A down-to-earth problem.

Homeowners might have more to fear from the ground beneath their house than they do from hurricanes, tornadoes, earthquakes or floods. At least, they have more to fear if their house is built on certain types of clay known as "expansive soils."

Expansive soils cause an estimated $2 billion in damages each year in the United States -- more than all the above-mentioned natural catastrophes combined. The problem, according to W. Kent Wray, a civil engineer at Texas Tech University in Lubbock, is that the clays swell or contract as soil moistures vary, which can cause the foundation of a house to buckle if it's a concrete "slab-on-the-ground" type.

Wray has been studying expansive soils at two experimental house foundations he has built in Amarillo and Bryan, Tex. Instruments attached to the experimental foundations gather data on soil surface movement, rainfall, temperature and the expansion characteristics of the clay.

Wray has found that a foundation will either "dish" (sink in) or "dome" (bubble up in the center), depending on whether the soil at the foundation's center is getting wetter or dryer.

"For expansive soils to be a problem," says Wray, "the ground moisture content has to change." Clay is particularly susceptible to moisture, he says, because clay particles have a great surface area for their size; water is able to penetrate among the particles and push them apart.

These clays are a particular problem in semiarid regions where rainfall is followed by long periods of no rain, says Wray, adding that Texas and California record the most annual damage. It's lucky, he notes, that England has such a rainy climate. "London clay," he says, "is potentially the most damaging in the world."