Al Ambs: Sinkhole – Thy Name Is Hernando

January 25, 2015

http://accordingtoal.com/

Special thanks to Al Ambs for this post on his website (link above).

In the kingdom of sinkhole, Hernando reigns supreme. Did you ever wonder why?

First, let’s examine the facts as they relate to sinkholes in Florida. Unlike Manhattan, which sit on a foundation of granite, Florida sits on limestone. Limestone is soft, porous, and dissolves in acidic solutions. The porousness means the limestone acts as a very hard sponge which is invaluable as a reservoir for our drinking water. We have no other source other than capturing rainwater in cisterns. Rainwater is filtered through the sand which covers most of our state and enters the limestone aquifer at places such as the Green Spring.  As we need water to support our needs and lifestyles, we withdraw it from the aquifer via wells. The rainwater being slightly acidic dissolves the limestone as it passes through. The reason we have hard water in Florida is the result of limestone being dissolved in it.

When enough of the limestone is removed a subterranean cavern occurs. If the cavern is several hundred feet below the surface no one knows it exists since the material covering the limestone will support the roof of the cavern. This cover material is composed of sand, clay, loam, rocks, roots and debris. These items weave together and form a pretty strong support structure if they are thick enough. Here is Hernando’s problem. The top layer is so thin as to be almost nonexistent. In all of Florida the limestone substructure is closer to the surface than anywhere else. Remove the limestone and the thin cover material is inadequate to support itself, much less any structures place upon it.

Now let’s look at limestone mining. What they do is strip mine their properties. This involve three steps. First, they remove the top layer. The less they have to remove the less it costs them. Next they place charges and explode the limestone. Lastly they gather up the fragments of limestone that resulted from the explosions and take them to the grinder for processing.

How does this type of mining operation impact sinkholes. The explosions fracture the rock sufficiently so that it can be gather up by a front-end loader. These fracture do not stop at the edge of the blast zone, but continue through the rock for some distance, even into neighboring properties. So the porous stone now has a new set of fractures in it, and it is easier for water to travel through the foundation of the county.

Step three of the mining process – gathering up the broken limestone – leaves a hole where the limestone used to be. Water collects in these holes and washes through the recently fractured remaining limestone. If there is no water in these newly formed depressions, than the water has already washed through the limestone. Left in its undisturbed state rain water will sheet over the top layer and end up in the Gulf. Strip the top layer off and you are creating an environment conducive for sinkholes.

Since the limestone is so close to the surface in Hernando County mining operations have been here for a long time, and so has the sinkhole problem. Every year when you pay for the sinkhole rider on your homeowners policy you can thank the mining operators. Most of them have quarried the stone and left town. Cemex is the sole major mining operator still in the county. Instead of allowing them to expand their operation the county should be working to shut down their existing mines. These mines need to be returned to their prior condition, including elevation, to ensure proper surface drainage.

The dozen, or so, jobs at the mining operation are not worth the hundreds of thousands of dollars in damage inflicted on Hernando homeowners because of their strip mining operation. Since property assessments are reduced by 50% whenever a sinkhole is present, the County’s tax revenues take a big hit due to mining induced sinkholes. The mining operation is only good for the mining company at the expense of the County and its citizens.

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