HS2 ‘green tunnel’ takes shape at Chipping Warden
HS2 Ltd provided a first view of the Chipping Warden ‘green tunnel’, in Northamptonshire, on August 4.
The structure is the first in the UK to use the ‘green tunnel’ design which blends the highspeed railway into the landscape, incorporating experience gained from the recent construction of French high-speed lines.
The 1.5-mile tunnel, scheduled for completion in 2024, differs from a normal underground tunnel in that it is being built on the surface, using a pioneering off-site manufacturing approach to speed up construction times and improve efficiency.
The tunnel is in an M-shaped double arch, giving it separate halves for southbound and northbound trains. Each will 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.
Instead of casting the whole tunnel on site, five different concrete precast segments will be slotted together using a crane to achieve the double arch - comprising 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.
The segments are manufactured by Stanton Precast in Ilkeston (Derbyshire), before being transported to Chipping Warden and then assembled on site. When complete, the tunnel will then be covered by earth, with trees, shrubs and hedgerows planted to cover the site, which was previously open fields and scrubland.
The local A361 road, which will be temporarily re-routed, will subsequently be re-laid over at an almost central position some 3.6 metres above the new tunnel, at the point where the concrete segments are at their thickest to bear the weight of the road traffic above.
The ‘build off-site’ method was developed by HS2’s main works contractor EKFB (Eiffage, Kier, Ferrovial Construction and BAM Nuttall).
Chipping Warden is the first ‘green tunnel’ to be built on Phase 1 of the HS2 project, which will link London, Birmingham and the North. Similar ‘green’ examples will also be built at nearby Greatworth, at Wendover (Buckinghamshire), and Burton Green (Warwickshire), stretching for a combined total of more than four miles.