Green light for HS2’s Chiltern tunnel chalk grassland project

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

The ‘Colne Valley Western Slopes’ 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.

Chalk grassland – a type of calcareous grassland - is a habitat of international conservation importance mainly found on limestone and chalk valleys of south-east England and the Isle of Wight.

Lime-rich, but low in nutrients, the thin soil holds little water and heats up quickly. These conditions encourage a huge variety of smaller herbs and wildflowers and over 40 species can be found in one square metre of grassland, including some of the UK’s rarest orchids and invertebrates. Only 700 hectares of chalk grassland exist across the whole of the Chilterns AONB.

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.

This 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 areas to large parts of the site, which sits between the Colne Valley Regional Park and the Chilterns AONB.

Cllr Phil Williams, Lead Member for Environmental Services, Climate Change and Sustainability, at Three Rivers District Council, said:

“We welcome the plans to transform this site which will significantly enhance local biodiversity as well as providing a great new visitor attraction and amenity for people in Three Rivers and further afield.

"The design has evolved from the original proposals thanks to the work of council officers and engagement with the Colne Valley Regional Park Panel, which includes a range of local groups, working collaboratively to achieve a more distinctive and sensitive outcome in the Colne Valley.”

The plans were approved by Three Rivers District Council (which covers more than 99% of the site) and Buckinghamshire Council under Schedule 17 of the HS2 Act 2017. This followed extensive pre-application engagement and collaboration with the planning leads at Hertfordshire County Council, Three Rivers District Council and Buckinghamshire Council, as well as other key stakeholders.

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.

The first of two tunnel boring machines (TBMs) began excavating the 10 mile 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.