A few years ago, there was a great deal of press coverage about sinkholes. But setting aside the sensationalism, what are sinkholes and what can be done to repair the damage caused when they affect homes and businesses? Here are four facts about sinkholes and sinkhole repair:
Many Different Factors Can Cause Sinkholes
Sinkholes can be caused by people. For example, in areas where mining or oil and gas extraction is common, sinkholes can be caused by collapses of underground mines or underground oil or gas reservoirs. Similarly, man made sinkholes can be caused when water or sewer lines break and the leaks cause the ground to become unstable due to erosion.
However, in Florida most sinkholes are natural phenomena. Natural sinkholes are typically caused when water dissolves rock and creates voids. The overlying soil settles into the voids and causes depressions known as sinkholes. In Florida, the typical soil profile has a layer of lime rock about 30 to 60 feet below the surface. Water, particularly acidic water, dissolves lime. This can weaken the rock layer creating “Swiss cheese holes” that allow the overlying soil to settle into any voids opened up in the dissolved rock. This process is known as “suffosion.”
Florida is Known for Sinkholes
In central Florida, the lime stone layer is anywhere from 15 million to 20 million years old. Because the lime stone layer has been subjected to dissolution for so long, many sinkholes have opened up in central Florida. By contrast, the lime stone layer along Florida’s coast is around 120,000 years old. Since the coastal lime stone layer is much younger, it has not been subjected to the chemical processes that dissolve lime stone for nearly as long. Consequently, sinkholes are fairly rare along Florida’s coast.
Among the most famous sinkholes in Florida is Devil’s Millhopper in Gainesville, Florida, which is the site of a state geological park. The Devil’s Millhopper sinkhole is 500 feet wide and 120 feet deep.
Another well known sinkhole in Florida is the Winter Park sinkhole. This sinkhole, which filled with water and is now known as Lake Rose, is best known for its extensive media coverage. The sinkhole opened up in 1981 and was the subject of news stories across the world. Through the summer of 1981, Winter Park’s residents promoted the sinkhole as a tourist attraction, selling souvenirs and t-shirts to commemorate the event.
Sinkholes Can Be Destructive
The Winter Park sinkhole swallowed a house, the city swimming pool, five cars, a pickup truck, and parts of two streets. The city estimated the damage at $2 million to $4 million. In 2014, a sinkhole under the floor of the National Corvette Museum in Bowling Green, Kentucky swallowed eight rare Corvettes. The museum spent $3.2 million to repair the damage caused by the sinkhole.
There exist options for ground stabilization to reduce or avoid the damage caused by sinkholes. When early signs of sinkhole damage are discovered, such as cracks in the floor or walls and misaligned doors and windows, quick action can be taken to stabilize the ground.
One method for sinkhole repair is compaction grouting. In compaction grouting, grout is injected around and below a foundation. The grout occupies space in the ground, causing the loose soil to compact. The compacted soil is stronger with fewer voids and, thus, less susceptible to collapsing or eroding into a sinkhole.
Another method for sinkhole repair is chemical grouting. In chemical grouting, structural polyurethane foam is injected below the surface of the soil. The foam expands, which causes the loose soil to compact. When foam is injected under a settled foundation, the foam can lift the foundation as it compacts the loose soil. Chemical grouting is often used when the soil to be treated is relatively shallow. As with compaction grouting, the soil compacted by chemical grouting is stronger with fewer voids and less susceptible to collapse.
In sum, sinkholes are a natural occurrence in Florida due to its soil profile. Although nothing can be done to prevent sinkholes from happening, the damage from sinkholes can sometimes be avoided by taking action when the early signs of sinkhole activity are discovered. Compaction grouting and chemical grouting both compact and stabilize the soil so it can support the structures resting on it.