Caving in Sinkholes and their Formation

Caving in Sinkholes and their Formation

Florida is famous for sunshine and beaches, hurricanes, and alligators. However, it is also known for something less attractive: sinkholes. A sinkhole is a depression or hole in the ground caused by the natural collapse of a surface layer. According to Florida Office of Insurance Regulation, Florida has more sinkholes than any other state in the United States.  Three counties in the Tampa region are even known as the “sinkhole alley” – Hernando, Hillsborough, and Pasco counties. To understand the occurrence of sinkholes in Florida, one must first understand how they appear.

To answer this question, one must first understand the geological structure of Florida. The map on the left, from the U.S. Geological Survey, shows the variable water-soluble rock types underlying different areas of the United States, where sinkholes could potentially occur. The size of the sinkhole typically varies by different soil types. The Florida peninsula, for example, is composed of porous carbonate rocks such as limestone, and dirt, sand, and clay sit on top of the carbonate rock. Over time, the soluble bedrock is gradually removed by erosion from the water below the surface and the saturation of acidic rainfall. This creates voids under the limestone roof. When the material at the surface becomes too heavy for the roof, it collapses and forms a sinkhole. Although sinkholes are formed naturally, they can still be easily triggered by sudden heavy rainfall, droughts followed by storms, and human activity. The most common human causes are pumping groundwater from reservoirs, creating landfills, and leaking water pipes.


Solution sinkholes
These sinkholes form slowly when the bedrock of limestone is exposed at the surface level. The acidic rain directly drops on to the ground surface and creates a depression through saturation and erosion of the bedrock.

Cover-subsidence sinkholes
This type of sinkhole also forms slowly with sand on top of the bedrock. When sand gradually slides down into the openings in the bedrock and lowers the land surfaces, depression starts to grow and eventually creates a sinkhole.

These sinkholes involve bedrock covered by substances such as dirt, clay or sand on the top. As the bottom layer of limestone is gradually eroded by moving water, the size of the cavern underneath the surface expands. Until only a thin layer remains between the underground cavern and the surface of the ground, the sinkhole forms rapidly as the thin layer falls in and swallows everything above it.

Fortunately, while thousands of sinkholes develop every year in Florida, only a few dozens of them swallow up buildings. In fact, as Anthony Randazzo, the founder of Geohazards, stated in his research on sinkholes in Florida, there have only been two sinkhole deaths so far  Only one tragedy has happened so far in Tampa, where a sleeping man was swallowed up when a sinkhole opened up under his house. “Usually you have some time,” said Randazzo. “These catastrophic sinkholes give you some warning over the course of hours.” Fatal incidents like this happen rarely, he reassures.

But why are sinkholes happening so frequently?

As record by the state Office of Insurance regulation shows, the incidents have been steadily increasing over the past decade. Although groundwater pumping is the main reason for depression and is sometimes the cause of sinkholes, there is currently no direct evidence proving that human activities are directly related to the rising number of sinkhole incidents. It is also important to remember the timescale of many geologic events spans thousands of years, indicating the majority of sinkholes started forming long before negative human intervention in the environment. Sinkholes are primarily natural phenomena, which humans can only occasionally influence.


In addition, if we compare the sinkhole map, groundwater depletion map, and the map of bedrock types, sinkholes are more strongly associated with the type of bedrock, rather than the severity of groundwater pumping. Especially in the state of Florida, there is almost no sign of water pumping activity, yet it is still the state with the highest number of sinkhole incidents.

Since humans have so little control over the occurrence of sinkholes, how can we prepare ourselves in order to both prevent and provide a solution to them? First, there are several ways to spot a developing sinkhole. According to the Southwest Florida Water Management District, slumping or sagging fences or trees are the most obvious sign. Doors and windows that do not close properly and small pools of rainwater forming where water has not collected before are also signs to be aware of. In addition, a newly developed remote sensing technology, the Interferometric Synthetic Aperture Radar (InSAR), utilizes satellites and drones to detect changes in ground elevations over time. The subsidence of the ground indicates the area may be vulnerable to sinkholes. Reducing activities such as drilling holes into the ground and groundwater pumping can also relieve the extra pressure on surface which may trigger sinkholes.

There are many companies that work to prevent sinkholes from occurring. Generally, these companies inject grout into the area to fill the cracks or voids underground in order to stabilize the foundation. Although sinkholes can be dangerous and devastating, if you pay attention to the signs of a sinkhole forming around home or neighborhood, you can take action before a larger sinkhole swallows up your property. Florida and Tennessee mandate sinkhole insurance, however, insurance companies outside of those states may offer a sinkhole coverage policy, as well.

An interactive map from the Department of Environmental Protection shows the locations of past subsidence incidents, swallets, and the types of sinkholes in the state of Florida. Click me.

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