Over the past two decades, stories of sinkholes appearing around the world are reported in the news about every two months or so, with major sinkholes that result in death or heavy loss of improved property making headlines three to four times annually. While sinkholes are certainly frightening to think about as it seems like the Earth is literally opening up to swallow everything whole, the phenomena is actually quite common and not all that scary. Geologists would even argue that if anything, the appearance of sinkholes is rarely, if ever, reported as they are so common, they are actually an almost daily occurrence when the phenomena is considered in a global sense.
There are three major varieties of sinkholes, as per National Geographic, and while one seems much scarier and more dangerous than the others, each is statistically rather benign. They are the solution sinkhole, cover-subsidence sinkhole, and cover-collapse sinkhole. Some may be massive, some may be only a foot or two in diameter. Each one is the result of a variety of natural, and increasingly, man-made, circumstances. The natural factors that can make an area a risk for sinkholes are easy to spot as it involves little more than being able to identify what kind of soil, sediments, and overburden an area contains.
A solution sinkhole is one in which rock is either exposed at ground surface, or only has a thin layer of soil covering it. These can often be seen as visible dips in the ground that have filled with water. Cover-subsistence sinkholes are the result of the overburden being heavily composed of sand, which doesn’t stick together very well. If the rock that lies underneath the sandy overburden develops a hole, the sediment around the hole will gradually trickle down leaving the surface weakened and vulnerable.
Cover-collapse sinkholes are the variety that is often reported in the news, according to Practical Engineering, such as the one that appeared on the White House lawn recently, or the 2010 Guatemala sinkhole that consumed a three-story factory killing 15 people. This is the type of sinkhole that occurs in areas in which the overburden is heavily composed of clay. As clay sticks together very well, it can bear weight to a degree, and it takes much more time to erode than sand for instance. Because clay overburden holds together so well, massive cavernous arches can form underneath it. When the weight on the arch, or arches, beneath the overburden can no longer bear the weight above it sufficiently due to the overburden thinning, it will collapse, often with no warming.
The way that holes and arches form in the rock that lies beneath overburden in the first place is simple and unavoidable, aside from man-made influence. Holes begin forming in certain types of rock, such as limestone, dolomite, gypsum, or other evaporites when there is no natural external surface drainage according to the USGS. While the foundation of any of those varieties of rock is solid, they dissolve slowly over time due to exposure to acidic groundwater, rain, or a combination of the two. It is a matter of gravity working in conjunction with natural erosion.
Of the man-made conditions that help lead to sinkholes forming, or hastening the collapse of existing ones, digging and drilling are major factors. Leaking water/sewer pipes, injecting water into the ground, is also significant aggravating factors that can lead to urban sinkholes, as are poorly designed and maintained storm drains. The Department of Environmental Protection also cites vibrations caused by blasting, or the use of certain types of heavy equipment, as another factor. Then consideration must be given to actual structural loads on the overburden, ie; buildings or other forms of significant man-made weight in a concentrated area.
While it is hard to determine if sinkholes are actually on the rise or not, it is certain that people hear about their occurrence more now than in decades past, if for no other reason than the existence of the Internet. The bad news is that there is nothing we can do to stop sinkholes from happening as their formation is a natural process. The good news is we can take steps to avoid, or lessen the role of humans in creating them, and that the use of Interferometric Synthetic Aperture Radar can identify small changes in the overburden which allow for identifying where sinkholes are, or the potential for them to form exists, according to NASA.