Work on the UK’s longest railway bridge has ramped up this week, with High Speed Rail Group members Align JV beginning production of 1,000 enormous concrete segments that will form the deck of the Colne Valley Viaduct.

The viaduct will carry high-speed trains some 3.4km across a series of lakes and waterways just outside London, with the design inspired by the flight of a stone skipping across the water. The design was chosen to enable views across the landscape, minimise the viaduct’s footprint on the lakes and help complement views across the natural surroundings.

The huge deck segments – which weigh up to 140t – are being made on site at a temporary factory built specifically for the project. Each segment will be a slightly different shape depending on where it fits into the viaduct, a modular approach chosen to ensure quality, safety and efficiency in production and to avoid having to transport the segments by road.

HS2 Ltd is on track to cut the amount of embedded carbon in the viaduct by at least 28.4%. Applying lessons from the construction of the latest European high speed railway bridges, HS2’s design team has cut the amount of embedded carbon in the viaduct by 63,300t CO2e. The production of concrete and steel are major sources of CO2 emissions, so narrowing the width of the viaduct allowed a significant reduction in carbon. It also helped reduce disruption for local residents by reducing the number of HGVs on local roads.

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. This approach – where each segment is poured against the previous one – will ensure the whole arch fits perfectly when reassembled on site.

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

Work has also begun on the 56 giant piers that will support the viaduct, working from north to south. Each pier is designed to support the full weight of the deck above 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.

Once complete, the team will then use a specialised ‘launching girder’ resting on top of the piers to lift the deck segments into position.

The team has also completed the construction of the first two of four jetties across the lakes to get equipment into position to support the construction, taking construction vehicles off local roads. 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.