David Harro, P.G., reports that sinkholes in Florida are a major concern to engineering and environmental studies. The use of geophysics for the identification of sinkholes in Florida have focused on two types of geophysics, Ground Penetrating Radar and Multi Electrode Electrical Resistivity. Applications of GPR and MER have been successful for areas where the cover material over the limestone is less than 30 feet. However, in much of the state this material can reach to depth. Sites with limited distances can limit MER significantly leaving only GPR. In most cases GPR and MER do not reach the limestone where sinkhole activity occurs or identify the sinkhole throat. Two projects were focused on the identification of sinkholes. The FDOT was performed on a section of alignment of highway where relic sinkhole conditions were identified. A total of 26 test borings comprised of Standard Penetration Test (SPT) and Cone Penetrometer (CPT) were conducted to identify the relic sinkhole. G3 conducted a MERIT survey consisting of 28 surface and 28 implanted electrodes to 48 forty-eight feet.
The results from the MERIT survey show the geometry of the relic sinkhole feature to 170 feet and identified the throat. A depression caused by a sinkhole and infilled by surface sands, shown in red can clearly be seen reaching 70 feet. Corresponding GPR survey performed over this area penetrated 25 feet. The second project shows the results of the MERIT survey performed at a former gas station site. Installation of a deep monitor well identified a void area. The project site lies within an area that is prone to sinkhole activity. A GPR survey was performed and reached a depth of 25 feet and did not identify sinkhole anomaly. The result of the MERIT survey shows the upper sands infiltrating into the lower clay unit and reaching the top of limestone at 55 feet bgs identifying the location of the sinkhole throat.
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