Sinkholes: When the ground collapses!

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Man swallowed by sinkhole, dies in Florida

On Feb. 28, 2013, the earth opened up in Seffner, Florida, and Jeffrey Bush fell to his death. Local authorities decided they could not safely recover the body: excavating the rock and sand would cause more collapse and more danger.

Man's face crumpled in grief.

Photo: Daily News
Jeremy Bush, who tried to save his brother, Jeff, says his parents are ‘going through hell’ after Jeff died in a Florida sinkhole Feb. 28.

Then, on March 12, an Illinois golfer fell through a sinkhole — possibly associated with an abandoned mine — on the 14th hole. Mark Mihal, 43, survived an 18-foot fall with just a sore shoulder, after his golfing companions fished him out with a rope.

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House that is partially caved into ground with large cracks through roof.

Photo: 2009, Richard Elzey
This house suffered severe damage in Brooksville, Fla., 50 miles north of Seffner, site of the recent tragedy.

The incidents are graphic examples of the dangers of taking geology for granted. Earth is not always as solid as a rock.

In large parts of Florida and other states, a soluble subsurface geology called karst is conducive to sinkholes. Karst occurs in rock dominated by gypsum, limestone or salt — which can dissolve, leaving underground streams and cavities.

The U.S. Geological Survey says about 20 percent of the United States overlies karst terrain; the worst sinkhole damage occurs in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania.

Classically, sinkholes occur in locations where water, unable to flow laterally, percolates through soluble rock, creating caverns and cavities. Often, the surface will gradually subside, causing a cover-subsidence sinkhole. And as we’ll see, other forms of collapse are popularly called sinkholes as well.

Cover subsidence sinkhole

Sinkhole forming as overlying land slowing erodes downward to fill underground hole.

Gradual movement of soil grains into cavities in karst can result in a widespread form of sinkhole that is damaging, but not especially dangerous.

More rarely and dramatically, when the roof span becomes too large, the outcome is a “cavity collapse” sinkhole. That was apparently the fatal flaw in Seffner, Fla.

Cover collapse sinkhole

Sinkhole forms suddenly after underground sediment layer between land surface and underground cavity thins.

The undetectable underground changes can take centuries, until the earth gives way with a sudden, dangerous collapse.

Subsidence can also occur

Above mines and underground streams

In organic soils that shrink when they decompose

After routine activities like pumping groundwater up for drinking and irrigation, which removes water from an aquifer and reduces the volume of sand and gravel, causing subsidence

In cold winters in the strawberry fields east of Tampa, Fla., farmers pump groundwater so they can spray their crops to prevent freezing. “In 2010, this opened about 140 sinkholes,” says Mark Stewart, professor of geology at the University of South Florida. “Most of them did not damage homes, but several did, and there was some damage to an interstate highway.”

Sinkholes in Florida

Cover-collapse sinkholes found in Panhandle, near Tampa, and in Northeast Florida.

Modified from original map by Florida Geological Survey
Cover-collapse sinkholes dominate in the blue and purple regions.

Because wells are a known trigger for sinkholes, many subdivisions in the sinkhole-prone North Tampa area prohibit private wells, Stewart says. “You can’t have a well for irrigation or water supply. You have to be on city water.”

The many sides of subsidence

Guatemala City sinkhole: 2010

Photos: 1.) horslips5, 2.) horslips5
At least one man was killed after a three-storey building fell into this 200-foot-deep hole after Tropical storm Agatha dumped more than three feet of rain on Guatemala. Rollover to see a street-level view.

If removing water can cause collapses, so can adding water through leaking water pipes, sewers and storms. Storm runoff was blamed for a dramatic hole-in-the-ground that formed in Guatemala City, Guatemala, in 2010. The city is built on a volcanic ash plateau, and groundwater, sewer systems and storm drains all feed the fragile ash. “The result is erosion of the material, which creates cavities that can collapse,” says Stewart. “It can be really catastrophic.”

Large diagram of landscape shows different subsidence risks after mining.

Pennsylvania, riddled with expired coal mines, has major problems with subsidence.

In New Orleans and other river deltas, subsidence occurs as organic material in soil decomposes and new sediment, which would sustain ground level, is blocked behind levees. Parts of the Sacramento-San Joaquin river delta in California have fallen to more than 15 feet below sea level.

Sinkholes can result when industrial and water-storage ponds get so heavy that they trigger a collapse.

Minding mining

And then there is mining, which is a major cause of subsidence in Pennsylvania, West Virginia and Kentucky. Those states have about 60 percent of U.S. abandoned coal mines; overall, the nation has about 14,000 active mines, and up to 500,000 abandoned mines of all sorts.

Asphalt road with chasm; half of road has sunk several feet Finding that sinking feeling.

A collapse at the Retsof mine in Upstate New York had broad repercussions for the economy, landscape and groundwater.

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Corner of house with long horizontal crack from foundation to doorway.

A telltale of subsidence. Don’t buy this house ’til you find out what caused that nasty crack!

In 1994, an earthquake collapsed a 500-foot square chunk of roof rock above a huge salt mine south of Rochester, N.Y., and water began flooding in. Since the Retsof mine opened in 1885, it had grown into the world’s second-largest salt mine, covering 10 square miles.

Within 21 months, the mine was inundated and closed. Water levels dropped in wells as far as 10 miles away, and two sinkholes formed, each about 50 by 200 feet. Further subsidence is expected as groundwater dissolves more salt; eventually, as the mine roof continues its slow-motion collapse, the ground above the mine is expected to fall eight or nine feet.

Finding that sinking feeling

Sinkholes can be surprising, but they don’t appear at random. In sinkhole-prone states, state geological survey maps should provide at least a general guide to risk; the USGS also has a national mapping facility.

Although a cover-collapse sinkhole may come without warning, experts say people in karst terrain should look for these signs of subsidence:

Cracks in the walls and foundation

Doors and windows that refuse to close

Settling around the foundation

Recognizing safe building sites in areas prone to sinkholes entails a multipronged approach, writes sinkhole expert Francisco Gutiérrez, professor of geology at the University of Zaragoza, Spain.

Identification techniques include field surveys and geomorphological mapping combined with accounts from local people and historical sources. Detailed sinkhole maps can be constructed from sequential historical maps, recent topographical maps, and digital elevation models complemented with building-damage surveying, remote sensing, and high-resolution geodetic [Earth-measurement] surveys. On a more detailed level, information from exposed paleosubsidence features (paleokarst), speleological [cave] explorations, geophysical investigations, trenching, dating techniques, and boreholes may help in investigating dissolution and subsidence features. Information on the hydrogeological pathways including caves, springs and swallow holes [where streams disappear belowground] are particularly important … .1

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Expanse of rocky cliff showing many holes and large hollows.

Photo: Minerve, Hérault Department, France, Wikimedia Commons
This karst landscape shows how water carves conduits and cavities in limestone and other soluble rock. The river through the village of Minerve in southern France disappears into a swallow hole in the karst below town!
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1967 newspaper headline with map of Florida showing sinkhole locations.

The New York Times
Sinkholes are nothing new around Tampa.

Yet as the recent Florida case shows, cover-collapse sinkholes usually come out of the blue. You might think that a state laced with sinkholes would want to zone development away from danger, but the Florida Board of Realtors “is not interested in any kind of hazard zoning,” Stewart says. “To get a mortgage, you must have homeowner’s insurance, and then it would be extraordinarily difficult to get insurance, so banks would be very hesitant to give mortgages, and that would greatly affect property values” in sinkhole areas. Rather than blacklisting areas likely to subside or collapse, the trend is to “leave the risk to the homeowner.”

Better take out some insurance

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Several people stand, looking at chasm in grassy field along road.

September 2003, U.S. Geological Survey
This cover-collapse sinkhole occurred in limestone near Frederick, Maryland. Many sinkholes occur along highways where rainwater runoff concentrates in storm drains and ditches and erodes the subsurface. That storm sewer pipe may have played a role in creating the sinkhole.

The Florida insurance industry, buffeted by sinkhole claims, has pushed through a law requiring cases to be settled by arbitration, rather in court, Stewart says. “There was a substantial loss to the insurance industry, so the legislation was changed.”

Homeowner’s insurance in Florida does cover sinkholes, he says, but not other causes of subsidence, which can cause slow-mo damage. Insurance companies “typically call in a geotechnical firm to do an investigation. In many, many cases, it’s not due to a sinkhole, so the homeowner hires their own geotechnical firm, which says it is, and the issue goes to arbitration.”

One source of data, ground penetrating radar, which reflects off different layers in the subsurface, “can be interpreted over a broad range by experts,” says Stewart. “The level of professionalism among people in the industry has not been at its highest level. Until recently there was a lot of money to be made by lawyers, geotechnical firms and the insurance industry.”

In 2010 alone, there were more than 6,600 sinkhole claims in Florida, Stewart says. “It’s not possible to estimate how many were legitimate. There are geotechnical experts on both sides, so there is no final determination whether it was a sinkhole.”

In a sense, sinkholes, like lightning are frightening natural phenomena that seem to strike from nowhere. “A hole opened under the bedroom and the man went down and was buried while sleeping,” says Stewart. “The Earth isn’t supposed to open up under you. It’s like shark attacks; they get press even though they are extraordinarily rare. Because of the emotional attachment, people see too much risk where there is in reality low risk.”

– David J. Tenenbaum

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Terry Devitt, editor; Emily Eggleston, project assistant; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer; Amy Toburen, content development executive

Bibliography

  1. Identification, prediction, and mitigation of sinkhole hazards in evaporite karst areas, F. Gutiérrez et al, Environ Geol (2008) 53:1007–1022
  2. Sinkholes
  3. In the news: Florida’s sinkhole problems
  4. USGS’s take on the science of sinkholes
  5. Florida’s sinkhole geology
  6. 12 amazing holes in The Earth
  7. What happens after mines close?