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Dry clay soils can shift buildings

By Bonnie Coblentz

MISSISSIPPI STATE -- Ground that gets dry enough can shift and displace buildings, a fact building owners across the drought-ridden state are seeing as they deal with cracking foundations and split walls.

Larry Oldham, soil specialist with the Mississippi State University Extension Service, said the clay content of soils is the driving force for this movement.

“There are many types of clay. Some clays absorb up to 30 times their volume in water, and others can absorb as much as 300 times their volume in water,” Oldham said. “When these clays absorb water, they expand in size and hold water in place.”

Oldham said clays are the smallest primary particles in soil, so small that they can only be seen with magnification. Despite their size, clays have the most influence on soil properties.

“When we have a period of normal rainfall, the clay particles store a significant amount of water in the soil,” Oldham said. “When we do not have rain or irrigation to recharge the water content of soil, this water is released to plants or evaporates from the surface.”

As water is depleted from the soil, the clay particles shrink, creating voids that are filled with air. These voids appear as cracks in the ground, and can range from microscopic in size to several feet long and several inches deep.

Jim Thomas, Extension agricultural engineer, said what happens below ground can dramatically affect what is above ground.

“Soils are very strong from the standpoint of being able to move things,” Thomas said. “As they shrink away from a foundation or a footing on a conventional foundation, they move it.”

Thomas said this shifting of foundations is what makes doors bind, sheetrock crack and walls split. On roads, it makes pavement buckle and roadbeds grow uneven.

“Foundations typically are poured when there is decent soil moisture and the ground is neither baked dry nor soggy,” Thomas said. “Foundations either get wet or dry after that, and they move, causing structures to crack.”

One way to prevent some movement problems is to put deep footings down, usually 36 inches deep to what is considered a more constant moisture zone. Such a foundation costs more and would not necessarily prevent all soil movement problems.

A simple solution for existing buildings is to keep a constant moisture level around the base of the structure. Run a soaker hose around the perimeter of the building to wet the soil. Thomas said a week to 10 days of soil soaking could bring the cracks back together in buildings.

“If you can keep those soils around the footings wet, you keep them swelled up to their normal soil moisture. This alleviates a lot of the drag on doors and the foundation cracks,” Thomas said.

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Released: Aug. 24, 2006
Contact: Dr. Larry Oldham (662) 325-2760 or Jim Thomas (662) 325-7346

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