HS2’s m-shaped double arch tunnel is a tasty alternative for local burghers

IMage: HS2

HS2 Limited, the company delivering Britain’s high speed railway between London and Birmingham, has begun work on the first of a series of ‘green tunnels’, designed to carry the railway though environmentally sensitive regions. The cut-and-cover method revives a technique more usually employed in urban settings, but used here to enhance rural settings. When the projects are completed, they will be landscaped with locally native trees and shrubs to reestablish and benefit the surrounding countryside.

The first tunnel, designated Chipping Warden, will run under the Northamptonshire countryside for a distance of one-and-a-half miles (2.5 kilometres). The design intends to blend the high-speed railway into the landscape and reduce disruption for communities. The lining and infrastructure of the tunnel is being prefabricated to speed up the construction and the fitting out phases of the project.

Five thousand giant segments

Unlike a normal underground tunnel, say the HS2 company, the Chipping Warden green tunnel is being built on the surface using a pioneering off-site manufacturing approach to speed up construction and improve efficiency. They say this approach will see more than five thousand giant concrete tunnel segments made in a factory in Derbyshire before being assembled on site.

The completed tunnel will then be covered by earth, with trees, shrubs and hedgerows planted to fit in with the surrounding countryside. The Chipping Warden Tunnel is one of five ‘green tunnels’ that are being built along phase one of the HS2 project.

Reduced carbon embedded in construction

Applying lessons from the construction of the latest French high speed lines, the off-site approach was developed by HS2’s main works contractor, EKFB – a consortium of fabrication and engineering companies Eiffage, Kier, Ferrovial Construction and BAM Nuttall. The tunnel segments are being made by Stanton Precast in Ilkeston, in Derbyshire. The work is part of a contract which is set to create up to 100 local jobs.

“The Chipping Warden green tunnel is a great example of what we’re doing to reduce disruption for people living close to the railway”, said Rohan Perin, HS2’s Project Client. “It’s fantastic to see the first arches in position. Our trains will be powered by zero carbon electricity but it’s also important to reduce the amount of carbon embedded in construction. The off-site manufacturing techniques being used will help cutting 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.”

First of five pioneering projects

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.

An overhead shot of everything that will be going underground – the sections being assembled at the factory in Derbyshire (HS2)

The whole project is using lightweight materials to further reduce the carbon footprint during construction. “This three-year construction programme will benefit from off-site manufacturing making the green tunnel build more efficient than the traditional on-site building method”, said Jeremie Martin, EKFB’s Project Manager. “The HS2 green tunnels are a first of its kind in the UK. A twin arch ‘m’ shape 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.”

Specially designed ‘porous portals’

The tunnel will be built in sections, with construction expected to be complete in 2024. 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 four miles (6.5 kilometres). 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.

All 13,290 segments for Chipping Warden, Greatworth and Wendover are being made by Derbyshire-based Stanton Precast Ltd, in a deal that is set to create up to 100 jobs at their Ilkeston factory – an increase in their workforce of around 50 percent.

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Author: Simon Walton

Simon Walton is UK correspondent for RailTech.com and Railfreight.com

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