What are expansive soils?

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.  The 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.  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.  This results in soil shrinkage.  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.

Why do so many houses in the Greater Houston Area show signs of damage due to foundation movement? 

There are several reasons why the Greater Houston Area includes large numbers of houses that show signs of damage due to foundation movement: 

When you put the above facts together, it is surprising that there are not even more residential foundation problems than what we see.

What areas of the Greater Houston Area are affected? 

Most of our soil consists of various types of clay with some sand mixed in.  The South, Southwest, East and Central Areas of Houston are characterized by what is known as the Beaumont formation.  This soil formation contains significant amounts of moderately to highly expansive clays.  The north and some parts of west Houston are underlain by the Bentley and Lissie formations.  The Bentley and Lissie formations consist of sands and sandy clays; these soils generally have a low to moderate shrink/swell potential.

Some of the sandy soils in the north and west parts of the Greater Houston Area also contain silt and can become very weak when they are wet. In many cases, the silty, sandy soils on the surface are underlain by an impermeable clay strata that can hold water creating what engineers call a perched water table. The perched water table can hold water allowing the underlying clay soil to soften which can result in the clay soil losing much of its bearing capacity resulting in foundation problems. In some situations, the underlying clays are expansive making a bad situation even worse.

The web page titled Houston Area Soils and Areas provides more specific information concerning the variety soils types and risks damage due to foundation movement found in the Greater Houston Area.

The bottom line is this: although some areas may not be as bad as others, or may have a different mix of problems than other areas, there are very few subdivisions in the Greater Houston Area that that do not have the potential for foundation movement that can cause damage to a conventionally constructed house.