http://www.guardian.co.uk/world/2013/mar/04/what-causes-sinkholes-florida-man
What causes sinkholes?
It's the stuff of nightmares: last week, the ground opened up and swallowed a
Florida man as he lay sleeping in his home. But why do these sinkholes occur
and how widespread are they?
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a.. Jon Henley
b.. The Guardian, Monday 4 March 2013 17.35 GMT
c.. Jump to comments (39)
Link to video: Sinkhole house in Florida demolished
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.
A sinkhole in the centre of Guatemala City that swallowed a three-storey
building in May 2010. Photograph: Luis Echeverria/AP 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."
Jeremy Bush, right, is consoled as he sits outside the home where a sinkhole
swallowed his brother Jeffrey Bush. The house has since been demolished.
Photograph: Chris O'Meara/AP 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.
This sinkhole appeared overnight in a house in Guatemala City in 2011. It was
12.2m deep and 80cm in diameter. Photograph: Johan Ordonez/AFP 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.
Link to video: Huge sinkhole in Guangzhou swallows buildings
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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