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Balancing ponds which are part of the dewatering at our Chipping Warden Green Tunnel site.
Balancing ponds which are part of the dewatering at our Chipping Warden site.

Water supply and management

Protection of the water environment through which the railway passes is a fundamental commitment that our designs and construction techniques must address.

Whether this is ensuring that water quality remains unaffected or that the water HS2 requires for construction doesn’t add further stress to the environment or affect the security of public water supply, everything we do follows the requirements of organisations such as the Environment Agency, local authorities, and water companies.

HS2 is acutely aware of the importance of the chalk aquifer that forms the Chilterns and underlies the Colne Valley. As well as providing water for public supply, groundwater from the aquifer also supports flow in chalk streams.

To protect this valuable resource, we have worked closely with the local water company and the Environment Agency to agree working methods and protective measures before the start of construction. This is supported by detailed and targeted monitoring.

Further Information

Chilterns Tunnel

Tunnelling through the aquifer requires careful assessment of the risks involved. This has informed the design of the tunnel boring machines (TBMs) and how they operate. The Chilterns TBMs have been designed to be optimal for the local geology, which has been investigated in extensive detail, and to protect groundwater quality through being as clean as possible. They use water to carry away excavated material as a slurry.

Our engineers are confident that construction will not affect the flow of chalk streams like the River Misbourne, under which the tunnels pass. As the TBMs and their operation are tailored to the chalk geology, the integrity of the chalk at depth beneath the Misbourne is maintained.

Nevertheless, we are undertaking detailed monitoring before, during and after construction in order to fully assess any impacts and the effectiveness of our protective measures. Note that these rivers have a history of natural intermittent flows.

Find out how the River Misbourne is flowing by viewing an Environment Agency measuring station’s information.

The footprint of the HS2 Chilterns tunnels is tiny compared to the aquifer unit it will pass through and on which the local rivers depend. The Mid Chilterns Chalk groundwater body covers an area of 730 square kilometres. The tunnels are 16 kilometres long and each of our tunnelling machines is only 10.2 metres wide.

Colne Valley Viaduct

Construction of the Colne Valley Viaduct has required piled foundations to be constructed into the chalk. As with the planning for the Chilterns Tunnels, we have worked closely with the local water company and the Environment Agency to agree working methods, monitoring and precautionary protective measures that need to be in place in advance of work commencing.

Some of the foundations for the viaduct have been constructed within the lakes of the Colne Valley, which presents additional challenges. Cofferdams were built to isolate lake water from the construction of those foundations and provide a dry working environment. Nevertheless, seepage into the cofferdams occurs and this wastewater must be managed carefully as it comes into contact with the cement used for the foundations.

Find out more how the water in this area will be managed.

Managing water quality

In order to deliver the level of protection of groundwater and rivers required, and to provide recycled water for the above construction activities rather than take additional water, a large and sophisticated water treatment plant has been built at the tunnel south portal. Here wastewater is collected and treated so that it can be reused or returned to the environment in accordance with quality standards.  For example, the majority of the water used by the TBMs is recycled.

This treatment processes reduce levels of heavy metals, such as Chromium 6, to well below drinking water limits. As a result, our long-term monitoring has shown no discernible impact on water quality around our work sites.

Beyond our work sites there has been no impact on public water supplies, either in terms of quantity or quality, nor on river flows. As mentioned above, HS2’s vigilance and monitoring continues, in addition to that undertaken by the water company and the Environment Agency.

Currently the Chilterns Tunnel TBMs are past the halfway point and the foundations for the Colne Valley Viaduct have been completed.

HS2 and Bentonite

Bentonite is a naturally occurring clay that is very widely used in a wide range of industries including cosmetics, agriculture and brewing.

Bentonite is of low toxicity, is not defined as a hazardous substance, and is safe to use in all these applications if managed appropriately.

On HS2, bentonite mixed with water is used in the construction of assets such as tunnel ventilation shafts and piled foundations to stabilise excavations before the concrete is pumped in.  It is then removed, cleaned and reused. Bentonite is used in this way in building sites across the country every single day of the year.

In the Chiltern Tunnels, bentonite is mixed in small quantities with cement to form a rapidly setting grout which is injected into a small zone immediately around the outside of the tunnel lining to help stabilise the structure. It is therefore trapped within the grout – which gels in around 12 seconds. The use of quick gelling and setting grout was a deliberate choice to ensure minimal migration in the aquifer, in the same way that our choice of tunnelling machines reflects our focus on minimising the risks to groundwater quality.

Powdered bentonite would be damaging if released into surface waters as it can cause turbidity in the water, block sunlight and cover water-based flora. Our contractors are not using powdered bentonite anywhere near surface watercourses where wildlife could be affected. The use of bentonite, as with all other elements of our construction activities, has come after many years of planning and scrutiny of our design and methodologies.

We take our environmental responsibilities incredibly seriously and we have a world-leading team of engineers, hydrogeologists, and environmental scientists in place to ensure that our construction operations afford the right protection for the environment and is completed as safely and efficiently as possible.

Find out more how the water in this area is being managed.

Further information

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Further information

  • Join our mailing list

    To receive email updates about the HS2 project, including how it’s being planned and constructed

    Sign up here

  • Contact us

    If you contact the Helpdesk, our Community Engagement Advisors will try to answer your questions immediately.

    Contact us