Just inside the M25 motorway in northwest London is a huge worksite for the HS2 railway, building two tunnels heading north, but also the viaduct bridge that will span the Colne Valley as the railway heads towards London.

Colne Valley Viaduct (c) HS2

At the moment, it’s a vast construction site, but when all the mucky boots and hard hats have been hung up for the last time it will be landscaped into a nature reserve. But that’s a few years away yet and at the moment it’s a hive of activity building the HS2 railway.

So, on a very wet day last week where the staff kept reassuring us that on a clear day you can see this from here, or that from here, all was lost in the dank rain that obscured everything and seeped into every crevice and unsealed gap in our safety overalls. Even a visit to a new staff relaxation area with a roof terrace was simply to remind people how grey and wet everything was on the day.

However, one thing that no amount of rain could conceal was the gigantic hanger of a building that we were here to see – the factory that is making equally gigantic concrete decks that will be assembled to form the UK’s longest railway bridge, leaping over the Colne Valley in northwest London.

The bridge is made up of 56 piers being built into the ground and between them are the deck segments, which are being built at this factory. Once ready, they will be taken to the nearby valley site, and another huge machine will lift and install each of the 1,000 deck segments in place.

The concrete decks are reinforced steel covered in concrete, so the steel frames need to be assembled, and even today, that’s largely a manual job. So lots of people work outside building vast steel cages. As the design of the decks is to create pleasing to look at slopes under the railway bridge, it means they can’t stand upright during construction, so they hang from the top, unearthly desiccated skeletons creaking and swaying slowly in the wind.

They won’t be like that for long though, as next to the skeleton assembly line is the factory that will coat them in concrete. A vast, and exceptionally noisy building with three moulds side by side and dozens of ant-sized orange clothed people scurrying all over these giant machines.

Each of the concrete decks is very slightly different to allow for the curved slope that they need to build, so after each deck is created, they have to make slight changes to the mould for the next deck being installed, and carfully clean each mould to ensure the next casting is perfectly smooth. To ensure the decks join up correctly, when one is cast, the back of it is used to cast the front of the next deck.

The moulds are quite interesting, as they have moving parts to adapt for the changing shapes of the bridge decks, and some of the inner linings also can fold inwards to make removing the concrete deck afterwards. Then they go outside to finish curing.

Vibrating the concrete into the mould is also exceptionally noisy, so everyone has to wear headphones to block out the noise. When it’s at full capacity, they expect to make around 10 bridge decks each week – so about two years of work on this site.

Outside the finished mould sit for final curing, and there was something ethereal about these silent giants looming overhead in the grey rain. These modern-day Easter Island heads standing here lined up and waiting for the UK’s heaviest lifting crane for their final journey to the Colne Valley.

For us, that’s a bus ride away though.

Over at the Colne Valley end, a giant red machine is being prepared to start building the viaduct.

This huge machine has been brought to the UK from other railways where it has been building viaducts, and will slowly move forward over the next couple of months until the front sits over the gap between the preinstalled piers and start installing the deck units that will eventually make up the railway bridge.

We are standing at the northern end of the Colne Valley viaduct, where the bridge connects with land, and the land here is an embankment made from soil taken by flattening some land between this spot and the tunnel entrance. They’ve also used some spoil from the Chiltern’s ventilation shafts to save carting it away elsewhere.

A large concrete box has been built which will be the start of the viaduct, and you can just about see under the green tarpaulin the first of the free-standing piers that are being built for the viaduct.

Each pylon needs exceptionally deep piles to support them in the soft gravel soil that the viaduct will pass over — some going as much as 55 metres into the ground. About 45% of the 292 piles have been installed now, and they are working on building the concrete piers up on the first of the bridge sections to be completed.

Once the concrete bridge decks are installed, they will be cleaned up, and the bridge is then designed to age gracefully over the next century or so.

A subtle reminder that this construction project is building a railway that’ll still be carrying passengers in 200 years time, and beyond.

The viaduct factory

Assembling the wire frames

A cast deck ready to be removed

A flurry of activity on top of the deck

Heading up to work on the mould

Manually cleaning each mould after use

London is there in the distance, somewhere.


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  1. Stephen says:

    All that re-bar work looks ideal for some sort of robot to cut/bend/fix 24 hours a day. Has this ever been looked into?

    • ianVisits says:

      The fact that we’re still hiring humans to do the job suggests that it’s not ideal for robots.

    • Chris Rogers says:

      There are power tools to tie rebar and cut the tie after but a good steel fixer can still do it faster by hand, have seen videos of it

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