The Science Behind Sinkholes

by Michael Mosher on February 19, 2013

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Something that can only be described as ‘really freaky’ happened last week near a subway construction site in Guangzhou, China’s third largest city. At first, local residents complained of loud, cracking sounds emanating from the earth; a few also reported minor tremors. Then, a little after 4pm, a building suddenly collapsed below the street. The culprit — a massive sinkhole that covered roughly 3,200 square feet and stretched more than 10 meters deep. Authorities scrambled to evacuate apartments and office buildings, and no deaths or injuries were reported. However, it was too late to save the buildings; three more structures plummeted into the sinkhole later that evening. The alarming video footage below was posted by NBC News.

Terrifying, yes, but certainly not unprecedented. Two years ago, a 70-foot sinkhole in Leesburg, Fla., claimed a beauty shop, a corner store, a dumpster and an oak tree. Another stateside sinkhole — this time in Ohio — was discovered last November, but not before it swallowed a sizable section of state highway. And in 2010, a 20-meter sinkhole appeared in a housing development in Schmalkalden, Germany, forcing 25 residents to relocate and destroying one automobile.

Sinkholes — which are also known as sinks, swallets, and cenotes — have appeared across the globe in all shapes and sizes, but they are particularly common in China. They occur when layers of soluble bedrock dissolve and collapse — a naturally occurring phenomenon known as a ‘karst process’. However, the size of sinkholes is greatly exacerbated by human activities, such as subterranean drilling or pumping, drainage diversion, or, in this most recent case, subway construction. Underground structures also play a role; in Beijing, where numerous bomb shelters have been installed over the years, nearly 300 sinkholes were reported between 2007 and 2009, and severe flooding last summer caused 99 sinkholes to appear in a span of just three weeks.

David Weary, a research geologist with the U.S. Geological Survey, told ABC News that areas with heavy rainfall are particularly susceptible to sinkholes. ”Usually you get a collapse because something occurred that transported the material, creating the void that the sinkhole falls into,” he said. “If there’s a cave underground filled with soil and sediment and you get an episode of high rainfall or a change in groundwater flow, the dirt that filled the void will be hollowed. Once it hollows out close to the ground surface, it becomes thin enough that it can’t support what is on top of it and it falls in.”

Sinkholes are always destructive, and in some cases they have also proven deadly. A woman in Beijing was killed last April after the sidewalk beneath her feet collapsed and she fell into a subterranean pool of boiling water; a rescue crew was able to extract the woman, but 99 percent of her body was burned and she later succumbed to oxygen failure. Two more people were killed by a sinkhole in the Chinese city of Harbin four months later. And in 2010, a sinkhole in Guatemala swallowed a three-story factory and claimed the life of a security guard working inside the building.

So, how do we protect ourselves from cavernous depressions that seemingly appear out of nowhere?  According to a report by Florida’s Suwannee River Water Management District (which has recorded its fair share of sinkholes), there are a few warning signs. These include:

  • Newly exposed fence posts, structural foundations or tree roots
  • Doors and windows that will not close properly
  • Fresh cracks in walls, floors, and pavement
  • Ponds that suddenly appear after a rainstorm
  • Patches of land where vegetation appears to be wilting
  • Well water with large traces of mud or silt

And just so we’re clear — if you see a sinkhole, it’s in your best interest to contact your county emergency management coordinator, and then get the hell out of Dodge. If the sinkhole is in your backyard, you may want to quickly rope off the area since you’ll be liable for any injuries that occur on your property — but in doing so, keep your distance. “Don’t get near the edge because commonly, the void underneath it is bell-shaped, so it’s really unsafe to be anywhere near the edge in case it continues to collapse,” Weary said.

Full article…here

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