About Sinkholes

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Science of sinkholes

Tara Muck
News-Leader

Nixa — It’s just an empty lot now. Any sign of what used to stand there is long gone. Cars pass by without a second thought about what happened nearly three years ago.

The 75-foot deep, 60-foot wide Nixa sinkhole that swallowed up Norm Scrivener’s car, garage and part of his house in August 2006 has baffled geologists. Never in southwest Missouri had such a phenomena occurred in an urbanized area.

After a couple years studying the formation, Missouri State University Professor Doug Gouzie tried to provide some answers to the area’s most infamous sinkhole.

Surprise

The beauty of southwest Missouri’s landscaping also contains very porous rock — limestone.

Limestone, made up of calcium and carbonate, dissolves where water flows because of the acidity in rain water, Gouzie, a geology professor at MSU told a crowd of about 40 last week at a lecture at MSU.

About 60 percent of Missouri is underlain with this rock, which is why the state boasts elaborate caves.

It’s also why sinkholes are so common.

As groundwater moves through the soil over time, top rock is eroded away first and then dirt, leaving a hollow hole in its place, Gouzie said.

Gouzie, who has studied sinkholes for nearly 30 years, said Scrivener’s sinkhole was surprising on a couple levels.

A study conducted by an MSU professor in the 1970s documented about 270 sinkholes in the Nixa Sinkhole Plain, which runs from Missouri 14 north to where James River crosses U.S. 60, Gouzie said. But of those 270 identified, only about 10 were collapsed sinkholes, where the soil actually collapsed in, rather than a gradual sinking.

A gradual sinkhole would have the appearance of a saucer, while a collapsed sinkhole is more thought of as an ice cream cone shape, Gouzie said.

“When you see any collapse that’s significant in size — 15 to 20 feet in diameter — then that’s pretty unusual right there,” Gouzie said. “That’s maybe 5 to 10 percent of the total number that anybody studies is the collapse.”

But it was one missing element that really baffled the geological crew at the site — rock.

Gouzie said the most surprising thing to him and other geologists was there was no rock to be seen anywhere in the sinkhole. Dirt erodes much faster than rock, so if the rock is gone, the soil should be as well.

“We’ve seen some thick soil around here … but for the most part 10 or 15 feet is pretty good,” Gouzie said. “If you have the irregular top of the bed rock, you might get 30 feet of soil somewhere, but to have 75 (feet), it’s probably twice than what we would have expected anywhere.”

Origins

Many times, large sinkholes, such as the one in Nixa, open up to a cave and are formed by underground moving water.

Tests usually performed to find origins of sinkholes weren’t plausible in Nixa, Gouzie said. That’s because Scrivener’s garage, along with his car, went down the hole, making it impossible to run dye into the bottom of the hole and detect it in area springs.

Another way would to be to go through a cave in the vicinity to see where it led. But the cave Gouzie and other geologists feel lead to Nixa’s sinkhole — the Saunders Valley Cave — hasn’t been searched deep enough.

So Gouzie and other professionals made an educated guess that the sinkhole drains out to Blue Spring, northwest of the site near the Saunders Valley Cave.

But while that’s the geological origins, there are also other factors Gouzie feels could have expedited the process in Nixa.

Perfect storm

Because Scrivener’s home was built in 1969, it originally had a septic tank, Gouzie said, which was visible after the garage collapsed into the sinkhole.

If the septic tank leaked even after it was closed off in the early ’70s, it could have helped the erosion process already in play by rainwater, he said.

Also exposed around the sinkhole were tree roots that lead to the stumps of two directly behind the house that had been cut down years earlier. Gouzie said when the tree roots’ water source died because of the trees being cut, the roots eventually shriveled up, leaving more empty spaces.

There was also a rain gutter hose that was placed on the uphill side of the house, which meant rainwater washed sediment down to the other side of the house. It was on the uphill side of the house that the sinkhole occurred, Gouzie said.

That doesn’t mean Scrivener or the builder is to blame.

“I think this collapse was going to happen anyway,” Gouzie said. “Whether it would’ve happened in 20 years or 50 years or a hundred years or a thousand years, that’s unfortunately where I’m still doing research.”

sinkholes today

The hole has been filled — on top of Scrivener’s car and parts of his garage — with 165 truckloads of rock.

Patches of grass grow over the once-hollow ground.

And while homes are still occupied on both sides of the former house, the earth continues to settle over time, causing two small sinkholes to appear.

“When you fill it up like that, there’s usually some settling that occurs the first couple of years,” Gouzie said of the rock that filled the hole.

“What’s happened is the soil that was put on the very top as the very last thing … is still washing down into the rock, and that’s going to happen for a while.”

The idea of that type of formation occurring again in the area is of concern. Could the spring that caused the original sinkhole create another along its path?

Possibly, Gouzie said. But sinkholes are more likely to develop farther down the stream as the water gains momentum. That is why the Nixa sinkhole was so surprising, because it was actually on the ridge of the Nixa Sinkhole Plain.

But planners at the city and county level are using technology to spot sinkholes, such as Ground Informational System, which the county uses, said Bob Atchley, Christian County’s acting planning and zoning administrator.

With this, the county is able to detect sinkholes so development of water and sewer, streets, and buildings can avoid a potential hazard. According to county ordinances, no new construction is allowed within 30 feet of the rim of a sinkhole.

“Any time we have any type of proposal in the county … we do a brief environmental assessment before we issue a permit,” Atchley said. “That starts with looking on the GIS system to see if it shows any mapped sinkholes.”

In Nixa, the city has looked to updated technology to help open up more space for development. The city’s use of a Ground Penetrating Radar (GPR) enabled the city to change its setback ordinance from prohibiting construction 30 feet from a sinkhole rim to 10 feet.

That’s because the GPR is better able to detect the precise location of sinkholes, City Planner Travis Cossey told the board in October when the ordinance was passed.

Still, Gouzie and Atchley both said the best way to prevent a sinkhole disaster is to be vigilant regarding water runoff, as well as look for signs of sinking or cracking in the soil or foundation.