What are sinkholes, how are they formed and why did one appear in a Surrey street?

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Residents of Godstone in Surrey have been evacuated from their homes due to concerns that two large sinkholes in their main shopping strip could trigger a gas explosion. Here we examine what causes sinkholes, whether they are becoming more frequent and what can be done to prevent them.

What are sinkholes?

Sinkholes are big holes that appear in the ground when the underlying rock either collapses or gets worn away, dissolved in water or breaks down through other processes. In the UK, they're usually 5 to 10 metres deep, but can go down more than 50 metres.

How do they form?

Some sinkholes develop when rock like chalk, gypsum or limestone dissolves due to acidic rain or underground water, resulting in the ground above collapsing. Others occur when underground caves collapse. Additionally, some sinkholes form when loose material under the surface is worn away by moving water.

What can trigger sinkholes?

Heaps of rain, surface flooding, and drought can all cause sinkholes by weakening underground voids. Mining, burst water pipes, and building work, which affects how water drains off the surface and the weight the ground has to handle, can also be a factor.

Sinkholes: A Look Around the World – in Pictures

Are there certain areas that are more likely to experience sinkholes?

No worries. The Mendips, bits of Wales, the Peak District and the northern Pennines, which sit on carboniferous limestone, a hard sedimentary rock made of calcium carbonate, are all hotspots. Areas of Dorset, Hampshire and the Chilterns, where clay and sand sit on chalk, are also at risk. The riskiest spot in Britain is around Ripon in North Yorkshire, which sits on gypsum, a soft mineral that dissolves much more easily than limestone.

Was Godstone vulnerable?

No worries. Godstone's built on weak sand that's easily washed away by water. Andrew Farrant from the British Geological Survey reckons it's not usually a problem unless there's a big change in the underground water level. That could happen if someone digs a deep hole below the water table, or if there's heavy rain, flooding or a burst pipe lets in heaps of water.

What about Godstone's historic sand pits?

In 1872, a discovery was made near the site of the Godstone sinkhole, with Farrant finding an old sand mine entrance. Farrant also found two other old mine entrances south of the sinkhole, located on the eastern side of the road. It's possible, he said, that there could be more sand mines in the area that aren't marked on maps.

What do you reckon went down in Godstone?

You can't be certain without more info. “It's likely that the sinkhole's caused by a burst water pipe flushing out loose sand beneath the road over time, creating a hole that collapsed when it was big enough,” Farrant said. But the sinkhole might've been caused by a long-abandoned sand mine collapsing, he said. A leaky pipe could've weakened the rock and led to a mine collapse, if one existed under the road, though it's possible the mine roof collapsed and busted the water pipe.

Dr Arya Assadi Langroudi from the University of East London thinks the burst water main theory is a good one. "The high pressure water from the damaged pipe likely 'boiled' the sandy soil under the surface, making holes that caused the ground to collapse," he said. A second hole in the ground opened up on the main street soon after the first one.

Are sinkholes appearing more frequently?

Tough to say. A lot of sinkholes go unrecorded, especially since most occur in remote areas of Wales, the Peak District, and the Pennines. However, there are occasional increases in sinkhole formation, often linked to prolonged periods of heavy rainfall. The wet winter of 2012 sparked a cluster of chalk mine collapses in south-east England. “I'd be very cautious of saying that the number of sinkholes appearing across the UK has gone up over the past five years or so,” Farrant said.

It might change with global warming and more extreme weather. Geologists think that heavier rainfall will make sinkholes worse, but also old mine shafts, sewers, drains and culverts will collapse more often. “Many of our old Victorian culverts and drains just weren't designed to handle some of the changes we're seeing due to climate change and the growth of cities and populations,” Farrant said.

Can we prevent sinkholes?

In crowded built-up areas, it's vital to make sure there's good drainage, says Assadi Langroudi. "If the ground can be stabilised, investing in proper drainage infrastructure can prevent the nasty mix of water and loose sand from causing further collapses," he said.