March 8, 2013
Many people were shocked to hear of the death of one Florida man last week, when a sinkhole opened up and swallowed his home while he slept. These occurrences are rare in the United States, but in Pennsylvania sinkholes are becoming more common. The state is among the top seven most at risk for this type of ground subsidence.
"Pennsylvania has a considerable area of the state underlain by limestone, and sinkholes are almost always associated with limestone terrain," said William White, Penn State University emeritus professor of geochemistry.
The reason that areas with porous rock layers—limestone, gypsum, salt bed, and domes—are susceptible to sinkholes is because the acidity in groundwater can dissolve them, forming empty spaces and even caverns below the surface, according to Science Daily. Lack of drainage is key to the sinkhole formula, and liquid has nowhere else to go expect through the rocks.
White said residents should not feel worried when they hear the word “acidity” associated with groundwater. It occurs naturally as carbon dioxide evaporates in the atmosphere and returns to the earth with rainfall. Then, rainwater picks up organic matter in the soil, causing more acidity.
What is alarming is that these caverns can form underground over long periods of time only to collapse in a dramatic fashion when the earth above can no longer support itself. This is called a cover-subsidence sinkhole. Cover-collapse sinkholes can both form and collapse in a matter of hours, Science Daily reports.
White said the riskiest areas for sinkholes in Pennsylvania include the limestone valleys through central PA—around State College and also from the Maryland line up through Harrisburg and east to Allentown and Lancaster. But nature isn’t the culprit in other parts of the state.
“Very little limestone is exposed at the surface, for example in Erie, Pittsburgh area, and Indiana area,” he said. “They do have a sinkhole threat. It’s not from the limestone, it’s from collapsing coal mines which cause problems particularly in the Pittsburgh area.”
According to the Department of Environmental Protection, ground subsidence in relation to coal mines is caused when the mines are close to the surface and cannot support the weight of materials above them. Other manmade causes of sinkholes include aquifer systems and road construction.
White said the warning signs to look for are doors and windows that no longer close properly, cracks in your foundation, or a hollow sound below your basement floor.
“I don’t think Pennsylvanians are in any great danger of the kind of horrible thing that happened to the poor fellow in Florida, but you can get quite extensive property damage from these things,” he said.
For Pennsylvania homeowners living above abandoned mines, DEP offer mine subsidence insurance. Go to their website for maps of old mines to see if you qualify.