HS2 reveals ambitious plans to create one of the largest areas of chalk grassland on the edge of the Chilterns in the Colne Valley

Chiltern Tunnel South Portal Aerial View

The Colne Valley Western Slopes project will be a major contributor to help meet our carbon reduction targets by delivering significant ecological gains. It will also reduce the carbon footprint of the project by cutting road haulage and waste treatment activities and arable land will be replaced with natural habitats.

Plant species and habitats are being carefully selected to be resilient to a changing climate. Almost 65,000 trees and 32 species of shrubs will be planted, together with nearly 3.5km of new hedgerows. Around 90 hectares of calcareous grasslands, which once thrived on the valley slopes, will also be established and areas of historic woodland reinstated. The project, being designed and delivered by our civils contractor Align, will provide wider health and recreation benefits for neighbouring communities, including providing new connected green spaces and around 4.5km of new footpath, cycling and horse-riding routes.

Arable land that is initially needed for a major works compound to construct the Colne Valley Viaduct and Chilterns Tunnel will be transformed by reusing existing soils and recycling three million m3 of chalk taken from the tunnel excavation, as well as concrete and limestone aggregate materials used in the construction process. Reusing materials instead of transporting them away by road will significantly reduce carbon, while the excavated material will be used to re-establish the distinctive local chalk valley landscape, replicate natural drainage flows and establish the right growing conditions for calcareous grassland across the site.

Calcareous grasslands, which develop on shallow soils overlying chalk or limestone, are a valuable, scarce and rapidly declining habitat in the UK, with this decline reflected in both the Colne Valley and adjacent Chiltern Hills. The neighbouring Chilterns Area of Outstanding Natural Beauty is currently thought to support around 700 hectares of chalk grassland.

This collaborative project will provide a unique opportunity to create one of the largest extents of such habitat in this area. New planting and seeding will create an extensive mosaic of habitat that will potentially be colonised by hundreds of species of flora and fauna, including invertebrates, birds, mammals, reptiles and amphibians.

Align is currently preparing the site for the launch of the two tunnel boring machines that will excavate the 16km-long Chiltern Tunnels. Once launched in 2021, the site will 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. At peak, around 1,200 people are expected to be employed in the design and construction of the Chiltern tunnels and the viaduct, with 50 opportunities for apprentices.

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You will be able to see the full plans at our dedicated ‘You Said, We Did’ online webinars which are taking place on:

Please also visit our dedicated Chiltern Tunnel South Portal and Colne Valley Western Slopes webpage

If you have a question about HS2 or our works, please contact our HS2 Helpdesk team on 08081 434 434 or email hs2enquiries@hs2.org.uk.

Read more here.