Into the Earth: Sinkholes in the Ozarks

Scrivener was the victim of a sinkhole, the result of the Ozarks’ unique topography. His house was built over an undetected erosion dome, and water had slowly moved away the soil from underneath the ground until it collapsed under its own weight.

While sinkholes are common in the area due to the region’s Karst topography – a layer of underground limestone, highly susceptible to dissolving in slightly acidic water – those rapid collapses are a small portion of the sinkhole picture.

“Sinkholes fall into two types,” said Doug Gouzie, geology professor at Missouri State University. “One is the broad solution valley. That’s where the water, bit by bit, eats away at the surface rock and forms the gentle coffee-saucer shape. Those typically have a slope of a few degrees, but if you stand in them and look very closely, you realize that all the water in them drains into one area – it’s like a bowl.

“The ones that we see like the one in Nixa are much more rare.”

Safety first

Sinkholes exist in other parts of the country, but they are more prevalent in southern Missouri. Developers in the Ozarks treat the issue differently.

Jonathan Robison is the regional manager for SCI Engineering, which has done the site work at the Nixa sinkhole. He said that in St. Louis, where sinkholes also have been mapped, developers are bolder in their building procedures due to the smaller amount of open land available for development.

“Because the price of land is so high, you won’t see a five-acre common area because there’s a sinkhole in the middle,” Robison said. “You had to figure out a way to work properties in there closer to it.”

In southwest Missouri, however, there are regulations. In Springfield, if a sinkhole is visible, the builder must hire a geologist to determine its boundaries and buildings must be located away from the rim. Greene County requires a 25-foot setback for new commercial buildings, parking lots and sanitary sewers.

“We take a three-tiered approach,” said Kevin Barnes, Greene County storm-water engineer. “We avoid sinkholes where it’s possible, we minimize their impact when it’s not possible, and we mitigate them as a last resort.” Barnes added that the county tries to make as little impact as possible when existing development goes through a sinkhole, such as on National Avenue south of Springfield, where a retaining wall was built when the road was widened.

Filling the hole

The restrictions don’t make any land with a sinkhole off limits. Remediation techniques can be used to effectively fill sinkholes to support new development.

But that also must be done properly – filling the hole with dirt won’t cut it.

“The problem is that when you fill it in with backfill, you can limit the sinkhole’s ability to properly drain to surrounding property,” said David White, manager of geologic services for Sunbelt Environmental Services. “If you fill it in with fill material and build a home on it, the hole will continue to pipe that dirt away underneath. It will form a cavity that will eventually subside, and that causes foundations to crack and serious structural issues in the home.”

Improper filling also can create new problems, according to Environmental Works Principal Scientist David Vaughan.

“You have to backfill them with different size rocks at different levels, so you can allow water to flow through it, but at a controlled rate,” Vaughan said. “If you block a sinkhole, that water is going to somewhere, and you’re likely to open another one.”

Paving is also an issue – increased runoff from impervious surfaces such as concrete or asphalt leads to changes in water flow, which can create additional issues.

Despite the unpredictable nature of the ground, engineers all agree on one point: The prevalence of sinkholes in this area underscores the need for developers to plan adequately prior to building. MSU professor Gouzie cited a 1970s study of the region between Nixa and the James River that showed 266 significant sinkholes.

“The big thing would be to consult with a geologist,” Sunbelt’s White said, citing the Walgreen’s drug store at highways 160 and 14 in Nixa. That store, he said, was built on a site with a sinkhole, leading to extra money spent on securing the building to the underlying bedrock.

“If they would have had a simple geologic study done prior to building, they would have saved a lot of money,” White said. “They probably wouldn’t have even bought the lot.”[[In-content Ad]]