A Geologic Framework for Evaluating West-Central Florida Sinkholes

Sinkhole activity is a covered loss under Florida's current regulatory statutes regarding property insurance, and several thousand subsidence investigations are completed annually for homeowners with cracking distress and other possible indicators of sinkhole activity. The majority of these insurance claims are located in Florida's gulf coast communities, north of and including Tampa. Here, the limestone is shallow and is overlain by sedimentary and wind-blown sand and clay layers. In addition, there is often, but not always, a vertical downward gradient from the surficial aquifer to Floridan aquifer. In Florida's geologic past, before the sand and clay layers were laid down, changes in the ocean level caused weathering of the limestone surface, creating an uneven surface where subsequent infills of loose sands and soft clays have still not consolidated. This has created subsurface conditions that appear to be sinkholes, but are not. Sinkhole formation is chiefly the result of raveling of overlying granular soils into voids, cavities and caves within the limestone. Proper analysis of these cases requires the geotechnical engineer to understand the regional geology, and the geologist to understand the engineering conditions under which damage can occur to a structure. The case studies presented allow the reader to distinguish the subtleties between general subsidence and subsidence due to true sinkhole activities.