Chances of sinkholes in San Antonio?

by Michael Mosher on July 8, 2013

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SAN ANTONIO– “There’s approximately 600 caves in northern Bexar county” that’s according to the Edwards Aquifer Authority’s Chief Tech Officer Geary Schindel. 

Where there are caves, there are potential sinkholes.

If a sinkhole collapses causing the ground to cave in, structures and people could be in danger.

Sinkholes can appear suddenly, but are thousands of years in the making.

What really makes them dangerous is the fact that they’re unpredictable. 

“Occasionally there are caves, the roof of the cave is so thin that it can not support the weight of the cave or maybe something built over top of it. 

Those caves are actually very rare, but there are a few examples of them in San Antonio but very few” say’s Schindel. 

They form so slowly, that without thorough geological or geophysical research, you can’t really tell if something’s changing.

This is why collapses have dramatic, unexpected effects, especially in urban settings.

Cub Cave, in the Stone Oak area, is one of the hundreds of caves all around San Antonio that used to be made up of solid limestone. 

Over millions of years, water has carves out limestone creating beautiful caverns and caves. The problem is these walls are supporting what’s being built above.

Many homes in San Antonio are actually built on top of caves and caverns, especially north of 1604 where the Edwards Aquifer is. 

Sinkholes do happen. One was discovered at the ACYSO soccer field from the storms Monday night. 

Just below the surface of the ground is made up of limestone filled with caverns and caves that could give way… and that’s where it becomes dangerous.

“It makes development more difficult. They have to perform a geological assessment before they develop to identify all the caves or what are also termed karst feature” say’s Schindel.

Sinkholes are almost always prone to karst areas. 

Schindel explains, “Karst is a word that describes a terrain or topography where caves and sinkholes and sinking streams occur.”

And the Edwards Aquifer that spans across our Hill Country is a karst aquifer.

Sinkholes are usually caused by rain water gradually dissolving porous rock below the surface.

As water drains and collects, that area is stressed and then weakend. 

Eventually, the ceiling collapses because the overburden cannot be supported, hence, a sinkhole is formed.

While we have seen sinkholes here collapse under the weight of a car,

According to geologists it’s less common for “structures” to collapse in San Antonio.

“The rock here is much much older, more time to consolidate, that’s one of the reason why you haven’t heard of a sinkhole collapse” Schindel says. 

We spoke to Allstate Insurance and in Florida, where sinkholes are very common, insurance companies are required to offer sinkhole or ground shifting coverage in their standard policies — at an additional cost.

But right now that isn’t the case in Texas – as of yet. Allstate say’s none of their policies cover earth movement and neither does most other insurance companies.

Full article…here

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