Florida Sinkholes: Science and Policy

6. Detection and Mapping

6 Detection and Mapping People are naturally afraid of sinkholes. This is partly a result of the uncertainties surrounding sinkhole insurance and sinkhole policy. For a few people, though, this fear borders on “sinkhole phobia.” They are terrified the earth will swallow their home whole, sucking loved ones and possessions into the underworld after it. While catastrophic sinkholes do happen from time to time, sometimes with tragic consequences, most sinkholes are far from being Dante-esque events. In fact, it is likely that most sinkholes in Florida go unreported due to the subtle nature of their expression, and when homes are affected by sinkholes, the damage is typically not of a magnitude that should induce dread about natural earth processes. Where are sinkholes located? How are they mapped? These seem straightforward enough questions, yet difficulties are involved with answering both for a number of reasons. First, because the processes leading up to sinkhole formation are located underground—typically as the expansion of void spaces formed from the solution of limestone—detection isn’t always possible without special equipment. Similarly, detection of the causes of any particular property damage is costly due to the high-tech nature of the methods employed in the assessment process. Second, sinkhole mapping is problematic because sinkholes’ topographic expression varies tremendously. Some sinkholes are quite deep and fall readily within contour intervals used in traditional mapping techniques while many have minimal relief and are thus not detectable using common topographic maps. Another problem with mapping sinkholes is that their size varies a great deal, from very large ones that show up clearly on commonly available maps to very small ones that aren’t even identified on maps. Of course, due to the fact that sinkhole formation is a common phenomenon in the state, it is difficult to keep maps up to date. Indeed, as noted above, most sinkholes are probably not reported, and there are benefits to not reporting sinkhole occurrences near or under homes. Detection and Mapping 135 Yet there is still a clear need for a better understanding of the location of sinkholes and the voids that cause them. Also, there is keen interest in mapping sinkholes in the state. This chapter reviews how sinkholes are detected in the field, summarizes the way sinkholes are mapped using both traditional mapping techniques and remotely sensed data, and concludes with a summary of the status of sinkhole mapping in Florida. Sinkhole and Void Detection Sinkhole and void detection techniques are used as research tools by karst scientists to better understand sinkhole formation and by geotechnical firms to assess claims made by homeowners of sinkhole damage. The techniques include simple field observation to detect voids, ground-penetrating radar, and drilling. Field Observation There are a number of potential causes of land subsidence in Florida. For example, buried organic deposits can oxidize to cause the land surface to shrink. This can occur when buried peat deposits oxidize due to the changing nature of aquifer systems. There has been some degree of concern about this phenomenon as regional aquifer levels began to decline in the late 1990s due to the pumping of aquifers and a severe drought. Another cause of subsidence induced by oxidized organic matter results from poor preparation of construction sites. Sometimes in the site preparation process, downed trees, tree stumps, or wood construction debris is left on the sites where homes will be built. When this occurs, the buried organic matter oxidizes over time, leaving a void space into which part of the foundation of the home can slip or collapse. Another cause of land subsidence is the action of shrink-swell clays. These clays, typically within the montmorillonite family , shrink and swell as they get wet and then dry. The mineralogy of these clays makes them suitable for embracing water in the crystalline structure during wet times of year. During dry periods, the water is shed from the mineral structure, causing the clays to decrease in size. Some of these clays can increase or decrease in size significantly. When they are located within an aquifer, there is little problem because the clays are permanently wet and thus of constant size. If the clays lie within a zone of wetting and drying , however, or, if due to regional groundwater withdrawals they become Florida Sinkholes: Science and Policy 136 aerated, the ground will subside, particularly if the deposit is thick or regional in extent. In...