By Dawn Hewitt
Southern Indiana is pockmarked with sinkholes, characteristic of the karst geology beneath us: dissolvable limestone bedrock.
A few weeks ago, a sinkhole suddenly opened under the bedroom of a house in Florida, and within a few seconds, sucked a man sleeping in his bed into the earth, apparently killing him.
Could it happen here?
Not likely, says Indiana Geological Survey research associate Richard Powell.
“There are some geologic and hydrologic differences between the karst and cavernous regions of Indiana and Florida,” Powell said via email.
According to the U.S. Geological Survey, the most damaging sinkholes in the country tend to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania. Not Indiana.
The USGS describes Florida’s karst as limestone “swiss cheese,” topped by a thin layer of sand. In Florida, karst limestone is near the surface, and so is the groundwater, which is used for agricultural irrigation. When the level of the water table changes, caverns that were full of water can become underground voids.
The sudden sinkholes that have infamously opened in Florida and Guatemala — where, in 2011, a 60-foot wide, 200 feet deep sinkhole swallowed a three-story building, killing 15 people — occur where the groundwater drains “through a semiconsolidated or poorly cemented sand formation overlying a cavernous limestone,” Powell said.
Sinkholes usually form after heavy rains, when subterranean drainage systems change. When the water table is close to the surface, then drops, it can take with it whatever is above it.
“Areas where water levels have lowered suddenly are more prone to collapse formation,” the USGS says.
“The sinkhole [karst] areas in Indiana are developed on well cemented, dense limestone formations,” Powell said. Indiana’s karst, in most areas, is covered with thick clay, and the groundwater is deep below the surface.
“In areas where cover material is thicker or sediments contain more clay, cover-subsidence sinkholes are relatively uncommon, are smaller, and may go undetected for long periods,” according to a USGS fact sheet.
The limestone here is construction-grade. While it does dissolve — as the abundance of local caves proves — the process is very slow. While the ceiling of a cave can suddenly collapse and cause a sinkhole above it, in Indiana, there are no known cases of sinkholes suddenly opening up and sucking buildings into it.
Human activity can cause sinkholes, too, if it causes changes in groundwater.
“Sinkhole formation is more prevalent where groundwater tables fluctuate,” according to the Monroe County Comprehensive Plan. “Fluctuation occurs in and around mining and quarrying operations and where surface water drainage is changed by construction.”
Aging underground infrastructure, such as sewer drains, water mains or other structures that collapse, can cause sinkholes, too.
“While collapses are more frequent after intense rainstorms, there is some evidence that droughts play a role as well,” according to the USGS fact sheet.
A sinkhole developed under a runway at the Monroe County airport in 2011 that will cost millions of dollars to repair, but airport manager Bruce Payton said the problem was first recognized while the runway was being built in the 1960s. Engineers did their best to repair the hole at the time, but in 1989 or ’90, the ground sank again, and was repaired again.
“Technology keeps improving,” Payton said regarding current plans to address the sinkhole under the runway that will allow groundwater water to flow without affecting the soil and asphalt above it.
Similarly, a sinkhole could develop under a Bloomington area home or business. It could cause serious structural problems, but it could take months or years to develop, giving the property owner time to repair the problem.