HAVE you ever wanted the ground to swallow you up? If you live in Ripon, in the north of England, your wish might come true more often than you鈥檇 expect.
The Yorkshire town is something of a world capital for sinkholes. Sinkholes are caverns that form when circulating water dissolves rocks rich in carbonate or evaporites. Ripon is built over gypsum, which dissolves readily, and over the past 150 years more than 30 major sinkholes have appeared, often dragging buildings and gardens down into them. The sinkholes of Ripon are thought to have been Lewis Carroll鈥檚 inspiration for the passage to the underworld in Alice鈥檚 Adventures in Wonderland. The fantasy has turned into a financial nightmare: to date, the repair bill exceeds 拢1.3 million.
Residents of Ripon aside, should we be worried about sinkholes? Yes. Worldwide, damage from sinkhole collapses appears to be on the increase. The US suffers more than the UK, particularly along its eastern seaboard. Among cases this year is an 11-metre sinkhole that swallowed a car at a gas station in Citrus Springs, Florida, and a 12-metre sinkhole that closed the interstate highway through North Carolina near Charlotte. The runway at Capital City Airport near Harrisburg, Pennsylvania, is frequently affected.
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Why are the experts worried? The problem, they say, is urbanisation. Sinkhole collapses are more prevalent near expanding cities and towns, because buildings and roads disrupt the natural drainage system. Rainwater runs off roads and pavements and drains away in a few specific areas, where the increased flow washes out the soil and makes any sinkholes beneath more liable to collapse. Also, when more water is piped from aquifers to meet growing demand, this increases the flow of water through the rock and hence the rate at which it dissolves. Tony Waltham at Nottingham Trent University in the UK estimates that 90 per cent of new sinkholes are caused by human activity.
How quickly can a sinkhole form? It depends on the type of rock. Carbonates dissolve at about 1 centimetre a year, but evaporites such as gypsum can dissolve more than a hundred times faster. Sinkholes can reach a considerable size before collapsing. In 1981 at Winter Park, Florida, a sinkhole ate an entire parking lot, half a six-lane highway, a house and some trees, washing them down with half a public swimming pool 鈥� all in 24 hours.
Is there any way to reduce the risk? Better planning would help avoid building in highly vulnerable areas, but if it鈥檚 essential to build there, houses, bridges and roads can be specially strengthened to help protect them against ground collapses.
One final piece of advice. If you live in an area with fine limestone caves and you wake up one morning to find a slight dip in your garden, don鈥檛 go too close.