The UK’s longest railway bridge is being built to carry HS2 2.1 miles (3.4km) over a series of lakes and waterways across the Colne Valley just outside London.

The deck of the Colne Valley Viaduct will be constructed from 1,000 enormous concrete segments, each weighing up to 140 tonnes. They are being made on-site at a temporary factory that was built specifically for the project. The mammoth building is 100m long and is visible from the M25 has an internal volume of 105,000 cubic metres, making it larger than the Royal Albert Hall.

Every segment will be a slightly different shape, depending on where it fits into the viaduct. A modular approach was chosen to ensure quality, safety, and efficiency in production and to avoid bringing them in by road. At the peak of construction, around 12 segments, each the size of a double-decker bus, will be cast every week using a ‘match-casting’ technique. Using this approach, each segment is poured against the previous one so that when reassembled on-site the whole arch fits perfectly.

The design of the viaduct was inspired by the flight of a stone skipping across water, with a series of elegant spans up to 80m long carrying HS2 about 10m above the surface of the lakes, River Colne, and Grand Union Canal. Views across the landscape were inherent in the design to minimise the viaduct’s footprint on the lakes and help complement views across the natural surroundings.

The project is being led by engineers from HS2’s main works contractor Align JV, which comprises a team made up of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick.

To support the viaduct, there will be 56 reinforced concrete piers, each 6m tall and weighing about 370 tonnes. The piers are being cast on-site using specially designed formwork to create the shape of the structure. This is removed after four days to reveal the final product. Where the viaduct crosses the lakes, the piles are being bored directly into the lakebed, using a cofferdam to hold back the water while the pier is constructed.

Each pier is designed to support the full weight of the deck and rests on a set of concrete piles going up to 55m into the ground. This foundation work began earlier this year and will require the construction of 292 piles and 56 pile caps across the whole length of the viaduct. When all piers have been constructed, a specialised ‘launching girder’ resting on top of the piers will be used to lift the deck segments into position.

Once the viaduct has been completed, the factory and surrounding buildings will be removed. The whole area between the viaduct and the Chiltern tunnel will then be transformed into an area of chalk grassland and woodland as part of HS2’s ‘green corridor’ project.

In another visible sign of progress, the first two of four jetties across the lakes have been constructed. These will enable equipment to be moved into position to support the construction, thereby taking construction vehicles off local roads.

HS2 Project Client, David Emms said: “We’re already making strong progress on the Colne Valley Viaduct – sinking the foundations, building the first piers, and now starting production at the new modular viaduct factory. Once complete, this iconic structure will carry trains at up to 200mph – improving connections across the UK, freeing up rail capacity, and offering passengers low carbon travel options.

“It’s great to see how much progress has been made and I look forward to seeing work on assembling the deck structure later this year.”

Daniel Altier, Align Project Director, said “Seeing the first deck segments being cast in the factory marks a significant milestone for the project. The design of the viaduct is such that each segment will be unique, delivering a structure which I have no doubt will be one of the most striking elements of HS2 once complete.

“I would like to thank all the Align team and our supply chain partners that have enabled us to get to where we are today and in particular VSL, Danny Sullivan, Sendin, and Tarmac.”