What are expansive soils?

 Hint: Clay soils are the culprit

Expansive soils contain clay soil. Clay soil particles are very small and are shaped like very thin plates; due to the thin plate shape, clay particles have a lot of surface area for their size. Clay particles are electrically charged and bond to each other like small magnets. The electrical bonding force is relatively weak and can be easily broken by water molecules that become inserted between the clay particles.

Changes in soil moisture drives the swelling and shrinking

As the soil becomes wetter, more and more water molecules attach themselves to the plate shaped clay particles and the water molecules push the clay particles further and further apart. (Think of the plate-shaped clay particles being like a deck of cards where  the cards are being pushed apart making the card deck appear thicker.) This results in the apparent volume of the soil mass growing so that we have soil heave or expansion. As the soil dries out, the process reverses; as the water molecules evaporate and become detached from the clay particles, the clay particles move closer and closer together. Soil shrinkage results.

In a sense, expansive soils act like a sponge; the apparent volume of the sponge increases as it takes on water and shrinks as the water evaporates.

Expansive soil shrink-swell potential

The degree to which expansive soils shrink and swell is characterized by the shrink-swell capacity. Most of the soils in the Greater Houston Area have moderate to high shrink-swell potential.