HS2 is investigating a sinkhole that has appeared above a completed section of the Chiltern tunnels in Buckinghamshire.
The sinkhole is in Bazzards Field, near to Amersham. HS2 said it was “likely to be linked to pre-existing ground conditions”, adding that “investigations are ongoing”.
According to the Guardian, an email sent to residents by HS2 Ltd contractor Align on Saturday evening said that the sinkhole was “quite sizeable – around 6 metres in diameter and 5 metres deep”. However, HS2 has now said that its size is closer to 4 metres across and 3-4 metres deep.
HS2 is in discussion with the landowner about next steps, and the Environment Agency has been informed.
A spokesperson for HS2 Ltd said: “A ground movement has occurred within a field above the Chiltern tunnels. Investigations are ongoing, but this is likely to be linked to pre-existing ground conditions above the tunnels. The site has been sealed off and there is no risk to the public.”
The field sits on a chalk aquifer, which feeds the nearby Shardeloes Lake and the area’s rivers. Great Missenden parish councillor Catherine Bunting has raised concerns about the impact of tunnelling on the aquifer.
“We’re told HS2 know what they’re doing with their drilling, that our aquifer is safe and our water is safe,” she said. “Chalk is a very fragile material. What happens if there are further cracks and this lake is at risk? When are we going to ask HS2 to stop what they’re doing? Enough is enough.”
The location of the sinkhole is above a portion of completed tunnel and there has been no impact on ongoing tunnelling operations.
The twin-bore tunnel is the longest on the HS2 route between London and Crewe. It is 16km long and will carry passengers under the Chiltern hills.
Two 2,000-tonne tunnel-boring machines, named Florence and Cecilia, are being used to create the tunnel. They were launched from the south portal, next to the M25, in summer 2021 and have since completed 10km of tunnel.
According to HS2, approximately 2.7 million cubic metres of material will be excavated during construction and used for landscaping around the south-portal site.
Once construction is complete, this will help to create about 90 hectares of wildlife-rich chalk-grassland habitats. Chalk grassland used to be widespread across the hills of south-east England and is considered a habitat of international conservation significance, with just 700 hectares remaining across the Chilterns.