Getting to the bottom of the sinkhole phenomenon




Sinkholes have been appearing in Ireland and the UK with increasing regularity in recent months. What’s causing them, asks John Hearne

IT’S like something out of the Book of Revelations. The ground suddenly opens up beneath you and you get sucked into the earth, never to be seen again.

This was the fate of Floridian, Jeff Bush last February. The 37-year-old went to bed on a Thursday evening in his unassuming house in a quiet, residential suburb east of Tampa. At some point in the night, the earth beneath his bedroom gave way and he was sucked down into a hole that had not existed seconds before. Afterwards, his younger brother Jeremy, who was sleeping in an adjoining bedroom, told local news stations that the crash which woke him sounded like a truck hitting the side of the house.

When he got to his brother’s bedroom, everything was gone. “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.”

Despite his efforts, Jeremy Bush could not reach his brother. When the emergency services arrived, they lowered a camera and a microphone into the hole. Neither picked up any signs of life. Three days later, the rescue team announced that the search would have to be called off. Jeff Bush’s body was never recovered.

While a catastrophic collapse like this is extremely rare, sinkholes are a common geological phenomenon. The Geological Survey of Ireland has mapped over 6,000 in this country, and report that there are thousands more that remain to be mapped. Most are nondescript depressions in the soil; their collapse happens slowly over time. You will occasionally get sudden collapses, but these tend to be small, and happen away from public notice in remote uplands.

Dr Caoimhe Hickey of the Geological Survey of Ireland (GSI) explains that sinkholes tend to occur in karst landscapes; places where the rock can be dissolved by water. In Ireland, for the most part, that means limestone.

“We get acid rain from the carbon dioxide in the air or in the soil,” she says. “That acid can dissolve the lime in the limestone. Over time, that will leave voids or cracks in the limestone, and if you have overlying sediments on top of that, the sediments get washed down into the voids and cracks. What you get next is a gradual sagging of the surface into the depression underneath and that’s called a doline or a sinkhole.”

Sudden, or ‘catastrophic’ sinkholes occur when the overlying sediment is ‘clayey’. This means that it sticks together, and as the rock is dissolved away underneath it, this bonded clay creates a kind of bridge over the void. “At a certain time that bridge suddenly drops into the void underneath because it can’t support its own weight.”

It’s the extreme weather, says Hickey, that has been largely responsible for the spate of reported sinkholes around the country in the last two weeks.

Strand Road in Tramore, Co. Waterford had to be closed in January when a part of the road collapsed into a hole several metres across. This one wasn’t caused by acid rain however, but by violent seas, which battered a hole in the sea wall and then sucked away the sand on which the road was built.

The hole that suddenly appeared the same day on the road leading to Long Beach in Ballybunion, appears to have been caused by the same thing. Rough seas broke through the wall protecting the road and quickly eroded the sand underneath it.

Something altogether different appears to have happened near the Galmoy lead and zinc mine in Co. Kilkenny, when a hole nine metres deep and fifteen metres across appeared overnight on land belonging to farmer Eddie Cavanagh. It was reported that the hole, which opened up above a shaft extending from the Galmoy site, was spotted by Cavanagh from his kitchen window last weekend. When he went to examine it, he was shocked to find just how big and deep it was.

John Walsh, professor of structural geology in UCD says that while it’s easy to jump to the conclusion that the collapse happened as a result of mining activity in the area, that may not have been the case. “It could be the mine, but a lot of Galmoy is karstified; it’s dissolved-out limestone, it could be a natural collapse.”

Whatever the underlying cause, the extreme weather conditions are once again the most likely culprit for the timing of the collapse, says Walsh. “High volumes of water can rush in from underneath, removing support, or the soil cover becomes thoroughly saturated, making it more likely that a collapse will happen.”

Initial assessments by Lundin Mining, who operate Galmoy, and the Department of Energy and Natural Resources, concluded that there were no public safety issues. It’s sobering to note however, that Mr Cavanagh and his son Brian had been working in the field two days prior to the collapse and had driven over the site in a tractor.

Meanwhile, a series of sinkholes in Sruwaddacon Bay in Co Mayo were first noticed last May, close to the site of the Corrib Gas pipeline. Again however, officials have been playing down their significance. Last July, the Communications Minister Pat Rabbitte told the Dáil that the ‘depressions’ as he called them, were very small and that they had been anticipated by the environmental impact report for the gas pipeline.

Despite the spate of sinkholes in the last few weeks, Ireland has nothing to rival some of the spectacular catastrophic collapses that have occurred around the world.

In Guatemala City, late in February 2007, a perfect circle of earth dropped 30 storeys, killing two people and forcing the evacuation of more than a thousand others. This disaster did not come as a result of any natural process, but because of the corrosion of a sewage system deep underneath the ground.

A sinkhole in Berezniki, Russia developed as a result of a flood in a potash mine back in 1986. What makes Berezniki particularly devastating is the fact that it’s growing every year. Right now it’s 200m deep, 80m long and 40m wide, but as it spreads and deepens, extensive tracts of land continue to collapse into it, along with large sections of the mine’s transport infrastructure.

There are many others which dwarf even these. The ‘heavenly pit’ in Chongqing, China, is 662m deep. Croatia’s 530m ‘Red Lake’ has steep vertical walls while the Minye sinkhole of Papua New Guinea is also over half a kilometre in depth.

Experts say that in this country at least, sinkholes in general pose very little threat to public safety. “The sudden ones that open up overnight are very rare,” says Dr Caoimhe Hickey of the GSI.

Professor Walsh in UCD agrees. “On the web, you’ll see some very high profile cases...They look incredible and it will frighten the life out of you but it really is extremely rare for something like this to happen.”

If you are worried, check out the GSI site at www.gsi.ie where existing sinkholes are mapped and you can see if you live in one of the vulnerable limestone areas.


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