Chilterns South Portal Chalk Grassland Project

The largest project in our Green Corridor programme 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.

As part of the ambitious 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 (nearly 2 miles) of new hedgerows on the western slopes of the valley of River Colne.

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.

Green Corridor construction

The plans have been developed by HS2’s main works contractor, Align – a joint venture formed of three international infrastructure companies: Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – working with its design partners Jacobs, Rendel-Ingerop, and LDA Design, that form Align D.

Two giant tunnelling machines – named Florence and Cecilia – were used to build HS2’s 10-mile tunnel under the Chilterns. The material they excavated was mixed with water to form a slurry before being pumped out of the tunnel.

It was then processed at a slurry treatment plant at the south portal site where the flint was removed and water extracted, to leave chalk cake which is then carefully laid out on site – together with recycled concrete and aggregates from construction works – in order to create a naturalistic landscape with all the temporary construction buildings removed.

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 AONB.

Field trials are ongoing, with seeding and planting of trees and shrubs are underway.

Green Corridor design

Since Spring 2018 we have been talking with the local community through a series of information events, to share details on the progress of the design of the largest project in our Green Corridor programme, to seek views and to respond to feedback.

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.