Work to turn a large HS2 railway construction site next to the M25 motorway into a large chalk stream wildlife haven has reached a milestone, with the millionth cubic metre of chalk laid out around the south portal of the Chiltern tunnel. Before HS2 turned it into a construction site, the area was mainly monoculture arable fields, but using the chalk dug out of the ground for the railway tunnels, they plan to turn the area into a wildlife haven dominated by the chalk landscape they are creating.
Visualisation of the final landscape (c) HS2
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, and at 90 hectares, the HS2 site will increase that by nearly 13 percent.
Around 2.6 million tons of chalk dug out by the tunnel boring machines will be used to landscape the area, and recently they reached the milestone of their millionth ton of chalk being spread out over the site.
The site now with chalk being spread out over former building areas (c) HS2
The two giant tunnelling machines – named Florence and Cecilia – are currently around 4 miles into their journey under the Chilterns. The material they excavate is mixed with water to form a slurry before being pumped out of the tunnel. It is then processed at a slurry treatment plant at the south portal site where the flint is 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.
Ninety hectares of chalk grassland will be seeded into re-profiled soil layers. 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.
Field trials are in preparation ahead of the final seeding, and planting of trees and shrubs in 2025.
The plans have been developed by HS2’s main works contractor, Align – a joint venture formed of three companies: Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – working with its design partners Jacobs, Rendel-Ingerop, and LDA Design, that form Align D.
