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A view across a tunnel ventilation shaft under construction. Large pipes enter the shaft and go down it side. A crane with a raised arm stand beyond the shaft.
A tunnel ventilation shaft under construction.

HS2 and Bentonite

What is 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 of these applications if managed appropriately.

How HS2 uses Bentonite

On HS2, Bentonite mixed with water is mainly being used in the construction of the ventilation shafts and foundation piling work to reinforce the excavation 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. It is not in any way controversial or unusual.

In HS2’s Chilterns tunnel boring machines, Bentonite is mixed in small quantities with cement to form a rapidly setting grout which is injected into a small zone immediately around the tunnel lining to help stabilise the structure. It is therefore trapped within the grout – which gels in around 12 seconds. Our two variable density tunnelling machines were specially designed and selected to operate within the aquifer and soft rock geology of the Chilterns.

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. HS2 Ltd is not using powdered Bentonite anywhere near surface watercourses where wildlife could be affected. Find out more about HS2 and the water supply.

HS2’s use of Bentonite in our tunnelling operations has come after many years of intense scrutiny of HS2 Ltd’s design and proposed construction methods.

HS2 take our 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 protection for the environment, and is completed as safely and efficiently as possible.