Question to the Department for Transport:

To ask the Secretary of State for Transport, whether the tunnelling of High Speed Two under the Chilterns is planned to involve Bentonite under pressure.

The operation of the Chiltern tunnel Tunnel Boring Machine will not use bentonite. As HS2 Ltd have always made clear, they will not begin works in the area until the Environment Agency is satisfied with their final approach.

However, it is currently proposed that Bentonite will be used in the construction of the diaphragm walls for the 5 intermediate shafts. Prior to the use of bentonite in these locations the construction methodology dictates that the ground surrounding the diaphragm walls will be grouted, therefore sealing and protecting the ground water from the bentonite.