Sinkhole activities, including ground subsidence or collapse, are major geologic hazards. In Florida alone, hundreds to thousands of subsidence cases are reported each year. Most occur in the covered karst of central Florida (Figure 1), where large anthropogenic changes in groundwater levels have increased the frequency of sinkhole activities (Tihansky, 1999). The main hazard induced by sinkhole activity is property damage. In August 2013 a sinkhole collapse destroyed a resort complex near Disney World. More minor events inflict a significant cumulative toll: the Florida insurance record for 2006-2010 indicates sinkhole related claims with a total indemnity of about $200 million per year (Florida Senate report, 2010). Sinkhole collapse can also cause a loss of life, as occurred on March 1st, 2013, when a resident of Seffner was "swallowed" by a sinkhole that opened beneath his bedroom.

Figure 1: Florida sinkholes. (a) Map of reported sinkholes since 1950. (b) Sinkholes in Hillsborough County and the town of Seffner. (c) The collapsed house in Seffner, where a resident lost his life, on March 1st, 2013. (d) A 'classical' house collapse in Central Florida.

Monitoring sinkhole activities is a challenging task, because most of the activity occurs in the subsurface. Geophysical imaging techniques, as Ground Penetrating Radar (GPR), provide very useful images of the subsurface, but are practical only over small areas (hundreds of square meters) and cannot image the entire sinkhole active zone (thousands of square kilometers).

Sinkholes activity in central Florida

Sinkholes develop above dissolution cavities in Florida's thick carbonate deposits. The cavities have developed over geological time scales by dissolution processes, mostly during low sea level condition, which exposed the Florida peninsula to karst processes. The dissolution cavities serve as important conduits for groundwater movement. Newly developed sinkholes occur mainly in western-central Florida (Figure 2a), because the geological and hydrological conditions in the area enable rapid sinkhole growth. In this area a 10-60 m thick layer of clay and/or sand covers the carbonate deposits (Tihansky, 1999). The surface response to the limestone cavity depends on the properties of the cover sediments. When the cover consists mostly of sandy deposits, its cohesion is low, resulting in downward granular flow, or piping, into fractures and voids. Slow subsidence results in gentle depressions (Figure 2).

Figure 2: Cover-subsidence sinkholes tend to develop gradually where the covering sediments are permeable and contain sand. (Tihansky, 1999).

However, when the overlying layer consists mainly of cohesive clay deposits, the top of the roof erodes upward as sediments spall into the cavity (Figure 3) (Tihansky, 1999). The thinning coherent roof could theoretically also subside. Two mechanisms for advancing subsidence include decreasing flexural strength, as the roof thins, and poroelastic response to groundwater draining through the void. Eventually the void expands to the surface, resulting in abrupt surface collapse.

Figure 3: Cover-collapse sinkholes may collapse abruptly (over a period of hours) and cause catastrophic damages. They occur where the covering sediments contain a significant amount of clay. (Tihansky, 1999).

Both types of sinkholes can become major geological hazards, as they can cause severe damage to property and latter even loss of life, as mentioned above.