What are sinkholes, how are they formed and why did one appear in a Surrey street?
Residents of Godstone in Surrey have been evacuated from their homes amid fears that two large sinkholes in their high street could trigger a gas explosion. Here we look at what causes sinkholes, whether they are becoming more common and what can be done to prevent them.
Sinkholes are large pits that appear in the ground when the underlying rock either collapses or is washed away, dissolved in water or degraded through other processes. In Britain they are typically 5 to 10 metres deep but can reach down more than 50 metres.
Some sinkholes form when soluble rock such as chalk, gypsum or limestone is dissolved by acidic rainfall or groundwater, causing the overlying ground to collapse. Others form when underground caves collapse. Yet more form when loose material under the surface is eroded by flowing water.
Heavy rain, surface flooding and drought can all trigger sinkholes by destabilising underground cavities. Mining, burst water mains and construction work, which affects surface drainage and the loads that ground has to bear, can also be a factor.
Absolutely. The Mendips, parts 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 vulnerable. 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.
Yes. Godstone sits on weakly cemented sands that are prone to erosion and flushing by running water. Andrew Farrant, the British Geological Survey's regional geologist for south-east England, said it was normally not a problem unless there was a sudden change in the hydrogeology. That could happen if someone dug a large hole below the local water table, or if there was heavy rainfall, flooding or a sudden ingress of water from a burst pipe.
Godstone sits on a network of tunnels and sand mines that date back to the 17th century. Maps published in 1872 reveal an entrance to an old sand mine very close to the site of the Godstone sinkhole, Farrant said. Two other old mines have entrances south of the sinkhole, on the east side of the road. There could be further sand mines in the area that are not recorded on maps, he added.
It is impossible to be sure without more information. 'It's probable that the sinkhole is due to a burst water main flushing out weak unconsolidated sands from beneath the road over a period of time, creating a cavity which then collapsed when it got big enough,' Farrant said. But the sinkhole might have been caused by the collapse of an old, unrecorded sand mine, he added. A leaking pipe could have weakened the rock and led to the collapse of a mine, if one extended under the road, though it's possible that a mine roof collapsed and broke the water main.
Dr Arya Assadi Langroudi of the University of East London favours the burst water main theory. 'The high pressure water from the damaged pipe likely 'boiled' the sandy soil beneath the surface, creating cavities that led to the ground collapsing,' he said. A second sinkhole formed on the high street shortly after the first.
Hard to know. Many sinkholes are never recorded, not least because the vast majority form in remote parts of Wales, the Peak District and the Pennines. But there are sporadic surges in sinkhole formation, usually linked to prolonged spells of heavy rainfall. The wet winter of 2012 triggered a spate of chalk mine collapses in south-east England. 'I would be very wary of saying that the number of sinkholes appearing across the UK has increased over the last five years or so,' Farrant said.
That may change with global heating and more extreme weather. Geologists expect heavier rainfall to exacerbate sinkhole formation, but also collapses of old mine shafts, sewers, drains and culverts. 'Many of our older Victorian culverts and drains simply weren't designed to cope with some of the demands placed on them due to changes in climate and increases in population and urban development,' Farrant said.
In vulnerable built-up areas, ensuring sufficient drainage is crucial, said Assadi Langroudi. 'If the ground can be stabilised, investing in proper drainage infrastructure can prevent the dangerous combination of water and loose sand from leading to further collapses,' he said.
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