Sinkholes can form anywhere there is soluble rock present underground. This is known as "karst terrain," according to Randall Orndorff, Director at the Eastern Geology and Paleoclimate Science Center of the United States Geological Survey. Soluble rocks that could potentially lead to sinkhole formation include limestone, gypsum and salt.
Significant rainfall is a key ingredient as to whether a sinkhole will open, because the water becomes acidic once it is underground and, without proper drainage, can pool in sinkholes. Florida is a prevalent state when it comes to sinkholes, but more than 20 percent of the country is above "karst terrain," according to the U.S. Geological Survey (USGS). Other states where sinkholes are prevalent are Texas, Alabama, Missouri, Tennessee, Kentucky and Pennsylvania.
Officials survey a gaping sinkhole that opened up a residential street on Chicago's South Side after a cast iron water main dating back to 1915 broke during a massive rain storm, Thursday, April 18, 2013, in Chicago. (AP Photo/M. Spencer Green)
Sinkholes can also be man-made even if rock solubles are not present underground. According to the USGS, a leaky faucet, old mine shafts and sewage malfunctions are three examples of how man-made sinkholes can form.
Sinkholes occurring in urban areas are dangerous because as the water and rock dissolve, spaces and caverns form underground until they are too big and a collapse occurs. Water and sewage lines are already present underground. As water soaks into the ground, it can dissolve and erode the pipes creating caverns to begin to form, according to Orndorff.
Infrastructure such as sewage pipes and water main pipes are beginning to age and erode, which could contribute to sinkhole activity, Orndorff said.
Natural sinkholes can also form due to heavy rainfall and droughts. In the case of a drought, the water table can drop losing the stability that it once had. Also, when limestone dissolves, it forms a claylike soil which holds a lot of water. When the clay soil dries, it loses its cohesive bond, which could potentially cause the ground to drop.
Three cars fell the victim to an opening sinkhole in Chicago this past week, after heavy rainfall inundated the city. One man was injured and taken to the hospital. The two other cars were parked and did not have anyone inside.
An increase in sinkholes may or may not be occurring according to Orndorff. The media reporting on sinkhole activity may be to blame; however, Orndorff also said that sinkhole activity may be increasing due to population growth and development.
"In certain areas we are probably seeing an increase, due to human activities," he said.
The USGS maps the nation and monitors sinkholes. By monitoring a property, it is possible to see if the land is susceptible to sinkholes. To do this, survey the land for holes or cracks in the soil, and check with local government, or the USGS to see if the property is above a "karst terrain."
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