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Dry as dust

Drought making Yazoo clay an issue

Years ago, Dr. Larry Oldham used to frequently make the drive from Jackson to Vicksburg on Interstate 20. Like all travelers on that stretch of I-20, he had to negotiate the uneven, undulating roadbed.

“One day I realized that the humps were moving; they were in different places,” Oldham said.

Oldham is a soils specialist with the Mississippi State University Extension Service, and he knows the root cause of the roller coaster-like ride I-20 offers — Yazoo clay.

Yazoo clay is a soil type found across a large swath of Central Mississippi. It contains a special quality that allows it to greatly expand and contract depending on moisture. When it rains, the soil works as a sponge, and can swell 200 times that of other expansive soils. In dry weather, it quickly shrinks.

This shrink-swell action plays havoc on foundations, roadways and other infrastructure, and is the bane of builders and owners. Unless properly accounted for, the soil can buckle and crack foundations or roadbeds, and on slopes can create “slumps,” a severe erosion that causes overlying structures to simply crumble.

In mid-October, scientists were recording historically dry soil moisture readings, and some experts say they expect to see some problems caused by shifting Yazoo clay. The heavy rains in late October offered drought relief, but will only serve as fuel for Yazoo clay expansion, they say.

Dr. David Dockery, chief of the Surface Geology Division at the Mississippi Department of Environmental Quality, has already observed some new damage from the drought on I-20. A section of the shoulder between Jackson and Clinton has broken away from the roadbed.

“It looks like a big truck or something has just smashed it down,” Dockery said. “It appears that if they don’t do something about it, it’s going to just crumble away.”

Dockery said the main issue is the upper, weathered Yazoo clay that can reach a depth of 40 feet. The bluish clay below this layer is much more stable, less prone to shrink-swell.

Tommy Dunlap, P.E., a principal at the engineering firm Burns Cooley Dennis, said Yazoo clay is especially problematic when it moves unevenly. Professionals refer to this as “differential.” For example, Yazoo clay under one part of a foundation swells or shrinks while the rest of the foundation does not. This can cause major foundation problems, and in extreme cases can affect a structure’s soundness.

The best solution, particularly in large structures, is to drive pilings into the non-weathered clay, put a pile between the pilings and the floor and replace the weathered clay with fill dirt. It is very expensive, but will save money — and headaches — later, Dunlap said.

Dockery related a story of a Jackson hotel owner who, when building his hotel, was told he needed to dig out the Yazoo clay and replace it with fill dirt. The owner decided to forego that advice — and expense. A few years after opening, the hotel’s first floor buckled so badly that it forced the owner to close for a year. Workers had to jack hammer the floor, undo all of the previous construction then do the fill-dirt work that was recommended on the front end.

“It ended up costing him way more than it would have, plus he lost all the money when he was closed,” Dockery said.

The Yazoo clay formation in Central Mississippi is wide, but not consistent. For instance, the City of Clinton lies on a “divide.” Generally, Yazoo clay is absent on the west side of the city.

“But if you’re building on the east side of Clinton, you might have problems,” Dockery said.

Dunlap said the drought might not have as much direct impact as feared. He said a tree near a foundation could be more threatening than the drought. The tree sucks moisture out of the clay, causing it to contract. In drought conditions, however, that same tree might draw a larger percentage of the moisture from the clay than the norm, posing a bigger threat to overlying structures.

It is not just Yazoo clay that cause serious problems to foundations, roadways and other infrastructure. A good example is the new Clinton High School. Dockery said the school reported foundation problems after opening, and some suspected Yazoo clay as the root cause. However, further testing showed the culprit was Vicksburg limestone, a soil that expands and contracts much as Yazoo clay.

Other problem soils are Porter’s Creek clay found in Northeast Mississippi and a clay in the Mississippi Delta commonly called “gumbo.”

But experts say these soil types are mere nuisances compared to Yazoo clay.

“Truthfully, nothing is quite like Yazoo clay,” Dockery said.

Oldham said sarcastically, “We’re blessed with the stuff.”

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About Wally Northway

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