Two new sinkholes have opened up in a field above the route of the HS2 Chiltern Tunnel in Buckinghamshire.
The first sinkhole, adjacent to Frith Hill on the outskirts of Small Heath in Buckinghamshire, was found during routine monitoring work above the tunnel next to Frith Hill on 7 February.
A second, larger sinkhole, was found in the same field on Monday 12 February by HS2 surveyors.
A spokesperson for HS2 said they were likely caused by a combination of disturbances from the ongoing tunnel boring works, recent rainfall and pre-existing weak spots.
They added: “The Chiltern Hills are made of chalk bedrock which is naturally vulnerable to sinkholes. Water erosion creates weak spots, which can collapse suddenly. Construction disturbances and heavy rain can accelerate this process.”
Other sinkholes were reported in the area last year.
TBM breakthrough postponed
Although tunnelling operations are ongoing, a TBM ‘breakthrough’ scheduled for today (20 February), along with a press visit to the site, was postponed.
HS2 said the delay had no relation to the appearance of the sinkholes.
The spokesperson said: “We have always recognised that the geology of this part of the route is particularly challenging, with poorer quality, less stable chalk, susceptible to ground movement in the final approach to the North Portal. We are working closely with the landowner and tenant, temporary fencing has been installed, and extra monitoring put in place.
“Throughout the excavation of the Chiltern tunnels we have been engaging with all key stakeholders including Buckinghamshire Council, Affinity Water and the Environment Agency, and we will keep them updated as we go forward. Safety remains our top priority.”
The Chiltern Tunnel is the longest tunnel on the HS2 route between London and Crewe. The twin-bore tunnel is 10 miles (16 km) long and will carry passengers under the Chiltern Hills.
It is being built by HS2’s main works contractor, Align, a joint venture comprising Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick
