Work on the UK’s longest railway bridge ramped up earlier this week as HS2 began production of the first of 1,000 huge concrete segments that will eventually be used to form the deck of the Colne Valley railway viaduct. Stretching for 2.1 miles across a series of lakes and waterways just inside the M25, the bridge will be made up a number of shallow spans up to 80 metres long that leap across the lakes.
The huge deck segments – which weigh up to 140 tonnes – are being made on site at a huge 100m long temporary factory that was built specifically for the project. The factory is on a large construction site that is also boring the two tunnels under the Chilterns, and has the advantage of having enough space to construct the viaduct segments locally rather than bringing them in on lorries.
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.
Complicating the matter though, each of the 1,000 segments needs to be a slightly different shape depending on where they fit into the viaduct.
Work has also begun on the 56 giant piers that will support the viaduct, working from north to south. Weighing in at around 370 tonnes, the first 6m tall reinforced concrete pier was cast on site by a team of engineers who used a specially-designed formwork to create the shape of the structure. This was then removed after 4 days to reveal the final product.
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.
In another visible sign of progress on the project, the team have also completed the construction of the first two of four jetties across the lakes to get equipment into position to support the construction thereby 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.
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 a public park.