SINKHOLE DETECTION IN FLORIDA USING GPR AND CPT

A portion of Florida's abundant annual precipitation percolates downward through the soil overburden into fissures and cavities in the underlying limestone, often carrying soil with it. This "sinkhole " activity affects transportation facilities, as well as many other public, residential, and commercial structures. To minimize foundation design costs, engineers usually perform a site investigation with only a small number of borings or penetration soundings, and seldom use geophysical methods. The reluctance to use geophysical techniques results partly from the lack of measured engineering properties, and partly from previous negative experience with cumbersome equipment and difficult interpretation. Improved equipment and analysis procedures now provide cost-effective geophysical tools to supplement a site investigation. Many soil profiles in Florida include distinct clay and limestone contacts, which provide good contrast for a Ground Penetrating Radar (GPR) survey. Depressions in these contacts may indicate zones of subsidence, which the engineer can check with Cone Penetrometer Test (CPT) soundings. The CPT provides results superior to Standard Penetration Test (SPT) borings because of better reproducibility and more definitive soil property correlations. Examples illustrate the combined use of CPT soundings and GPR surveys. Both methods are quick, relatively simple to interpret, and essentially non-destructive