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HS2 Ltd – Landscaping by Use of Chiltern Tunnels’ Excavated Chalk

HS2 Chiltern Site

In order to create grassland habitats in the Chiltern Hills, Align JV is using chalk dug from the High Speed 2 Chiltern tunnels.

During the current week this environmental project on High Speed 2’s (HS2) largest construction site attained the 1M.m3 landmark, while the location of chalk’s focus was around the south portal of the tunnels.

Building of these tunnels with 16m length will produce almost 2.6M.m3 of chalk and it is due to be used to landscape the site, which is set to become a 127ha wildlife haven.

This will include the largest areas of chalk grassland in the Chiltern Hills.

Comprising a joint venture formed by Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick, HS2’s main works contractor, Align has presented this plan; and collaborating with its design partners Jacobs, Rendel-Ingerop and LDA Design, which form Align D, it will advance the works of this project.

According to HS2 Ltd’s environment director Peter Miller: “The south portal project is one of the most important parts of our Green Corridor program 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.”

At the moment the Florence and Cecilia TBMs are working on the Chiltern tunnels, whereas passing about 6.4km of their journey. Align recently also completed the first tunnel cross passage under the Chilterns.

The work process commences by mixing the excavated material with water and generating a slurry before being pumped out of the tunnel. Then the processing phase is done at a slurry treatment plant at the south portal site where the flint is removed and water extracted. This leaves a chalk cake which is then 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.

Align D design director Chris Chantler said: “This innovative, multi-discipline project is providing a unique opportunity to create a huge expanse of new grassland using chalk being excavated from the Chiltern tunnels.”

“With the first million cubic meters of chalk having already been placed to create the grassland, we are well on the way to creating a landscape that will be both rich in diversity and a place for people to explore and connect with the natural environment,” added Chris Chantler.

As a habitat of international conservation importance mainly found on limestone and chalk downlands of southeast England and the Isle of Wight, Chalk grassland is a type of calcareous grassland.

While its thin soil is lime-rich but low in nutrients, it holds little water and drains well.

At the HS2 Chilterns site, 90ha 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.

Field trials are in preparation ahead of the final seeding, and planting of trees and shrubs in 2025.

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