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Mon August 02 2021

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HS2 Colne Valley landscaping approved

22 Jun HS2 has obtained local authority planning approval for a landscaping scheme using chalk excavated from the Chiltern tunnels.

Planned landscaping around the southern portal of the Chiltern tunnels
Planned landscaping around the southern portal of the Chiltern tunnels

The Colne Valley Western Slopes project  has been approved under Schedule 17 of the HS2 Act by Three Rivers District Council and Buckinghamshire Council.

It will create 127 hectares of new chalk grassland, woodland, wood pasture and wetland habitats around the tunnel’s south portal near Maple Cross, just inside the M25 motorway.

Align JV has begun the first drive of the 15.75km twin-bored Chilterns tunnel. The second tunnel boring machine (TBM) is expected to begin her drive next week.

At the front of each TBMs is a 20-metre long corkscrew pulling spoil out to the back of the machine. With a watery compound pumped from the rear of the TBM to the cutting head, the spoil that comes back again is in the form of slurry, conveyed through pipework along the length of the TBM. This slurry is then processed to separate the sand, silt and chalk. The sand and silt are removed by centrifuge. The remaining chalk is then compressed to extract water and the cakes are set aside to dry further in a storage compound on site. It is these dried chalk cakes that make the raw material for the landscaping scheme to create a lime-rich calcareous grassland.

In total, around three million cubic metres of chalk will be excavated from the tunnels and reused in the landscape restoration alongside crushed concrete and limestone aggregate used in construction

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. The design of these soil profiles will be completed through collaboration with Cranfield University and Tim O’Hare Associates in a research study that will use the results from both laboratory and on-site trialling of the soil layers.

As HS2 cuts a swathe through southern England, stressing that the damage will be repaired is an important part of the political narrative.

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How much of the site looks now
How much of the site looks now

HS2 environment director Peter Miller said: “The Western Valley Slopes project is one of the most important parts of our green corridor programme 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. It demonstrates HS2’s approach to addressing many of the complex issues surrounding climate change and which are central to protecting our environment, and is a great example of how good design and planning can mitigate the effects of climate change.”

Cllr Phil Williams, lead member for environment issues at Three Rivers District Council, said: “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 have been developed by HS2’s main works contractor on this section, Align – a joint venture of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – working with Jacobs and LDA Design.

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

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