The sinkhole truth: who's to blame - man or rain?

After weeks of rain, residents of the South East have begun noticing a different problem: their front lawns and driveways are starting to resemble soggy lumps of Swiss cheese.

First a hole measuring 15ft (4.6m) across appeared without warning in a family’s driveway in High Wycombe, then days later a similar-sized crevice opened up in the central reservation of the M2 in Kent.

Since then, a hole deep enough to hide a double-decker bus has appeared in a garden in south-east London, another one 35ft-wide appeared in Hemel Hempstead and, most recently, a 20ft-deep hole was found 15 miles away in a garden in Croxley Green, Herts.

On Monday night three homes were evacuated in Ripon, North Yorkshire, after a 25ft-wide hole opened up.

Experts agree that heavy rain in the South of England is to blame for the “sinkholes” appearing at four to five times the normal rate.

But when is a sinkhole not a sinkhole? Should the chasms appearing in motorways and lawns across the South more accurately be termed deneholes, crownholes or just “collapsed subsidence”? And does it make a jot of difference?

For someone who has woken up to find a giant hole where their Fiat Panda used to be, the answer to the latter is likely to be “no”. But to the neighbour anxiously wondering about the fate of their own driveway, the precise geological classification of the car-swallowing gulf across the fence is of the utmost importance.

Despite their seemingly random pattern, each of the holes to open up in recent weeks has actually been located on chalkland, and not in more heavily flooded areas such as the Somerset Levels.

Sinkholes appear naturally in chalk because it can be dissolved by rainwater, which becomes acidic as it interacts with layers of soil and clay. The acidified water slowly eats away the chalk underneath, creating vast cavities over hundreds or thousands of years.

Holes appear when the layers of earth, gravel and clay above are disturbed, perhaps by drilling or an earth tremor, or by becoming so saturated with water they can no longer support their own weight.

“The water seeps through and dissolves the chalk underneath,” Dr Tony Cooper of the British Geological Survey explains. “Then when the situation changes, it changes the nature of the underlying materials and they collapse.”

Most worryingly for those living by a sinkhole, the opening at the surface rarely reveals the full extent of the chasm below, and the holes often widen over time, or fresh ones appear nearby.

However, in many of the recent cases, there may be no immediate need to worry.

On the M2 in Kent, the 15ft-wide “sinkhole” appeared on land riddled with underground caves known as deneholes. These man-made caverns, comprising a narrow vertical shaft leading to a wider chamber, are thought to have been dug by early Danish invaders as hiding places, mines for agricultural chalk, or dry spots in which to store grain.

Dr Vanessa Banks of the British Geological Survey says: “The M2 hole is most likely a denehole. We have quite good evidence for that, because old Ordnance Survey maps show a circular type of feature which indicates it would well be one.”

In some countries there is no distinction between different types of sinkhole, but in Britain the term refers exclusively to those that form naturally, while man-made cavities are either deneholes or “crownholes”, where old mine shafts expand upwards to the surface.

The distinction is important because, while the appearance of a sinkhole is always cause for concern, a denehole indicates a much lower risk to the surrounding land.

Iain Stewart, professor of geoscience communications at Plymouth University, says: “If it’s a denehole, it’s not going to get any bigger. If it is a natural one, it is much harder to guess. Tens of metres down, the hole could be much bigger.”

Other incidents are the result of “collapsed subsidence”, or the caving in of a poorly supported mine roof.

In High Wycombe, where a family’s car disappeared into a hole in their drive, experts believe the rainfall collapsed an old chalk mine.

“According to our maps, there was a brickyard and some distance away there were chalk mines,” Dr Banks explains. “The chalk mines are different to deneholes – a denehole is just a shaft with tunnels that protrude out from it, so the central thing is the shaft. The chalk mine may be more extensive, and they tend to be more recent, whereas deneholes tend to be medieval.”

The vast hole that appeared beside a house in Hemel Hempstead on Saturday occurred above the site of a former brickworks, suggesting a similar man-made cause. Yet signs suggest the most recent incident in Croxley Green appears to be a bona fide, natural sinkhole, as is the hole in south-east London.

With the worst of the rain seemingly behind us, homeowners may be tempted to breathe a sigh of relief. But experts believe a second spate of sinkholes and deneholes could appear when the ground dries out.

While the recent cases were most likely caused by rain saturating the earth and making it up to 20 per cent heavier, a sudden drop in groundwater levels as the earth dries out can produce the same result.

Water that has built up in cavities in recent weeks gives the clay layer above more buoyancy, but when the water recedes it can cause a collapse.

Prof Stewart says: “I think we will probably see more of these things over the next week or so.

“We would not expect them across the whole country, but in the chalklands of the South East, and in limestone areas, people should not be surprised to see them.” You have been warned.