HS2’s first prefabricated tunnel takes shape

10 June 2022

Construction has started on HS2’s first ‘green tunnel’, designed to reduce the amount of embedded carbon in the structure.

The 2.5km tunnel at Chipping Warden in Northamptonshire is being built on the surface using an off-site manufacturing process to speed up construction, improve efficiency and reduce disruption for communities.

HS2’s main works contractor, EKFB, a consortium of Eiffage, Kier, Ferrovial Construction and BAM Nuttall, developed the off-site approach, taking lessons from the construction of the latest French high-speed lines.

The more than 5,000 concrete tunnel segments are being made by Stanton Precast in a factory in Derbyshire and are then assembled on site. The completed tunnel will be covered by earth and landscaped to fit in with the surrounding countryside.

Designed as an M-shaped double arch, the tunnel will have separate halves for southbound and northbound trains. Instead of casting the whole tunnel on site, five different concrete precast segments will be slotted together to achieve the double arch – one central pier, two side walls and two roof slabs. All 5,020 segments will be steel reinforced, with the largest weighing up to 43 tonnes.

By reducing the amount of concrete and steel needed for the tunnel, this lighter-weight modular approach is expected to more than halve the amount of carbon embedded in the structure. It also requires fewer people and less equipment on site, improving safety and reducing disruption for residents.

Chipping Warden is one of five ‘green tunnels’ being built on phase one of HS2, which is designed to improve links between London, Birmingham and the north.

HS2’s project client, Rohan Perin, said it was important to reduce the amount of carbon embedded in construction.

“The off-site manufacturing techniques being used will help cut the overall amount of carbon-intensive concrete and steel in the tunnel and make the whole process faster, more efficient and therefore less disruptive for the community,” he said.

EKFB project manager Jeremie Martin said off-site manufacturing made the green tunnel build more efficient than traditional on-site building methods.

“The HS2 green tunnels are a first of its kind in the UK. We have designed them as a twin arch ‘M’ shape which is more efficient than the standard box structure, reducing the amount of concrete required, which is a great example of how innovative engineering design can reduce carbon impact,” he said.

The tunnel will be built in sections, with construction expected to be complete in 2024. A relief road will take HS2 vehicles – and other local traffic – away from the centre of the village of Chipping Warden. This will later be extended, to take the A361 over the top of the green tunnel.

Similar green tunnels will also be built at nearby Greatworth as well as Wendover in Buckinghamshire and Burton Green in Warwickshire, stretching for a combined total of more than 6.5km. The tunnels will all have specially designed ‘porous portals’ at either end to reduce the noise of trains entering and exiting, along with small portal buildings to house safety and electrical equipment.

Tailored landscaping design plans will be developed for each tunnel, with thousands of native trees and shrubs typical to the local area planted to create woodland around the portals and hedgerows and field boundaries on top of the tunnel.

Stanton Precast is making all 13,290 segments for Chipping Warden, Greatworth and Wendover. New production sheds, casting and storage areas are being built at the factory to accommodate the work.