HS2 contractors ‘lose’ bentonite into aquifer

The Align join venture – formed of Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick – published a 45-page report into the “loss” of the construction material at the site of the Chalfont St Peter Ventilation Shaft (pictured) in Buckinghamshire.

A slurry was 97% water and 3% bentonite clay and was being used to temporarily support diaphragm wall panel excavation last year when the loss occurred. At one stage concrete was used to prevent further leakage of the support fluid.

A document from Scotland and Northern Ireland’s NetRegs environmental guidance body states that bentonite can be “highly polluting to water if released into the environment”.

Local campaign body Misbourne Environmental Protection applied for a judicial review of how the Environment Agency gave its consent for the Chiltern Tunnel portion of the £100bn HS2 scheme, claiming the body failed to hold adequate consultation and misunderstood environmental and water laws.

“Our intention is to force the Environment Agency to reconsider the entire tunnel project, take proper account of the Water Framework Directive, and consult interested parties before reaching a decision,” said the body.

“This should result in the tunnel running entirely through solid Chalk when passing under the River Misbourne, and external monitoring of the operation, to ensure an immediate response to any situation which might endanger the aquifer, Shardeloes Lake or the Chalk stream.”

The action body has raised more than £5,000 for its campaign.

The Align report said it was “likely” that the bentonite remained within 500m of the shaft and that the material was “unlikely” to spread further unless disturbed.

It said pre-treating the ground [by grouting] had significantly reduced losses. But it added that losses were still occurring because work had to stop at weekends due to planning conditions and because preventative measures such as excavating less on a Friday or pumping in a concrete mix would impact on cost and programme.

An HS2 Ltd spokesperson said: “Bentonite is a safe, naturally-occurring form of clay that is widely used across the construction industry as well as in agriculture and cosmetics. While we regret the loss at this location, monitoring has not identified any adverse effects on the aquifer or on water supplies, and the Environment Agency and water company have raised no concerns.

“HS2 Ltd takes its responsibilities incredibly seriously and we have a world-leading team of engineers, hydrogeologists, and environmental scientists in place to ensure that the tunnelling operation affords the right protections for the environment, and is completed as safely and efficiently as possible.”

An Environment Agency spokesperson said: “We take the protection of groundwater and watercourses, including Chalk streams, very seriously, as well as the protection of the wildlife and ecosystems that rely on them.

“The agency is working closely with HS2 Ltd, its contractors, local community groups, the water industry, and local authorities to ensure the environment is protected throughout the project.”

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