A sinking feeling
It sounds like something out of a Stephen King thriller. Just before 11 p.m. Feb. 28, Jeff Bush lay sleeping in his Seffner, Fla., home when the earth opened below him and swallowed him. Rescue workers, heavy equipment and specialized personnel arrived on the scene in hopes of pulling the 37-year-old to safety, but as the ground continued to erode and the chasm deepened, that prospect seemed unlikely. Microphones failed to pick up any signs of life, and two days later, Hillsborough County Administrator announced that authorities were discontinuing recovery efforts due to the unstable conditions.
“Any further work will need to be done from the perimeter…unfortunately we have not been able to determine the whereabouts of Mr. Bush,” he said during a press conference March 2.
The Seffner incident is only one of a rash of sinkhole reports to have gripped the nation in recent months. This spring alone, sinkholes have trapped animals, consumed cars and engulfed swimming pools; however, experts say there is little reason for widespread panic. Sinkholes are more science fact than science fiction and more common than people think.
“I think we are hearing more about them because of incidents like Seffner, but that kind of catastrophic event is extremely rare,” said Harley Means, assistant state geologist for the Florida Geological Survey. “Typically what we see around here is the slow subsiding of the ground that usually results in cracks to the foundation, ceiling or drywall and not a fatality.”
What is a sinkhole?
According to David Weary, research geologist with the U.S. Geological Survey, sinkholes are the result of ground surface falling into some kind of void in the subsurface. They occur naturally in areas where the rock below the surface is limestone, carbonate rock or other materials that are easily dissolved by circulating groundwater.
“Other types of sinkholes are the result of ground falling into voids resulting from human engineering activities,” he said, noting that the culprits are often aging or leaking storm drains, sewers or storm water retention ponds. If these structures leak, they can move particles of loose sediment or artificial fills and create underground voids that can eventually collapse. “These are usually the type of sinkhole that we see in developed areas that don’t have natural caves beneath them.”
He said that the frequency of sinkhole occurrence, both the natural and man-made variety, are known to go up in conjunction with unusual weather patterns, such as heavy rains, droughts, hurricanes and other events, that upset the water table and destabilize the ground. While there may have been an increase in the number of sinkholes that made the national news this year, Weary said that predominately due to the Florida case a rash of national reports emerged on a subject that would have ordinarily remained a local story.
“We (the USGS) received dozens of media inquiries following that incident, when normally there would be little interest in sinkholes, except locally,” he said.
Concern in the community
Means said another reason for the increase in sinkhole activity is the migration of the population to previously undeveloped areas that are at risk for sinkhole events. Since it’s unlikely that communities will stop developing on these areas, it is important for residents and leaders to understand the nature of sinkholes, calm the public fear and identify potential problem sites that may need attention before a collapse occurs.
“There is no typical event. Sinkholes can range in shapes and sizes, and there is really no way to predict them. Not to sound callous or anything, but the only way to avoid them completely is not to build on land where a sinkhole is likely to happen,” he said.
According to Weary, the key is to know where subsurface voids are located so they can be avoided or remedied prior to construction. Areas with known natural sinkhole risk, such as Florida, Texas and Louisiana, may require a geotechnical survey of properties using exploratory drilling or geophysical methods to characterize the subsurface of a potential building site.
Hayward Baker Geotechnical Construction, a company that works in Florida, Georgia, Texas, North Carolina and Tennessee, offers a different approach: dynamic compaction. In this process, heavy weights are dropped across an area to try and collapse any hidden voids that might be there prior to building; however, Weary said the USGS does not have any data showing whether or not the process works well in the long term.
Sinkholes are very costly to cities and states, causing millions of dollars in damage to buildings and roads across the country. While there are companies that specialize in sinkhole repair and remediation, Weary said it’s important for municipalities to address the root cause of the problem or run the risk of repeating history.
“If an area has the right geology and already has some sinkholes, it is likely that more will open up nearby in the future,” he said.
