Sinkhole could be first of many thanks to the floods

The Oxford Times: Hydrogeologist Melinda Lewis Hydrogeologist Melinda Lewis

A FOUR-metre-wide sink hole has opened up in South Oxfordshire.

The chasm at Nettlebed is thought to have been caused by centuries-old abandoned chalk mines beneath it collapsing, due to the high volume of water going into the ground through flooding and extended periods of torrential rain over the past few months.

And experts have said that South Oxfordshire’s chalky soil, combined with the underground chalk mines, make it a prime location for more sink holes to open up in future.

Nettlebed Parish Council chairman Barbara Lewis said the hole, which is believed to have opened up on February 7 or 8, was “dramatic”, but was not the first in the village’s history.

During storms in 1990, an enormous sink hole opened up and swallowed a 20ft beech tree, she said.

The most recent hole, on land owned by the Nettlebed estate, has now been fenced off by the landowner.

Malcolm Lewis, a member of the Nettlebed Conservators and chairman of the Neighbourhood Watch group said he did not want to give away the exact location of the hole for fear it would encourage what he described as “sink hole tourism”.

He said: “If a dog fell down you wouldn’t get it out, I would not want the fence taken down.”

Sink holes, like the 20ft-deep hole which swallowed up half a garden in Hemel Hempstead on Saturday, are caused by flooding. Rain water widens and erodes existing cracks and joints in chalk rock.

Water is more likely to do this when it is acidic, so the most likely places for a sink hole to open are where chalk soil is beneath acidic sandy or clay soil, as in the area around Nettlebed.

The Oxford Times:

The sinkhole ion Nettlebed

Hydrogeologist Melinda Lewis, of the British Geological Survey in Wallingford, said sink holes were likely in the South West of the county near Henley.

She said that areas where chalk and gypsum are present were most at risk.

“There are two main areas where they occur. One is in an area with gypsum dissolution, which is what happened in Ripon [where a 7ft sink hole opened] on Monday, and the other is in chalk soil,” she said.

“When you get rain it washes away clay deposits in chalk hollows.

“The other area you can get it is around chalk mine workings.”

Chalk was mined in South Oxfordshire up to 150 years ago by farmers for use as lime in soil and by brick kilns for making bricks more solid.

But Mrs Lewis added: “Most of Oxfordshire is outside the area [where sink holes are most likely].”

Oxfordshire Geology Trust vice chairman Lesley Dunlop, from Witney, said: “We could see sink holes anywhere with a chalky soil in South Oxfordshire. But the place we are mostly likely to see them is around Nettlebed and Woodcote, especially where there are ancient chalk mines beneath.”

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