Driving through Hall County, you probably don’t expect a huge hole to suddenly open up and swallow your car. But Florida residents are familiar with this scary phenomenon. It also happened in Chicago 10 days ago when the pavement buckled and three automobiles went down 10 feet into a gaping sinkhole.
Ground subsidence of this kind is common where one or both of two factors are present: A deep layer of soft soil, and limestone as the underlying bedrock.
Limestone is water soluble and produces caves. In Florida, they are below the groundwater table, producing a network of underground rivers and lakes. When a city expands, water is withdrawn for municipal use. If the level in a cave drops significantly, the cave roof can lose what little support it had from the water, and collapse within minutes or even seconds.
In Winter Park, Fla., you can visit such a sinkhole at the intersection of West Fairbanks Avenue. and South Denning Drive. Local resident Mae R. Williams noticed a tree disappearing in her yard on May 8, 1981, and was evacuated with her family before the hole swallowed her house. Parts of South Denning drive, a car dealership with several Porsches and other buildings also went down the drain.
This was the largest case of ground subsidence caused by geological conditions in the U.S. It served as a warning against excessive withdrawal of groundwater in unstable bedrock, especially during times of drought.
This past March, another Florida sinkhole produced tragic results when local resident Jeff Bush, sleeping in his bed, was swept into a submerged cave in suburban Tampa and disappeared in the watery darkness.
Calamities of this kind are unlikely in Hall County, where the bedrock is massive and the soil is relatively thin. Caution is advised in the Dahlonega area, though; 19th century gold mining has left old tunnels and mine shafts that don’t necessarily show up on maps.
Subsidence of the Florida type can happen in South Georgia. In the coastal plain, young limestone is overlain by thick layers of sand. Urban planners, builders and developers need to carefully consider subsidence risks before placing homes and businesses.
Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His column appears Sundays and at gainesvilletimes.com.