How Do Sinkholes Form
Sinkholes are naturally occurring in Florida, and they can occur throughout the state. Limestone underlies soils throughout numerous parts of Florida. Limestone is alkaline, and as rainwater, which is somewhat acidic, percolates downward through the ground, it causes the limestone to slowly dissolve over millions of years.
As the limestone dissolves, voids form which are often filled with water - almost like "bubbles" of water in the limestone. Slowly gravity pulls the layers of soil downward into the voids, a process called raveling.
In a solution sinkhole, a depression slowly forms in the land as the soils migrate, or ravel, downward into the voids left by the dissolving limestone. This raveling may cause a slight, sometimes barely visible, indentation in the soils above. See the image below in Graphic B. Sometimes this may occur gradually over years, other times the soils may shift in a matter of months depending on rainfall or lack of rainfall. Solution sinkholes are very common throughout Florida, and rarely lead to a catastrophic collapse. Often these sinkholes cause the ground to shift slightly, which places stress on concrete foundation slabs – one part of the concrete slab is shifting; the other part remains in place. This stress causes differential settlement to occur at the structure, and often manifests itself through cracking in the home.
Collapse sinkholes are often the ones seen on the news. They tend to occur suddenly and can be very catastrophic, sometimes swallowing cars, homes, and buildings. Frequently collapse sinkholes are triggered by extreme fluctuations in the water tables. For instance, in graphic A above, the voids in the limestone are below the water table, and the voids are thus filled with water. The water supports the walls and ceiling of the void, in a sense keeping the above surface from collapsing downward. But if the water table suddenly drops quickly to the point where there is not enough pressure to support the sides and ceiling of the void, a collapse can occur. See Graphic C below – the above soils have quickly collapsed into the void.
Collapse sinkholes are often on the news, and often swallow surface objects.
Additional information on sinkholes in Florida can be obtained from the Florida Department of Environmental Protection at http://www.dep.state.fl.us/geology/geologictopics/sinkhole.htm