Sinkhole Science for Beginners

sinkholes

Two giant sinkholes made big news in Florida over the last few weeks. The first sinkhole appeared in the backyard of a home in Windemere, Florida — an Orlando suburb. In north Florida, a couple who had lived in their home 37 years, awoke to a sinkhole that was well on its way to swallowing their shed. In both cases, the families hastily left their homes while the sinkholes continued to grow.

Now, these types of sinkholes make the headlines because of their unpredictability, their ominous size, and their potential to cause serious injury or even death to people who live nearby. As frightening as they are, however, “cover collapse sinkholes,” as they are referred to by geologists, are a fairly infrequent occurrence. If you are new to sinkholes, the language used to describe them can get pretty intense. You may get the feeling that you are reading a geological textbook rather than something you and I can understand. You may feel especially frustrated if there is a sinkhole on your property and you’re not quite sure what it’s all about. Scratching your head? Read on.

It starts with limestone…

Have you ever picked up a piece of limestone rock? If you have, you may have noticed that it is quite porous, meaning there are many small holes inside rock. It resembles Swiss cheese or a honeycomb. Now imagine that your limestone rock is buried beneath layers of sand, clay,  and loose earth. Suppose you took a watering can and poured water over it all. The water will gradually seep into the layers of sand, clay, and loose earth until it reaches the limestone rock. Depending on how much water was poured, each hole in the limestone rock will eventually fill with water. In addition to being porous, limestone is also quite soft compared to other rocks like granite, for example. 

Over time, (a very long time), the mild acid in the water begins to dissolve and weaken the limestone rock. That’s because limestone is largely made up of calcium carbonate, the same substance as blackboard chalk. Some of the holes in the limestone collapse and form bigger holes that are interconnected like channels. Now you have a mini aquifer. The water is held or preserved in the holes of your limestone rock just as it is held under the ground in the immense Floridan Aquifer or the smaller Biscayne Aquifer.

Because of the high rainfall in Florida there is plenty of water that is absorbed into the limestone layer beneath the ground. Remember, water has mild acids in it. As water seeps through the layers of sand, clay, and earth, it gathers even more acids. As the limestone weakens, water pressure is often what keeps the walls of big subterranean limestone caves from collapsing. A prolonged drought or rainy season can both cause instability in the soils above the limestone. Once the limestone is severely weakened, or eroded, (if it is near the surface of the ground), it will collapse and a sinkhole will form.

And ends in sinkholes…

Most people tend to think of sinkholes as bad but they are an integral part of the Florida landscape. Without them, we would not have our vast fresh water reserves. In your research, you may have come across the term, “karst.” Florida has a karst landscape, which means it is a landscape that developed on top of a limestone layer, although karst landscapes can also develop on other types of rocks, such as gypsum and salt. Karst landscapes tend to be dotted with sinkholes, caves, springs, and subterranean streams. 

If you live in Florida, particularly in central Florida, you must learn to live with sinkholes. Not all sinkholes are created equal, however. Below are descriptions of each type of sinkhole. 

Cover Collapse Sinkholes

Cover Collapse sinkholes, as mentioned before, make the news because they appear out of nowhere, tend to be huge, and are very costly to repair. This type of sinkhole occurs when the soil cover over soluble bedrock (limestone and dolomite, for example) becomes so heavy that it collapses into a crevice in the bedrock. In other instances, the soil cover is directly over the roof of an underground limestone cave. When the load becomes too heavy the roof of the cave collapses. The soil cover falls into the cavity down below. 

Solution Sinkholes

This type of sinkhole develops slowly. Solution sinkholes occur when there isn’t a lot of top soil or vegetation over the limestone layer. In this instance, rainwater and wind wear down the limestone and a sinkhole forms. Because solution sinkholes develop gradually over time, the damage they cause is preventable and easily repaired.

Subsidence Sinkholes

If you have sinkhole damage on your property, in many cases it is due to a subsidence sinkhole. This type of sinkhole occurs when limestone, that is close to the surface, begins to dissolve and is replaced by loose sandy soil. The limestone collapses little by little and is filled with sand that does not allow water to escape. Many Florida ponds are formed this way.  You may begin to see bowl-shaped depressions or spongy areas  in your yard. If the sinkhole develops close to your house, you may see cracks in the foundation or around windows and doors.

Call an expert if you think you have a sinkhole problem…

Only an expert can determine whether or not damage to your home is due to a sinkhole. Most people start by contacting their insurance company. Once sinkhole damage is confirmed, homeowners must decide who will perform the repairs. If you have a confirmed sinkhole on your property, contact the experts at Helicon. Call us today at 813-567-1065 or visit us online at heliconusa.com. We will put you in touch with one of our project specialists who can assist you.