In late August, a sinkhole gobbled up a chunk of roadway and closed the northbound lanes of U.S. Highway 17-92 in DeBary. Then, on Sept. 17, a vehicle-sized sinkhole opened up in a grassy field on the Stetson University campus.
Let’s get to the bottom of sinkholes.
Why and how do sinkholes form?
“Rocks that underlie Florida and other places are primarily made up of carbonate, mainly limestone and dolostone,” said Ben Tanner, a geology professor at Stetson. “When carbonate interacts with weak acidic solutions, like rainwater and some groundwater, it dissolves. Over time, pockets develop, and every now and then those pockets will collapse.”
Tanner notes that the underwater caves at Blue Spring in Orange City are an example of these pockets.
The addition or subtraction of water is major factor in sinkhole formation. Water that previously stabilized a pocket could disappear in a drought, causing a collapse.
In an especially rainy time, such as during a hurricane or the rainy season, groundwater can rush in and be equally destabilizing.
Other factors, like development and population growth, most likely also contribute to sinkholes, Tanner said.
“Additional groundwater withdrawal and added weight from an increasing population could lead to the formation of more sinkholes,” Tanner said. “It’s a hypothesis that makes a lot of sense.”
What caused the recent DeLand sinkhole?
In the case of the recent sinkhole on Stetson University property, the cause seems to be the nature of the particular area.
“The good thing is that where it is on campus is a grassy area that isn’t used for anything,” said Al Allen, facilities manager at Stetson.
The site along Florida Avenue, near the West Volusia Historical Society’s DeLand House Museum, is actually lower than the rest of its surroundings, he said.
“The geo-tech engineer, Thomas Bechtol, told us that water naturally flowed there as the lowest spot, and just sat there with all its weight and sunk in,” Allen explained.
Is there any way for me to tell if a sinkhole might form on my property?
“There are ways to identify areas that have increased risk,” said Tanner, “but it would be expensive.”
According to Tanner, identifying the pockets that lead to sinkholes requires a survey of a property, along with ground penetrating radar or exploratory boreholes. Boreholes, narrow shafts made in the ground, could identify a pocket.
On the other hand, such a survey captures only a moment in time. A pocket could form later due to the nature of the limestone.
According to a 2017 report by the Florida Division of Emergency Management, Volusia County — in particular West Volusia — is one of the more favorable locations for sinkhole formation on Florida’s East Coast.
Efforts to prevent sinkhole formation, outlined in the report, include regulatory policies like construction requirements, stormwater management, and proper and comprehensive planning and design of developments or renovations.
Visual cues that could indicate a potential sinkhole may include cloudy or muddy well water, exposure of foundations, formation of ponds after rain, interrupted service of underground utility lines, and dying or wilted vegetation.
“However, just because a sinkhole forms in a place doesn’t mean you’ll get more activity,” Tanner said. “The sinkhole at Stetson is probably stabilized, and usually these events are isolated. You could think of them as more or less random.”
How do you fix a sinkhole?
The sinkhole at Stetson was assessed by Bechtol, the geo-tech engineer, as well as engineers from the City of DeLand.
“On advice of our geotech engineer, two truckloads of a soft cement went in to stabilize the hole,” said David Rigsby, manager of the grounds at Stetson and a landscape architect.
Rigsby said 15 cubic yards of the soft cement was used.
“The engineer came back and did further tests and determined it is no longer an active sinkhole. On his advice, we filled the rest over with soil, and we’ll monitor it for about a week,” he added.
Allen added, “This is the standard technique to fill sinkholes.”
In the case of the sinkhole on U.S. Highway 17-92 in DeBary, the road reopened Sept. 14 after a contractor working for the Florida Department of Transportation pumped 350 cubic yards of grout into the cavern.
Can I buy insurance to protect against sinkhole damage?
Florida law requires insurance policies to include catastrophic ground-cover-collapse coverage, but sinkholes don’t necessarily meet the criteria for coverage as “ground-cover collapse.”
Four specific criteria must be met for coverage.
“Abrupt collapse, a depression clearly visible, structural damage, and the home has to be condemned,” said Phyllis Mattheson, vice president of The Page Agency, an insurance company in DeLand. “In other words, the city has to come in and say it is no longer safe to live there.”
According to Mattheson, some policies offer additional coverage simply called “sinkhole coverage,” which, she said, usually covers only the property surrounding the home.
“In that case, I believe the standard deductible is 10 percent of the value of the property, and it must first pass a geological inspection,” Mattheson said.
During a geological inspection, according to a flier produced by the Florida Department of Financial Services, “If sinkhole activity is present … the insurance company may decline coverage.”
What if you have sinkhole coverage for your yard and catastrophic ground-cover-collapse coverage for your home, and a sinkhole cracks the foundation of the still-livable home?
“They won’t pay for it,” Mattheson said.
Find more information at www.myfloridacfo.com.
Are sinkholes becoming more common in Central Florida?
According to an article in the May 2018 Smithsonian magazine, there are such things as “sinkhole seasons.”
Water — whether a lack or an overabundance — is the greatest natural cause of the formation of sinkholes. A swing from one to the other, such as a heavy period of rain that follows a drought, creates optimal conditions for sinkhole formation.
A major man-made effect is overburden on topsoil caused by development: literally, the weight of a structure.
As Central Florida expands in population and development, the burdens on our bedrock will increase.
Can we prevent sinkholes?
The short answer is no.
The long answer is that Florida’s bedrock of carbonate — created over millions of years from the remains of sea creatures left behind when water that once nearly covered the state receded — will tend to sometimes dissolve, and sometimes this will cause sinkholes.
Several steps can be taken that will mitigate the frequency of sinkholes, including policies that manage our resources and govern development with sinkholes in mind, and comprehensive planning that tries to prevent overburden on the ground beneath our feet.
In the meantime, at least we don’t have earthquakes.