When the concrete foundation for a new house has been completed and workers first strip off the forms, it’s hard to believe that anything could damage the massive walls that are revealed. But if the house occupies a site where expansive soils are present, this foundation could easily crack, buckle and shift even before the house has been finished. Expansive soil, also known as expandable soil, shrink-swell soil or swelling clay, has the potential to cause extensive foundation damage. According to some estimates, expansive soil is responsible for $2 billion to $7 billion in construction damage.
Although expansive soil damages roadways, side walks, retaining walls and other masonry structures, in this article we’ll focus more on how these soils cause problems in houses.
Clay is the culprit
You most likely have seen expansive soil before, but just didn’t know its proper name. Ever noticed dry soil with polygon-shaped cracks? Or, maybe wet soil that is unusually sticky, dense and malleable? Well, that’s expansive soil and clay is the ingredient responsible for such characteristics. More specifically, geologists have identified a certain class of clay minerals called “smectites,” that make up expansive soil. These super-fine clay particles expand and contract at high rates as they gain and lose moisture.
What goes up (in volume) must come down
When saturated with water, the expansion of a clay-rich soil can exert as much as 5,500 lbs. of pressure per sq. ft. on a house’s foundation. Such force can push laterally against a foundation wall, causing it to bow or crack. Expansive soil lifts up walls and concrete slabs, and then shrinks down away from the masonry when the soil dries out. The resulting voids leave slabs, footings and walls unsupported, so this masonry is likely to crack, settle or even break apart in response to swell-and-shrink cycles.
Some areas are more susceptible than others
Expansive soil is more common in some areas than in others. For example: Colorado, Texas, North Dakota, South Dakota and Montana have the highest proportion of these soils. In fact, Mississippi even has a special name for its unstable soil -Yazoo clay. Whether a state has a high or low concentration of expansive soil, no state is immune to the damage it can cause. Since clay is created by a natural process of erosion and weathering, its effects are likely to be found just about anywhere.
Foundation repairs offer solutions to expansive soil problems
A foundation repair specialist who is familiar with local soil conditions will probably know whether or not expansive soil has played a role in a foundation’s damage. But it’s sometimes necessary to call in a soil engineer. If unstable or expansive soil has been identified as a problem, the foundation contractor may recommend installing piers through the process of piering, in order to support repaired sections of the foundation and to also connect the foundation to stable soil at greater depth.
Controlling soil moisture is critical
Correcting the damage done by expansive soil is more detailed than making the repairs. After you’ve repaired the cracking, corrected the settling, and restored the footings, walls and floors to their original condition; there’s still more work to be done. An experienced foundation repair contractor will suggest ways to control the water content in the soil that surrounds the house. Here are some water control measures:
• Remove large trees that grow close to the house, since they may take too much moisture out of the soil.
• Install a drip irrigation system to water vegetation around the house. Drip irrigation uses a minimal amount of water and maintains consistent soil moisture content.
• Make sure that gutters, downspouts and (if necessary) drain pipes carry water away from the house foundation.
• Slope the ground away from the house foundation so that ground water won’t saturate the soil that surrounds the house.