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What causes sinkholes?



Powered by Guardian.co.ukThis article titled “What are sinkholes and what causes them?” was written by Jon Henley, for The Guardian on Monday 4th March 2013 17.35 UTC

Last week, in a quiet residential suburb east of Tampa, Florida, the Earth opened up and swallowed a man. Jeff Bush, 37, was tucked up in bed late on Thursday evening when his entire bedroom floor simply gave way with a deafening crash that his brother, in the room next door, later described as "like a truck hitting the house".

Jeremy Bush, 35, heard his brother’s scream and rushed towards his bedroom. "Everything was gone," he told local television stations. "My brother’s bed, my brother’s dresser, my brother’s TV. My brother was gone. All I could see was the top of his bed, so I jumped in and tried digging him out. I thought I could hear him screaming for me and hollering for me."

As the house’s floor threatened to collapse further into a gaping hole more than 9m across and 15m deep, a sheriff’s deputy who had arrived on the scene with the emergency services eventually pulled Jeremy to safety. Jeff remained trapped. "I couldn’t get him out," Jeremy said. "I tried so hard. I tried everything I could. No one could do anything."

As Jeremy and four others, including a two-year-old child, were led away uninjured, rescue teams lowered a microphone and video camera into the hole, but it was soon apparent that Bush could not have survived. By Saturday, the search for his body had also been abandoned. "We just have not been able to locate Mr Bush, and so for that reason, the rescue effort is being discontinued," a local official, Mike Merrill, said. "At this point, it’s really not possible to recover the body."

When the ground begins opening up beneath our feet and plunging unsuspecting mortals into the abyss, some may be tempted to reach for the Bible and start predicting the End of Times (and a quick online search reveals that several of the wackier sort of website have not hesitated to do just that). But biblical as the story sounds, the sinkhole – as the phenomenon is called – that caused Jeff Bush’s death was not an act of God but of geology.

Natural sinkholes – as opposed to manmade tunnel or cave collapses – occur when acidic rainwater seeps down through surface soil and sediment, eventually reaching a soluble bedrock such as sandstone, chalk, salt or gypsum, or (most commonly) a carbonate rock such as limestone beneath. In a process that can last hundreds, sometimes thousands of years, the water gradually dissolves small parts of the rock, enlarging its natural fissures and joints and creating cavities beneath.

As the process continues, the loose, unconsolidated soil and sand above is gradually washed into these cracks and voids. Depending on how thick and strong that top layer is (sand will not last long; clay can hold out for millennia), and how close to the surface the void beneath is, the land may be able to sustain its own weight – and that of whatever we build on top of it. But as the holes grow, there will come a day when the surface layer will simply give way.

"Once those caves start to collapse, the materials above will simply funnel in," says Dr Anthony Cooper, a principal geologist at the British Geological Survey, which maps the country for rock types susceptible to sinkholes and carries out surveys for developers, builders and individuals worried about the prospects of the land caving in beneath them. "It’s just like an eggtimer, really. That’s certainly what appears to have happened with this incident in Florida."

In the language of geologists, the process that causes sinkholes is "the creation of a void which migrates towards the surface". In the language of the layman, when there’s not enough solid stuff left underneath to support what is left of the loose stuff above, the whole lot collapses. The resulting depressions characterise what is known as a karst landscape, in which hundreds or even thousands of relatively small sinkholes form across an area that, seen from the air, can appear almost pock-marked.

Since around 10% of the world’s surface is made up of karst topographies, sinkholes are far from uncommon. The entire state of Florida, as the Bush family unfortunately learned, is classed as karst landscape, and sinkholes are so common that insurers are obliged by law to offer cover to home owners who ask for it (insurance was compulsory until 2007, when many home owners dropped it because of the rising cost). "If you look at a satellite image of the state, or even just a map," says Cooper, "you’ll see it’s peppered with little circular lakes and lots and lots of sinkholes. A great many of them are visible, but many more are covered in. It’s typical karst topography."

Elsewhere in the US, sinkholes are common in Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania. In Britain, the BGS says the carboniferous limestone of the Mendip Hills, the north of the South Wales coalfield, the Peak District, the Yorkshire Dales, the northern Pennines and the edges of the Lake District all host well-developed karst landscapes. Karstic features are also common in the UK on the chalk of south-east England, on salt in the centre and north-east of the country, and particularly on the gypsum that underlies parts of eastern and north-eastern England, especially around Ripon and Darlington, and in the Vale of Eden.

"Gypsum is the most soluble of all," says Cooper. "If you were to place a block of gypsum the size of a transit van in a river, it would dissolve completely within about 18 months." Ripon in North Yorkshire, Cooper says, is very susceptible to sinkholes, the most famous – some 20m deep – dating back to 1834. In 1997, four garages collapsed into a huge sinkhole that only just missed the front of a neighbouring house.

One of the more spectacular recent British sinkholes, a 7.5m-deep crater, opened up in 2010 beneath a patio in Grays, Essex. "It was like an earthquake. There was a rumbling and we both ran out to look and there just a couple of steps away there was this monstrous hole," the house owner, Ben Luck, said at the time. "It was there in a second. There wasn’t a bit of dust, and there was no sign of the crazy paving – it had all disappeared in the hole." Structural engineers said the hole was caused after water penetrated chalk some 25m down, causing tonnes of soil above it to shift.

Around the world, this process that produces sinkholes has created such striking natural features as the hills of Ireland’s western coast, the caves of Slovenia and the pillars of Guilin in China. Where the underlying limestone layer is thick and rainfall heavy, vast underground caverns and subterranean rivers have produced sinkholes of dimensions that make what’s happened in Florida or Essex look positively insignificant: the Xiaozhai tiankeng ("heavenly pit") in Chongqing, China, is 662m deep; the Dashiwei tiankeng in Guangxi 613m. Croatia has a 530m-deep hole, with vertical walls, called the Red Lake, while Papua New Guinea has the Minyé sinkhole (510m deep) and Mexico the Sótano del Barro (410m) and Sótano de las Golondrinas (372m deep).

What finally triggers a collapse? The most common factor, Cooper says, is changing groundwater levels, or a sudden increase in surface water. During long periods of drought, groundwater levels will fall, meaning cavities that were once supported by the water they were filled with may become weaker (water pumping, for factories or farms, can have a similar effect). Conversely, a sudden heavy downfall can add dramatically to the weight of the surface layer of soil and clay, making it too heavy for the cave beneath to bear.

Sometimes the trigger can be man-made. In chalky West Sussex in 1985, a burst water main caused an alarming rash of small 1m- to 4m-wide sinkholes to appear in Fontwell. "There  was also a man who emptied his swimming pool out on to his garden, and was soon confronted with a large sinkhole under his house," Cooper says. "And in Florida, automatic frost sensors have set off sprays fed from boreholes and intended to stop strawberry crops from freezing – but the result was more than 100 small sinkholes."

So how can you detect a developing sinkhole – and can anything be done about it once you suspect the process may be under way? In Britain, Cooper says, the BGS maps the country to locate rock types that may be affected by sinkholes. It also keeps an up-to-date National Karst Database recording visible sinkholes, springs, soakaways and known building damage. Using all manner of modern technologies, "we cut an awful lot of data, from rock types to slope angles, covering materials and drainage, and basically zone the country into datasets that can be used by property developers, local councils, the construction industry, insurers and the like," he says.

At the most basic level, people in a sinkhole-prone zone are best advised simply to "look around them, at the adjacent land and buildings". Telltale signs may include sagging trees or fence posts, doors or windows that no longer close properly, and rainwater collecting in unlikely places. Some developing sinkholes can be filled in; Anthony Randazzo, a former University of Florida professor who has spent his career studying sinkholes, now runs a profitable company that does just that, injecting grout to fill cracks that develop underground and shore up the foundations of buildings. "It’s like a dentist filling a cavity," he says.

But this is not always possible. The key is good drainage; you want to get water away from a vulnerable area. "Covering an opening up with concrete, or filling up a hole completely with solid concrete, may not necessarily help," warns Cooper. Sometimes, too, the hole may simply be too deep: 80m, perhaps, compared with the 12-15m height of a house. "On some occasions, we have had to point out to developers that a hole 20m deep and 30m wide is a lot bigger than a house," Cooper says. "That’s a hell of a lot of concrete."

Despite the frequency of sinkholes, linked fatalities are rare. Randazzo says he can recall only two other people besides Bush who have died because of them in the US during the past 40 years. Even then, he says, in both cases the people concerned had been drilling boreholes (and thus interfering with groundwater levels). "Usually, you have some time," Randazzo, who has lectured on sinkholes at Oxford University, told USA Today. "These catastrophic sinkholes give you some warning over the course of hours. This latest incident is very unusual, and very tragic."

In the UK, Cooper says, no deaths attributable solely to naturally formed sinkholes (as opposed, say, to the collapse of disused mine chambers) have been recorded in recent times. But, he points out, since extremes of sinkhole-affecting weather – long periods of drought, for example, followed by spells of unusually heavy and persistent rain – are widely predicted to become more frequent as the Earth’s climate changes, "we would certainly expect there to be more sinkholes in the future". It could be only a matter of time before Britain buries a Jeff Bush.

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