After a Florida man was killed in February when a 20-foot-wide sinkhole opened under his bedroom, many people wondered, Could that happen to me?
Thousands of sinkholes appear in the US each year—but sinkhole deaths are very rare. Only two other people are known to have been killed by sinkholes in the past 40 years in the US. Both were in Florida, and both people were operating heavy well-drilling equipment at the time. Serious injuries are rare, too, though minor injuries such as twisted ankles from small, overlooked holes do occur. In March, a man playing golf suffered a dislocated shoulder after he fell into an 18-foot-deep sinkhole on a golf course near Waterloo, Illinois.
While the risk to life is low, the risk to property can be considerable. What you need to know…
Sinkholes typically form when a relatively soft underground rock or bedrock—usually limestone but sometimes salt or gypsum—is eroded by water over time. This usually causes a gradual “subsidence” in the ground above the eroding bedrock as a bowl-shaped depression slowly forms. The process is called cover subsidence, and it’s the likely result when a sinkhole forms in an area with sandy soil. The process can take many years.
Occasionally the ground above eroding limestone remains almost perfectly in place before suddenly giving way, resulting in a cover collapse sinkhole such as the fatal one in Florida. This is particularly likely when the sediment overlying bedrock consists mainly of clay. Fortunately, even these sinkholes typically give way over a period of hours, not seconds, as the surface topples into the sinkhole below, providing enough time to get clear. When this happens beneath a home, there usually are signs of the problems—cracking in the concrete of the foundation or exterior walls, for example—before the floor falls away.
Florida is the sinkhole capital of the country because limestone is the predominant bedrock for most of the state. But other states have sinkhole potential, too. Limestone outcroppings can cause sinkholes in parts of Alabama, Kentucky, Missouri, Pennsylvania, Tennessee, Texas and elsewhere. “Evaporative rock” such as salt or gypsum is found under 35% to 40% of the country, according to the US Geological Survey, although much of this is at great depth, reducing the sinkhole potential. (Click here for a map of affected regions.)
Sinkholes can occur just about anywhere else, too, when a leaking underground water pipe erodes soil…or when wooden construction debris or tree stumps are buried beneath soil and slowly rot away…or when mining activity has occurred beneath the land.
Example: The collapse of a century-old mine caused a 30-foot-wide sinkhole under a Springfield, Missouri, neighborhood earlier this year, endangering two homes. Sinkholes can happen at any time, but they’re more likely when the groundwater table or hydrostatic pressure changes rapidly, such as when there’s heavy rainfall after a dry spell or a new water well is put into service.
Watch for the warning signs that a sinkhole is developing underneath your home or elsewhere on your property.
Possible signs include windows and doors that suddenly become difficult to open…rapidly expanding foundation cracks that are wider than hairline cracks (wide cracks that form a “stair step” pattern are particularly common with sinkholes)…or cracks in a section of the basement floor. However, these symptoms might instead mean that the home is simply settling.
Early signs of a sinkhole under your lawn or landscaping could include water pooling where it previously didn’t after rains…or trees or fence posts that suddenly start to lean. However, leaning trees and fence posts could instead be caused by heavy rains or high winds, among other causes.
If the sinkhole is less than a few feet across and less than a few feet deep and not under or immediately adjacent to your foundation (or your swimming pool or some other valuable structure), just fill it with sand or soil or have someone do this for you. If you discover rotting wood or other organic debris in the hole, clear this out first. Compact the new soil, and watch for further subsidence before laying down new sod.
If the sinkhole continues to expand after you’ve filled it, consider pouring concrete into the hole. This sometimes can act as a plug with a small sinkhole, preventing further problems.
If the sinkhole is larger than a few feet across or deep, however, it’s best not to try to fill it yourself—it might not be safe to stand near the hole. Rope off the area using wood stakes and yellow or orange warning tape to prevent accidents, and call in water, septic or sinkhole professionals.
If the sinkhole is near underground water pipes, call the local water utility. If you see a depression where you know water or sewer pipes run, or if you can see a pipe in the depression, it is best to call. Leaking water eroding the soil could very well be the cause of the hole—leaking water is the most common cause of sinkholes outside Florida. If the leak is coming from a municipally owned water pipe, the utility company should pay to fix the problem. If the sinkhole is in the vicinity of your septic tank, call in a septic system specialist to see if water from that is causing the issue.
If leaking water seems not to be the problem, then call in a professional geologist or engineer experienced with sinkholes. In Florida, these sinkhole-remediation professionals are common and well-advertised. In the rest of the country, you might have to contact a home inspector or real estate professional to ask if he/she can recommend someone.
It also is worth calling the professionals if the sinkhole is under or adjacent to your foundation. These pros can use tools such as ground-penetrating radar to find out what’s going on under the home and take steps to prevent further damage, such as injecting special grout into the ground to provide stability.
It’s usually not necessary or cost-efficient to call in an engineer, geologist or insurance agent for a small sinkhole that does not threaten a structure. The costs of these professionals can be steep. Also, having a known sinkhole on your property could lower its resale value, even if the hole is filled and fixed—the word “sinkhole” is enough to scare off some buyers. And insurance is unlikely to cover the cost of fixing the problem (although policies sold in certain states do provide some sinkhole coverage).
Source: Gerald Black, a licensed professional geologist and vice president of environmental field operations for Geohazards, Inc., a geological investigation and consulting firm in Gainesville, Florida. www.Sinkholes.com