Sinkholes: an act of geology, not God

Sinkholes are not an act of God but of geology. But what causes them and how common are they?

PUBLISHED : Sunday, 10 March, 2013, 12:00am
UPDATED : Sunday, 10 March, 2013, 2:53am

Last month, 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 a 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."

He stared hopelessly at a gaping hole more than 9 metres across and 15 metres deep. His brother's body was never found.

Such incidents have also occurred in China and around the world. A spate of incidents involving random sinkholes in at least nine mainland cities in August killed four people and injured several others.

Early last month, a massive sinkhole opened up at a subway construction site in central Guangzhou. A 150-metre by 50-metre sinkhole in the Dachegnqiao town of Ningxiang, Hunan province, destroyed at least 20 houses in June 2010, though no one was killed.

Five shops and a three-storey building were swallowed by the nine-metre-deep hole, which covered an area of 300 square metres. No casualties were reported, but local residents said it was the second large instance of subsidence since 2004. A 2008 report by the Guangdong Geology Institute found the surrounding area was not geologically stable, though plans for two subway lines passing through the area still proceeded.

Similarly a giant sinkhole swallowed several homes in Guatemala City in February 2007.

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. But biblical as the story sounds, natural sinkholes are not an act of God but of geology.

They 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.

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," said Dr Anthony Cooper, a principal geologist at the British Geological Survey (BGS).

Geologists describe it as "the creation of a void which migrates towards the surface". In simple terms, 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 pockmarked.

Since around 10 per cent of the world's surface is made up of karst topographies, sinkholes are far from uncommon. The entire state of Florida is classed as karst landscape. Sinkholes are also common in Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania. In Britain, the BGS says the carboniferous limestone of the Mendip Hills, in southwest England, the north of the South Wales coalfield, the Peak District, the Yorkshire Dales, the northern Pennines and the edges of the Lake District in northern England all host well-developed karst landscapes.

Karstic features are also common in Britain on the chalk of southeast England, on salt in the centre and northeast of the country, and particularly on the gypsum that underlies parts of eastern and northeastern England.

"Gypsum is the most soluble of all," said 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 20 metres 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 7.5-metre-deep crater, opened up in 2010 beneath a patio in Grays, Essex, northeast of London.

Structural engineers say the hole was caused after water penetrated chalk some 25 metres down, causing tonnes of soil above it to shift.

Around the world, this process that produces sinkholes has created such striking features as the hills of Ireland's western coast, the caves of Slovenia and the pillars of Guilin , 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 662 metres deep; the dashiwei tiankeng in Guangxi 613 metres; and Croatia has a 530-metre-deep hole, with vertical walls, called the Red Lake.

What finally triggers a collapse? The most common factor, Cooper says, is changing groundwater levels, or a sudden increase in surface water.

During 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, southern England, in 1985, a burst water main caused an alarming rash of small one-metre to four-metre-wide sinkholes to appear in the village of 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 said. "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."

For prevention, the key is good drainage - 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," said Cooper. Sometimes, too, the hole may simply be too deep: 80 metres, perhaps, compared with the 12 to 15-metre height of a house. "On some occasions, we have had to point out to developers that a hole 20-metre deep and 30-metre wide is a lot bigger than a house," Cooper said. "That's a hell of a lot of concrete."

Despite the frequency of sinkholes, fatalities are rare. Anthony Randazzo, a former University of Florida professor who has spent his career studying sinkholes, says he can recall only two other people besides Bush who have died because of them in America during the past 40 years. "These catastrophic sinkholes give you some warning over the course of hours. This latest incident is very unusual, and very tragic."

In Britain, Cooper says, no deaths attributable solely to naturally formed sinkholes 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.

The Guardian. Additional reporting by Alex Lo