HS2 tunnel should go deeper to protect water supplies says leading geologist

  |  Published: Jul 15th 2015
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Water supplies in the Chilterns could be threatened if HS2 is allowed to go ahead as currently planned, a leading geologist told MPs on the select committee looking into the proposed high speed railway.

Dr Haydon Bailey, Chiltern Society Geological Adviser & President of the Geologists’ Association, said that current plans to tunnel the railway under the Misbourne Valley involved boring through the upper and middle chalk layers.  

Dr Bailey was appearing as a witness for the Society which is petitioning Parliament for a three bore tunnel, similar to the Channel Tunnel, under the Chilterns Area of Outstanding Natural Beauty (AONB). 

He said: ”The proposed route crosses the Misbourne Valley where the tunnel crown will be at a level with less than six metres of competent chalk above it.  Above this will be gravel and weathered rubbly chalk and there is certainly the threat of ground failure at the valley crossing point close to Chalfont St Giles”.

“The chances of the River Misbourne surviving this must be close to zero with the resulting loss of environmental benefits, wildlife habitats and public amenities.”

He added: “The chalk aquifer below the Misbourne valley is a major regional water supply, with numerous public water sources/boreholes in proximity to the proposed route.  The public water supply at Chalfont St. Giles will certainly be jeopardised by the tunnel construction, and several other boreholes along the Misbourne valley at Amersham and Great Missenden will be threatened. 

“In addition to this, 22 per cent of London’s water supply is at risk should there be any damage to or pollution of the chalk aquifer in this and the adjacent Colne valleys.”

John Gladwin, presenting the petition on behalf of the Society, said a three bore tunnel was the only tunnel option that would considerably reduce the risk to the aquifer and eliminate any damage to the AONB because it would be built deeper in more solid chalk.  A middle emergency tunnel would also make it safer for passengers in the event of an incident in which a train had to be evacuated.

"We believe our case for a three bore tunnel is a matter of balancing the extra construction costs against the value of conserving the AONB and the environmental and other benefits, including increased passenger safety,” he said.  “Whilst this cost is quite considerable it represents only a tiny percentage of the overall construction cost while enabling the government to meet its commitments to conserve and enhance the AONB”.

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