As I write this article there is an ongoing rescue onsite of a condo high rise in Miami Florida that collapsed due to a Sinkhole. More than 150 people were unaccounted for after a high-rise condo partially collapsed near Miami Beach, leaving at least four people dead and 11 injured, officials said. We hear about Sinkholes happening in Florida but where else do Sinkholes occur?
According to the USGS, 20% of U.S. & 13% of the world surfaces, where Sinkholes occur contain land formations called “karst terrain.” where rock below the land surface is made of limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by groundwater circulating through them.
Sinkholes are not limited to Florida and according to the USGS, about 20 percent of U.S. land is susceptible to Sinkholes. The most damage from Sinkholes tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania. Sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by groundwater circulating through them.
What Causes Sinkholes
The most common causes of a sinkhole are changes in groundwater levels or a sudden increase in surface water. A natural sinkhole typically occurs when acidic rainwater seeps down through the surface soil and sediment until it reaches a soluble bedrock like salt, limestone, or sandstone.
According to The Department of Environmental Protection:
- Water dissolved minerals in the rock, leaving residue and open spaces within the rock. (This is called “weathering”.)
- Water washes away the soil and residue from the voids in the rock.
- Lowering of groundwater levels can cause a loss of support for the soft material in the rock spaces that can lead to collapse.
- Changing groundwater gradients (due to removing or introducing water to the system) can cause loose material to flush out quicker from the voids and the surface to collapse in response.
- Any change to the hydrologic system (putting more water in or taking it out) causes the system to become at least temporarily unstable and can lead to sinkholes.
- Sinkholes can result from seasonal changes in the groundwater table, freeze and thaw of the ground, and extremes in precipitation (drought vs heavy rain).
What Are the 3 Types of Sinkholes
There are three types of natural sinkholes:
Dissolution sinkholes-Dissolution of the limestone or dolomite is most intensive where the water first contacts the rock surface. Aggressive dissolution also occurs where flow is focused in preexisting openings in the rock, such as along joints, fractures, and bedding planes, and in the zone of water-table fluctuation where groundwater is in contact with the atmosphere.
Rainfall and surface water percolate through joints in the limestone. Dissolved carbonate rock is carried away from the surface and a small depression gradually forms. On exposed carbonate surfaces, a depression may focus surface drainage, accelerating the dissolution process. Debris carried into the developing sinkhole may plug the outflow, ponding water and creating wetlands Gently rolling hills and shallow depressions caused by solution sinkholes are common topographic features throughout much of Florida.
Cover-subsidence sinkholes
Cover-subsidence sinkholes tend to develop gradually where the covering sediments are permeable and contain sand. In areas where cover material is thicker, or sediments contain more clay, cover-subsidence sinkholes are relatively uncommon, are smaller, and may go undetected for long periods.
- Granular sediments spall into secondary openings in the underlying carbonate rocks.
- A column of overlying sediments settles into the vacated spaces (a process termed “piping”).
- Dissolution and infilling continue, forming a noticeable depression in the land surface.
- The slow downward erosion eventually forms small surface depressions 1 inch to several feet in depth and diameter. In areas where cover material is thicker, or sediments contain more clay, cover-subsidence sinkholes are relatively uncommon, are smaller, and may go undetected for long periods.
Cover-collapse sinkholes-
Cover-collapse sinkholes may develop abruptly (over a period of hours) and cause catastrophic damages. They occur where the covering sediments contain a significant amount of clay. Over time, surface drainage, erosion, and deposition of sinkhole into a shallower bowl-shaped depression.
Over time, surface drainage, erosion, and deposition of sediment transform the steep-walled sinkhole into a shallower bowl-shaped depression.
- Sediments spall into a cavity
- As spalling continues, the cohesive covering sediments form a structural arch.
- The cavity migrates upward by progressive roof collapse.
- The cavity eventually breaches the ground surface, creating sudden and dramatic sinkholes.
Sinkholes Human Induced-karst landforms which are a type of landscapes necessary to develop a sinkhole can happen naturally through the weathering process so a 
sinkhole can be considered a natural occurrence. But, human influence causes sinkholes to occur where they might not naturally have happened. Or, they may occur sooner or more abruptly than under natural conditions. Other conditions that can be considered man-made as reasons or prerequisites that help develop Sinkholes are:
- The decline of water levels – drought, groundwater pumping (wells, quarries, mines)
- Disturbance of the soil – digging through soil layers, soil removal, drilling
- Point-source of water – leaking water/sewer pipes, injection of water
- The concentration of water flow – stormwater drains, swales, etc.
- Water impoundments – basins, ponds, dams
- Heavy loads on the surface – structures, equipment
Sinkholes in Florida
About 20% of the United States and 13% of the world are susceptible to Sinkhole occurrences but they are common in the state of Florida because of the many types of Karst landforms that include underground streams. disappearing streams and underground drainage systems drainage that exist all around the state of Florida. It’s these types of characteristics that make them typical in this region of the country.
Karst is a generic term that refers to the characteristic terrain produced by erosional processes associated with the chemical weathering and dissolution of limestone or dolomite, which happen to be the two most common carbonate rocks in Florida. The dissolution of carbonate rocks begins when they are exposed to acidic water. Most rainwater is slightly acidic and usually becomes more acidic as it moves through decaying plant debris.
Limestones in Florida are porous, allowing the acidic water to percolate through their strata, dissolving some limestone and carrying it away in solution. Over enough time, this ongoing erosional process has created extensive underground voids and drainage systems in much of the carbonate rocks throughout the state.
Data was compiled by the Florida Geological Survey on the number of Sinkholes in Florida that database collected information on approximately 1900 Sinkholes located in Florida was compiled in 1995 for the Florida Sinkhole Research Institute. The Data is made available in the Florida Sinkhole Index.
Another great source for information on Sinkholes or suspected sinkholes is called the Subsidence Incident Report Map which shows in map live from data areas that are reported to the state or country and the types and sizes of sinkholes that are reported and being studied.
What are the Warning Signs of a Sinkhole
In most cases, Sinkholes don’t form suddenly but develop over time. They build up over months and even over years. There are signals for property owners to look out for. Limestone areas in a tropical, moist climate are much more likely to form sinkholes than those in dry climates. So if you are living in those regions of the world, you could be living on top of one or in the vicinity.
- Fresh cracks in the foundations of houses and buildings
- Cracks in interior walls
- Cracks in the ground outside
- Depressions in the ground
- Trees or fence posts that tilt or fall
- Doors or windows become difficult to open or close
- Rapid appearance of a hole in the ground
If you’re looking for signs of a sinkhole below a building, the University of Florida recommends
- Being aware of structural cracks in walls and floors, cloudy well water and doors and windows that won’t close properly.
- On the ground, there are likely to be more signs, including wilting or dying vegetation, previously buried things — like fence posts, roots or structural foundations — becoming visible, the formation of new and small ponds and slumping trees and fences.
- Should a sinkhole occur near you, the Southwest Florida Water Management District recommends evacuating the premises and then notifying your insurance agency and the city.
All histories of Sinkholes should be listed on any properties up for sale in any state and there are Engineers that are trained to test ground for loose soil in areas where some properties are at risk. If you are buying property in areas where there is a history of Sinkholes you can have the ground around your house inspected by a qualified Engineer that can certify it for personal and insurance purposes.
References: The Florida Department of Environmental Protection
The University of Florida-How to spot a sinkhole
