HS2 Colne Valley Western Slopes Project Gets Green Light

HS2 has obtained approval for a scheme that will use material excavated from the Chiltern tunnels to create 127 hectares of new chalk grassland, woodland, wood pasture and wetland around the tunnel’s south portal.

The Colne Valley Western Slope project, which was approved under Schedule 17 of the HS2 Act by Three Rivers District Council and Buckinghamshire Council, will see the transformation of what is now an HS2 construction site into one of the largest areas of new chalk grassland in the Chiltern Hills.

As well as the landscaping, the approval also includes the design of the south portal itself and associated service buildings. Going forward, HS2 and its contractors will continue to work closely with the planning authorities to develop the final restoration of the site.

Integration of the HS2 rail line within the Colne Valley Western Slopes.
Integration of the HS2 rail line within the Colne Valley Western Slopes

As part of the scheme, 90 hectares of chalk grassland will be seeded into re-profiled soil layers using the nutrient-poor subsoils on the site and mixing these soils with chalk from the tunnelling and recycled concrete and aggregates from construction works.

The design of these soil profiles will be completed in collaboration with Cranfield University and Tim O’Hare Associates in a research study that will use results from both laboratory and on-site trialling of soil layers.

This grassland will sit alongside new areas of woodland, wood pasture and wetlands, including almost 65,000 trees and shrubs of 32 species and nearly 3.5km of new hedgerows.

Around 4.5km of new footpath, cycling and horse-riding routes will give the public access to large parts of the site, which sits between the Colne Valley Regional Park and the Chilterns Area of Outstanding Natural Beauty (AONB).

These plans have been developed by HS2’s main works contractor Align, a joint venture formed of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – working with Jacobs and LDA Design.

Align is responsible for the Chiltern tunnels’ south portal compound – from where the tunnel boring machines (TBMs) are being launched.

In total, around 3m3 of chalk will be excavated from the tunnels and reused in the landscape restoration alongside crushed concrete and limestone aggregate used in construction – taking away the need to remove the material by road.

Peter Miller, HS2’s Environment Director, said:

“This project is one of the most important parts of our green corridor programme to establish better connected, sustainable and biodiverse landscapes along the route of the new railway and will contribute substantially to HS2’s carbon reduction target.

“It demonstrates HS2’s approach to addressing many of the complex issues surrounding climate change and which are central to protecting our environment, and is a great example of how good design and planning can mitigate the effects of climate change.”

The first of two TBMs began excavating the 10m-long Chiltern tunnel earlier this year, with the site now set to receive a continuous supply of chalk until tunnel completion in 2024.  Field trials are in preparation ahead of final seeding, and planting of trees and shrubs in 2025.

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