Sinkholes

Perhaps some people have an exaggerated fear of sinkholes. The thought of the foundation falling out from under our feet is terrifying when the seemingly stable ground is suddenly rendered unstable. There is something highly unnerving about the ground giving way into a gaping void that can swallow trees, cars, structures, and even people. How frightened should you be of sinkholes? While they can be catastrophic, they are not entirely unpredictable. But the landscape of sinkholes is complicated and deserves attention. 

There have been horrible sinkhole incidents that resulted in loss of life and acres falling into a chasm that wasn’t there yesterday. But sinkholes are widely misunderstood. They are also somewhat predictable which means you can avoid putting homes and buildings where a sinkhole collapse is a high probability. Because I have some measure of expertise in karst (terrain characterized by sinkholes) in the eastern U.S. over the past 20 years, let me try to provide a primer about sinkhole hazards so you can perhaps regain a bit more control over any anxiety.

Ok, I know this doesn’t help but, yes, people die from sinkholes. Yet, it is rare that this happens. Collapse is preceded by subsidence, cracking, and slumping. Usually, an imminent collapse is noticed. But there have been occasions where the collapse has been sudden due to human factors that masked the collapse or accelerated it.

First, what is a sinkhole? It seems pointless to explain this in detail as it’s all over the web. A good place to head to is How Stuff Works. It’s also important to know about collapses that are called “sinkholes” but don’t fit the narrower scientific definition. While there are outliers, generally, when geologists refer to sinkholes, they mean those that are a result of the surface expression of pre-existing spaces in the underlying rock – in other words, a karst process. That is, the rock below is not solid through and through, it has holes and pathways that make it secondarily highly porous. These spaces are often full of water or clogged with soft material like clay sediments or soil. When the subsurface voids are flushed, the water drained, or material is mobilized (by water) down there, then you have a potential for that void space to eat its way up to the surface. Limestone, dolomite, and gypsum bedrock are often prone to manifest surface sinkholes.

Florida is the top state for sinkholes due to the geological situation – karst bedrock and sandy soils, along with high rainfall and urbanization, is a recipe for disaster.

Land can also collapse in a similar fashion as the result of more modern processes that mimic karst processes. In fact, many of the huge catastrophes you see in “Top 10” sinkhole lists are from these processes. Sinkhole formation related to salt deposits (which is water-soluble) can be natural like those happening along the Dead Sea in Israel or the result of the misuse of salt dome caverns with Bayou Corne sinkhole being a dramatic example. Subsidence effects from abandoned or active underground mines are also called “sinkholes” but they are not “true” karst sinkholes.

In residential and urban areas, collapses also called “sinkholes” are frequently related to leaking or collapsed pipes, poor drainage, or settling of unstable deposits. In these cases, karst bedrock is not necessary. Sometimes several factors related to drainage, deposits and increased water flow are involved. Roads, vehicles, and houses can be affected by these types of ground movement. Septic tanks and drain fields also can collapse. Huge sinkholes in Guatemala are featured in “Top 10” lists of disasters. But these gasp-inducing holes are the result of poor urban planning and movement of loose volcanic material, not karst bedrock. While they may be very scary, sinkholes are typically more of a nuisance. Knowing the general cause of a sinkhole is important in assessing if the hazard is widespread or rare and what can be done about it.

Although an underground cave or cavern can collapse, most karst-related sinkholes are the result of the collapse of the soil-like cover material and called, logically, “cover collapse” sinkholes. These can happen slowly, reflected as depressions in the ground surface, or suddenly as the surface material can no longer span the void underneath and give way. Formation of these kinds of sinks are greatly accelerated by any kind of surface disturbance, concentration and redirection of stormwater runoff, injection of wastewater, and groundwater withdrawal.

A small opening in the bedrock surface can result in a much larger conical “hole” on the surface. These holes are often filled but they remain active and keep sinking as water into the depression moves the material through the underground piping system. If the material has some place to go, water will transport it.

Locals used these depressions as convenient landfills, dumping garbage, scrap equipment, appliances and even dead animals into them. This brings up the lesser-known hazard of sinkholes – groundwater contamination. The holes drain directly into the groundwater system which will most often have a zone of very rapid movement, spreading a plume of contaminants quickly without filtering to streams and potentially to drinking water supplies.

Karst is a complicated topic. Trained geologists and geotechnical engineers, who are experts in karst formation and remediation of sinkholes, admit that it is tough to work in these challenging conditions and manage the sinkhole risk. Sinkholes are difficult to fix permanently and where there is one sinkhole, there are typically more. Fixing one means the water blocked off there will find a way around and can cause a problem someplace else. Special conditions in karst aquifers present various difficulties, increased risk, and additional expense if proper characterization is not done.

Sinkhole prone areas are not hidden or secret, but sometimes they are ignored to our peril. Geological maps of the area will show known sinkholes. Old aerial photographs regularly reveal sinkholes that are easy to spot if you know what to look for. Therefore, sinkhole formation is not entirely unpredictable if you consider the rock type and take a peek into historical evidence. It’s pretty spooky is when old maps reveal giant holes in an area that is later covered by houses, roads, and commercial structures.

Just because it’s filled or paved over doesn’t mean it should be forgotten. It’s not easy to choke a big sinkhole and stop the movement. There are hundreds of examples yearly in the US alone that reveal inattention to a problem that could be managed much better. Now, that’s scary.

Visit this page periodically to see additional spooky info relating to sinkholes.

Death of Arkansas man in a river swallet hole
64-year-old Donald Wright of Searcy died on June 9 near Saddler Falls, Arkansas when he attempted to help boaters who were caught in swirling water of the Spring River where a sinkhole had opened in the riverbed. He received fatal injuries when his kayak overturned. (Washington Post, June 11, 2018)