Greatworth Green Tunnel

The Greatworth Green Tunnel is the longest green tunnel on the HS2 route and will stretch for 1.7 miles (2.7km) through West 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

Unlike a bored tunnel, the tunnel is being built using a ‘cut and cover’ process. This involves excavating a cutting, building the tunnel, and then burying it, with trees, shrubs and hedgerows planted on top to blend in with the surrounding countryside.

The construction of the tunnel is happening in stages, with the cutting first being excavated and then a layer of ‘blinding’ laid to form a concrete base for the structure. To speed up the blinding process, the steel reinforcing bars are now being delivered as a ‘roll mat’ with the parallel bars connected by mild steel tape so they can be quickly rolled out ready for the concrete pour.

The next stage involves the assembly of the five concrete segments that form each of the ‘M’ shaped sections of tunnel, with separate halves for northbound and southbound trains. Inspired by similar structures on the French high-speed network, the approach promised to be quick to assemble, with much of the work done offsite.

The first kilometre of the structure has been assembled and the temporary realignment of the B4525 will allow work to begin on the next stage of the tunnel.

Similar ‘green tunnels’ are being built at nearby Chipping Warden, 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.

Landscaping design plans

Greatworth Green Tunnel is designed to blend the high-speed railway into the rural landscape and reduce disruption for communities around Greatworth in West Northamptonshire.

Designed as an m-shaped double arch, the tunnel will have separate halves for southbound and northbound trains. Five different concrete precast segments will be slotted together to achieve the double arch, which is the height of two double-decker buses – one central pier, two side walls and two roof slabs.

Tailored landscaping design plans are also being developed, 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.