Sinkholes have become a rising problem – particularly in Florida and Texas. As more and more sinkhole cases arise, it’s important for homeowners to understand what causes sinkholes and the risks so they can safeguard their homes and families. Crosstown’s sinkhole repair experts took a few minutes to answer some of the most common sinkhole questions.

1. Sinkholes seem to just pop up out of nowhere. So, what causes sinkholes?

As you probably know, sinkholes are getting much more media attention these days. If you are like many Americans, you see the shock-and-awe examples of sinkholes around the country. Reports of sinkholes swallowing homes, an antique corvette warehouse or even moving vehicles on a highway. These horrific sinkholes make up about 1% of sinkholes that are actually detected. In fact, sinkhole activity can sometimes be a very small situation that may never be detected … or it may cause a hairline crack in your wall.

Groundwater plays a large roll in the sinkohole development process.  Not only does it contribute to downward soil erosion, but it also dissolves the limestone over time – this process is called solutioning.

Solutioning is more prominent in subterranean zones with preferential paths that concentrate the flow of water and in areas where groundwater volumes or velocities are high. Since the infiltration rates of rainwater are very high along the Gulf Coast, there’s more water to transport soil into voids and to dissolve limestone if present. In areas like Florida and Texas where there is an abundance of limestone, solutioning can become a major issue.

2. Are all sinkholes the same?

Sinkholes are very interesting because the various soil types, geologic conditions and physical environments where they develop make each sinkhole unique. While each sinkhole is different, there are three general types of sinkholes.

1. Cover-Collapse Sinkholes – This type is typical of catastrophic sinkholes and can develop in a short period of time (sometimes over a few hours). Sometimes, they’re caused when a dense layer of sand acting as an underground cave ceiling collapses.

2. Solution Sinkhole – These sinkholes develop over time and are a result of the solutioning of limestone. Solution sinkholes occur in areas where limestone is exposed at land surface or is covered by think layers of soil and permeable sand. Florida is a great example. If you look at an aerial of Florida, you’ll see a lot of circular lakes. Those lakes are actually large solution sinkholes!

3. Cover-Subsidence Sinkhole – This type is found in areas with high concentrations of sand and appear as a concave depression in the ground. The dissolving limestone is replaced by sand granules that fall into the depression and fill the gaps and channels in the limestone. Typically, these sinkholes are only a few feet in diameter and depth because the development of the cavities in the limestone is slow since they’re filled with clay and sand.

3. How hard is it to find a sinkhole?

Solutioning of limestone is a slow process and may occur in very small areas within the limestone unit which means an even smaller area of the ground surface may be affected. There are several methods for sinkhole testing, but there are some simple signs you can look out for, too. If you’ve noticed cracks in your foundation, pavement, walls, ceiling or floors, or if you’ve heard of sinkhole activity in your area, you should seek a licensed professional engineer’s evaluation. There is also historical data available to homeowners on sinkholes in Florida. This information is available for free using tools built for homeowners, like the Florida MoveMap from Movers Atlas.

4. How do you test for sinkhole activity?

Generally, the best way to investigate for sinkhole activity is to begin with a Ground Penetrating Radar (GPR) survey and follow up with a Standard Penetration Test (SPT) program. If your site is known to have a lot of clays within the top 20 feet, it may be best to skip the GPR.

Most Common Soil Testing Methods:

• Standard Penetration Test – This is the most effective way to test the soil for sinkhole activity. It measures densities and gathers actual soil samples at depth.

• Ground Penetrating Radar – GPR uses radar technology to gather geophysical data.

• Cone Penetrometer Test – In this method, we measure resistance and friction when tools are thrust into the ground.

5.  How difficult is sinkhole repair? How do you fix a sinkhole?

The short answer to this question is, “it depends on the sinkhole.” Engineering opinions will vary as to which combination of common remediation techniques should be used and to what extent each should be used for each situation. Generally, there are three common remedies for sinkhole repair include compaction grouting (deep soils), chemical grouting (shallow soils) and structural underpinning.