The concrete tunnels, in M-shaped sections to allow two trains on the tracks, will be put in place and covered in earth and planted. Work begins this spring. It is said they will be the longest of their kind in the country.
HS2 spokesman Alistair Cowan told the Banbury Guardian: "The 1.5 mile long tunnels will be pre-fabricated off-site and then built on the surface before being covered in earth and new planting. It will reduce noise and disruption for villagers and also maintain easy access to the countryside.
"Greatworth is the longest but Chipping Warden will start construction first, with the first segments being put into position in the spring."
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The YouTube film shows an early, proposed design for the south end of the Wendover tunnel which then cuts to drone footage from Chipping Warden, showing the cutting that is being excavated to form the base of the tunnel. It gives a sense of the scale of the operation. If you cannot see the film on this page go to https://www.youtube.com/watch?v=cG10NhSEG_I
The plan is part of a scheme to build three innovative ‘green tunnels’ across Buckinghamshire and Northamptonshire to help blend the new high-speed rail line into the landscape and reduce disruption for residents.
The one-and-a half-mile long Greatworth tunnel will be built in a factory in Derbyshire before being shipped to site and assembled over the railway line as it passes the village.
Applying lessons from the construction of the latest French high speed lines, an ‘off site’ modular approach was developed by HS2’s main works contractor, EKFB. The alternative would have meant concrete and reinforcing steel would have had to be brought to the site, load by load, for construction.
Designed as an M-shaped double arch, the tunnel will have two separate halves for southbound and northbound trains. Instead of the whole tunnel being cast 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 5,400 segments installed at Greatworth will be 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 fewer people and less equipment on site, improving safety and reducing disruption.
HS2 Ltd’s Project Client Rohan Perin, said: “The Greatworth green tunnel is a great example of what we’re doing to blend the new railway into the landscape and protect communities living close to the line. By adopting an ‘off site’ approach to manufacturing, we’re also cutting embedded carbon from the design, improving efficiency, safety and making the whole construction process less disruptive for the community."
EKFB Delivery Director Andy Swift, said: “The green tunnel design is a combination of innovation, international engineering expertise and thoughtful landscaping for its local communities to enjoy. Once the tunnels have been built, the original earth removed from the cutting to make way for the tunnel, will be repositioned, creating a green space which will blend into the surrounding landscape.”
Similar structures will also be built near Chipping Warden and Wendover in Buckinghamshire, stretching for a combined total of 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 and small portal buildings to house safety and electrical equipment.
Thousands of tonnes of rock and earth will be excavated during the initial stages of construction. These will be carefully separated and stockpiled on site for reuse later in the process, helping to keep trucks off local roads, HS2 says.
The company says 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.
All 13,290 segments for the three tunnels are being made by Derbyshire-based Stanton Precast Ltd, in a deal which has created almost 100 jobs at their Ilkeston factory.
As well as the green tunnel, Chipping Warden is also getting a new relief road to take vehicles away form he centre of the village which is being built in stages. The first part – past the village was completed last year and the next stage will take the new road over the green tunnel.