, that I hope they’ll get the watercolours out and be painting those over many years.
25:02
Fran Scott
For David, the project’s lead bridge and viaduct engineer, the main satisfaction is in seeing a design that he really believed in, being realised as intended. It didn’t get compromised when it collided with the real world.
25:16
David Smith
I get most of my satisfaction in this career out of seeing things that I’ve had an involvement in design wise being built and being brought into public use. And it’s, it’s wonderful to be able to see something that you’ve spent so long on not being changed too drastically. But the vision that you had at that time materialising in reality.
25:38
Fran Scott
And there is also the legacy for his sector, a redemption for the segmental construction of bridges.
25:44
David Smith
I want a legacy out of this to be able to say yes, the UK can do precast, segmental construction, it needs to address those particular deficiencies that were the main reason for the moratorium. And you can do that and get assurance of that by including the techniques and the design details that Align have already done.
We’ve incorporated additional redundancy in the design and additional facilities to be able to top up that pre-stress if we need to. All of that will be written up in technical papers and presented to the industry and be used to inform the technical standards going forwards.
26:22
Fran Scott
For Ludovic, the approach to the project has stood out. The adoption of an integrated team approach.
26:29
Ludovic Vergne
For me, it is the first time I experienced this kind of project where you have a fully integrated team that the client and main contractor share the same office. To work in an integrated team, as you have today, you remove a lot of stress and pressure from the production and you can focus 100% on the quality and safety of your personnel. And after you can see that you are doing a very good job and what you are doing is exactly what you plan to do.
Everyone is there to try to do something together. This one for me I appreciate.
27:11
Fran Scott
And for Billy, this is a project that feels like the true legacy of his professional life.
27:17
Billy Ahluwalia
For me, it’s actually having a design that will stand out, not just for HS2, but for the UK, and set the standard for similar high-speed rail around the world.
Infrastructure like this lasts for hundreds of years, and it’s a legacy. Working around the globe. And going back to the countries where we work and seeing that infrastructure in use, as David has expressed, is extremely satisfying. And it’s the legacy that really helps. That legacy also helps to build and inspire the next generation.
27:58
Fran Scott
Bridges, unlike any other part of the project, have an opportunity to not hide themselves away. But instead celebrate HS2 and its engineered beauty, and here the engineers and architects have taken that opportunity by the horns. Blending the latest engineering techniques with aesthetics. To provide a new phase of beauty to the ever-changing Colne Valley.
28:41
Fran Scott
Next time on How to Build a Railway…
28:45
Laura Kidd
We’ve got four great stations for Phase 1. And they’re all very, very different. And that’s cause they’re all in different places, doing different things.
28:56
Hala Lloyd
There’s one thing that Laura Kidd said when I first joined that has really stuck with me throughout, was that “The design vision principles that we have are People, Place and Time”.
29:07
Adrian Hooper
From the very onset, we set out to be BREEAM ‘Excellent’ – So the Building Research Establishment category rating of ‘Excellent’. We’re on course with the station design and the implementation to exceed that and achieve the ‘Outstanding’ rating.
29:23
Hala Lloyd
These structures, the assets that we’re leaving will have a long term of impact. The expectation is they’ll last for 100 years more, 120 years.
29:37
Adrian Hooper
The deep box here is over 900 metres long, 70 metres wide and 20 metres deep. So it’s an enormous space to find for that in West London.
29:48
David Lunts
This amazing new part plaza that HS2 have designed as the main entrance space to the new station. Which I’m told is going to be twice the size of Trafalgar Square.
30:03
Adrian Hooper
As we’ve developed the design over the years, its now much more of a place, a real catalyst of development for the future.
30:13
David Lunts
I mean, these are really big game-changing projects. Which as I say, have changed the face of their part of London. I think Old Oak and Park Royal is the next one of those game changing projects.
30:31
Fran Scott
Your host has been me Fran Scott
Thanks to our guests Laura Kidd, Billy Ahluwalia, Ludovic Vergne and David Smith.
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