HS2 tunnel boring has created two new sinkholes in Buckinghamshire countryside, the company building the rail project said.
The two new sinkholes were discovered in a field next to Frith Hill on the outskirts of South Heath, HS2 Ltd said.
The smaller feature was found next to Frith Hill on February 7 during routine monitoring work above the new Chilterns Tunnel on the HS2 line from London and Crewe.
The larger, feature was found in the same field on February 12 by HS2 surveyors.
HS2 Ltd said ‘safety was its top priority’ and that both sinkholes had been fenced off amid ongoing tunnelling operations.
The field where the new sinkholes are located is privately owned and there is no public right of way across it.
These additional ground collapses mean there have now been a total of four sinkholes caused by HS2 drilling in less than nine months.
The first sinkhole in the area occurred above the tunnel near Shardeloes Lake near Little Missenden in May before a similar one in November near Hyde Heath.
Both have now been repaired and water monitoring has shown no significant impacts from the sinkholes on water quality.
HS2 Ltd said the two latest sinkholes may have been caused by a combination of disturbance from tunnelling, and potentially recent rainfall, which weakened pre-existing weak spots.
A 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 HS2 tunnel is being drilled through the Chiltern hills, which are made of chalk bedrock that is naturally vulnerable to sink holes, HS2 Ltd said.
Buckinghamshire Council and the Environment Agency have been approached for comment.
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