Chipping Warden Green Tunnel

The Chipping Warden Green Tunnel will stretch for 2.5km (1.5 miles) through Northamptonshire. It is one of five green tunnels, which are shallower tunnels that will be finished with trees, plants, and shrubs on top – helping them blend into the natural landscape that are being built. Materials from the excavation are kept on site and then reused on the tunnel surroundings, reducing movement of spoil.

Tunnel construction

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

Concrete and steel are some of the biggest sources of carbon emissions within the construction industry and by reducing the amount of both materials 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 less people and equipment on site, improving safety and reducing disruption for residents.

Similar ‘green tunnels’ are being built at nearby Greatworth as well as Wendover in Buckinghamshire and Burton Green in Warwickshire, stretching for a combined total of more than four miles. The tunnels will all have specially designed ‘porous portals’ at either end to reduce the noise of trains entering and exiting the tunnel, 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 such as Silver Birch, Oak, Beech and Willow planted to create new woodland areas around the portals and recreate the hedgerows and field boundaries on top of the tunnel.