MISSISSIPPI STATE -- The damage isn't nearly as dramatic as that caused by an earthquake, but Mississippi soils have the capacity to harm foundations when they get dry.

Frances Graham, housing specialist with Mississippi State University's Extension Service, said shifting ground can crack foundations. This damage often shows up as cracks or separation around doors and windows or brick veneer, or as cracks in the cement slab of the carport or garage. This damage is especially evident during droughts.

"A hairline crack in brick or drywall does not necessarily mean there is a major problem, but you might have trouble if it begins to shift or form a gap," Graham said. "If there are cracks in the carport area, one can usually assume that it does not end there, but extends through the house."

In some areas, the ground expands and contracts with the changing seasons and amount of moisture available. Cracks that form during a ground expansion do not necessarily return to their original position when the ground contracts, Graham said.

Separation from a shifting foundation can threaten the safety of the house. Graham said owners should carefully watch trouble spots and bring in an engineer or foundation specialist to check serious problems. Damaged foundations can be rebuilt, but this is very expensive.

Graham said conventional foundations tend to suffer less damage from cracked ground than do concrete slabs in these situations.

Larry Oldham, Extension soil specialist, said soil moves because of clay's ability to absorb an enormous amount of water.

"Clays are like an accordion and some can absorb up to 300 times their own size," Oldham said. "When they absorb this water, they expand, but when that water is used or evaporated, the clays shrink and decrease in size. This is a very powerful force that forms cracks in the soil."

During a drought, the water that evaporates from soil or that is used by plants is not replenished, so the clay loses its moisture and size. As it shrinks, it forms cracks that can extend many feet deep and damage foundations.

"Foundations crack as they are stressed by the motion of the soil," Oldham said. "It is a direct function of the physics of the clay being dehydrated."

In some areas, a regular foundation watering program can ward off problems, but in other areas, water has to be drawn away from the foundation at times to prevent damage. Since Mississippi has several different soil types, contact local authorities for advice on the best way to prevent foundation problems.