Tuesday, October 10, 2000

How does Texas both shrink and swell?

Many of us in Central Texas live in homes that rest on soils that shrink during the summer and swell during the spring or fall rainy season. We see evidence of these changes when shrinkage cracks form in the earth during hot and dry months, and when swelling tilts telephone poles and buildings after heavy rains. What causes our soil to move like this?

The answer is found in the particles that make up the ground below our feet. At least one-third of the particles in these shrinking and swelling soils are very fine clay. Minute electrical charges on the fine clay particles attract water molecules to their surface. The added molecules cause these soils to expand and swell like a wet sponge. Swelling commonly moves the soil upward, shifting foundations of houses and tilting fence posts. When the soil dries out later in the year, it contracts and shrinks. Drying causes this soil to loose most, but not all, of its water.

How do you know if you live on this type of soil? Watch your soil closely as the seasons change. Most shrink/swell soils are sticky when they become wet and are very hard when they dry. The forces expanding and contracting these soils are so strong that you may notice cracks in old sidewalks and cement foundations produced by the shrinking and swelling of the soil.

Residents of central and eastern Travis and Williamson counties have learned to farm and build on these soils by understanding their properties. For many years cotton was grown on the Blackland Prairies east of Austin and Round Rock because the shrink/swell soils there held onto their moisture during the long hot and dry summers. Many houses in this area were built on cedar or concrete posts to allow their foundations to adjust to the movement of the soil.

Why are we blessed or cursed with these soils that shrink and swell? Much of the credit or blame goes to volcanoes that erupted in Central and South Texas 65 to 95 million years ago. Ash from these eruptions fell into shallow seas that covered most of Texas at that time. The ash was altered to very fine clay and became part of the chalk and shale deposits that underlie our soil. When these deposits were exposed at the surface they weathered to produce our "swell" soils. Shrink/swell soils are so common in Texas that our state soil, Houston Black Clay, belongs to this group of soils. Would you like to know more? An Earth science teacher, county extension agent, geologist, or civil engineer can tell you more about shrink/swell soils. Technical information on these soils can be found at, or at

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