Why are so many sinkholes opening up these days, and should we be scared of what carnage they can, and may, cause? Following the radical disappearance of the eight vintage Corvettes from the museum in Kentucky this week, there have been yet more episodes of gigantic sudden sinkholes.
Last night, in storm-battered Britain, a chasm that was twenty-foot deep and thirty-five feet wide appeared in the middle of a Hemel Hempstead housing estate. Homes were evacuated, uprooting 17 families, as the monster hole was assessed. Carla Rance, 35, a resident, had been worried when her front porch inexplicably sank six weeks ago. Her landlords had assured her all was fine. Today her next-door neighbour’s house hangs precariously over the drop. Assessors predict at least two of the homes will have to be demolished. They were only built six years ago.
Earlier in the month, a car disappeared overnight in Buckinghamshire, and its owner presumed it had been stolen. It had not, it had fallen victim to another sinkhole which swallowed the Volkswagon Lupo whole.
On Tuesday a major highway, the M2 had been closed off to traffic as a fifteen foot hole emerged in the central reservation. Originally thought to be a sinkhole, this one was in fact an old chalk well, a sequence of connected underground caves accessible through a vertical shaft. Chalk mining has been going on since the time of the Romans, and was also common in Hertfordshire, scene of today’s disaster.
Old and unmarked mines can often be the explanation for mysterious holes opening in ground assumed to be solid. This was the case last year in the Peak District, when previous lashings of torrential rains collapsed into a former lead mine in the Peak District, going down for 130 feet. These sorts of cavings-in are distinct from sinkholes.
This may explain other new UK sinkholes this week at a school playing field in (ironically) Rainham, and also at a nearby rugby pitch in Gillingham. It would not explain the entire resort complex near Disneyworld which collapsed last August into a hole that was twenty meters wide. No old mines there. Perhaps it was the case in Illinois where a golfer fell into an 18 foot sinkhole as the earth gave way beneath him at the 14th hole in Waterloo last March. He was brought to safety with a rope. Illinois is riddled with old mines, particularly in the south-west.
Sinkholes in urban and built-on areas are more common after intense rainfall, as the underground channels bellow the surface are slowly getting eroded, especially if they have high limestone or other soft rock content, gypsum or salt. This can cause caverns to form, and ultimately, the hollow is too big to support the weight of the topsoil above it. Then it implodes. These are called cover collapse sinkholes. The other type builds more slowly and the ground starts to depress as little as an inch at a time. These are cover-subsident. Are there more sinkholes than ever before, or do we just see them more often, because of growing world populations?
Drew Glasbenner, a Florida geologist, thinks it’s the latter. He says more people are living near them now, which means they get spotted more often. He will be featured in a Destination America broadcast on February 17th called Sinkholes: Swallowed Alive. Twenty five percent of American terrain is susceptible.
Kentucky, where the Corvettes got gobbled, is particularly rife, because it is rich with soft limestone in the soil. The limestone hasn’t hardened up yet. In Missouri and Tennessee, they are also prone, but the limestone is harder.
Glasbenner says developers have to dig deeper when they are doing ground surveys, and also watch for how the natural drainage occurs. They need to go right down into the bedrock not just 10 or 15 feet down. He predicts many more sinkholes in Florida as the state continues to be built upon. The Florida sun is like a magnet, especially for retirees, and as more and more people come to the live there, more homes will be built in the wilder, sinkhole-ridden outskirts. They are trapdoors awaiting the hasty builders and the would-be homeowners.
Florida is in fact a vast platform of honeycomb limestone. Groundwater continually flushes through it, aided and abetted by the humid climate with plenty of acid rainfall. All this produces underground cavities. When surface mud and grit get washed down too, the land gets all pockmarked, which is called karst. Underneath the surface, everything is being gradually eroded. On top all is lovely green lawns, swimming pools and sunshine. Below, it is a different story. 6,500 sinkhole insurance claims are lodged in Florida every year.
The word karst is an Old Slovenian word from the area, Kras, which covered much of Old Yugoslavia (now Slovenia and Croatia) which is a huge sinkhole area. So are huge swathes of China and Russia. Mexico and Belize are karst zones and so is a considerable part of Italy. Basically, it is a worldwide geological problem.
Man made sinkholes can by caused by broken pipes and old sewerage systems. Geological ones are affected by man’s activity, but more often, have been building up for many years, even thousands of years. They are also called dolines.
The world’s largest sinkhole is in Xiaozhai Tiankeng in China, and it is a stupendous 2000 feet deep. There’s another massive one in Guatemala City that took two buildings down with it in 2010. This followed another monster, almost perfectly spherical giant sinkhole in 2007, right in the heart of the city where two roads intersected. Another very huge sinkhole exits in Egypt. The Qattara Depression, as it is known, is an enormous 5o miles by 75 miles in size.
One of the scariest sinkholes in recent memory was the one that took the life of Jeff Bush, 37, in Tampa. On 28 February 2013 the ground opened up and took Jeff, asleep in his bed, down with it. The body was sucked into the depths and never recovered. It’s a scene that still haunts attending police officer, Douglas Duvall. He said at the time, “It was like this thing was live.” He reported it as churning, moving around and making growling noises. Duvall managed to pull Jeff’s brother Jeremy, from the pit, and narrowly averted a second life being lost. Jeremy was scrambling in the dirt determined to try to save his sibling.
A cover collapse sinkhole had lain directly underneath Jeff Bush’s bedroom, and when it collapsed, the suction was so great, it sucked up the concrete floor and wrenched away the entire contents of the room.
Some blame agriculture for the problems. Florida is the fruit basket for America, particularly with its famous citrus groves, but those are irrigated from groundwater. This pumping, especially when frosts threaten the strawberry crops, can reduce the aquifer by as much as ten meters overnight. This can spur a spate of sinkholes. There is no evidence however that this was the cause of the Seffner sinkhole which took Jeff Bush.
There are warning signs, as the lady in Hemel Hempstead noted, when her front porch began to subside. Commonly these are cracks appearing unexpectedly, doors and windows which suddenly won’t close, perhaps some ground shifting. These signs are not conclusive, and it is almost impossible to tell when a sinkhole will open. A lot depends on the type of rocks beneath and what their tipping point is.
There’s very little doubt that human construction adds to the incidence of sinkholes. As we put up more and more parking lots, housing estates and office blocks, there are changes to the hydrologic regime of the ground. Where once water was free to drain away, it gets concentrated and all gets channeled into one point. This cannot help but create a weak spot.
Other famous US sinkholes include the Porsche dealership in Winter Park, Florida in 1981, which took down a city block, and the Daisetta in Texas, the first ever caught on video. Remarkably, they have not been much associated with loss of life to date.
Is the frequency increasing? This is the million dollar question. There is no international database taking track of all the sightings. Farmers may have been accustomed to sinkholes and never bothered to report them. As the urban sprawl moves ever outward, it becomes more of an accident waiting to happen.
Should we be scared of sinkholes? There is no definitive answer. Should we expect to see more and more of them? Absolutely, yes.
By Kate Henderson