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.

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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.

An Environment Agency spokesperson said: “We are investigating sinkholes on private land at Frith Hill in Buckinghamshire to understand whether they have resulted in any pollution to the local environment.

“Our specialist officers work closely with HS2 Ltd and contractors to ensure all relevant legislation is followed. We also continue to talk to local people, the water industry and local authorities to ensure the environment is protected throughout the HS2 project.

“Anyone with new concerns about this incident can call our 24-hour incident hotline: 0800 807060.”

Buckinghamshire Council said it had held a meeting with HS2 Limited and its contractor following the appearance of two sinkholes in a farmer's field near Frith Hill. The land, owned by the Department for Transport and leased to a farmer, is currently being tunnelled under by HS2 Limited.

Martin Tett, Leader of Buckinghamshire Council, expressed concern about the safety and long term risk from HS2 tunnelling in the area.

He said: "Although this land is not publicly accessible, we have sought reassurance from HS2 Limited that the area is now safe, and that any damage done will be quickly repaired and by them, not the Council.

"We have asked for greater transparency and accountability from HS2 Limited and they have told us that they will be commissioning an independent study of the whole of the area being tunnelled from the South Portal to the North Portal.

"This will give us all a clearer idea of whether to expect further sinkholes along the line both during tunnelling and afterwards and how this will be managed, now and in the future."