Soil swelling occurs when minerals in the soil absorb water and get bigger. Some of these minerals, which are found in many different types of clay soils, can expand up to 30 percent in volume as they absorb moisture, and swelling soil can exert up to 30,000 pounds of force per square foot on structures or anything else that gets in its way as it expands. To make the situation worse, soil that has swollen during a wet period will shrink again as the soil dries, and repeated cycles of swelling and shrinking can be particularly damaging to structures.
Soils that are prone to swelling contain clay minerals such as bentonite and montmorillonite, particles of which are especially expansive when they're exposed to moisture. Signs that soil may contain these or similar minerals include cracks, fissures or a popcornlike texture on the surface of the soil, which results from the shrinkage that occurs when the swollen soil dries out. These signs will disappear when the soil is moist and swollen, so you should look for them during dry periods.
Soil swelling can be extremely destructive to structures and pavement. Where it pushes against buildings beneath the soil surface, swelling soil can cause foundation cracks or displacement. Repeated cycles of swelling and shrinking cause damage similar to frost heave, including cracks and buckling in driveways, sidewalks and roads. Swollen soils also restrict the flow of water through them, and this action can cause drainage problems and interfere with the function of septic systems.
One option for reducing the tendency of soil to swell is to excavate and replace the existing clay-laden soil with soil mixtures that contain sand and other components that are less prone to swelling. Adding compost to soil may also decrease its tendency to swell, although not as substantially as sandy fills. You can also lessen swelling without replacing the soil by installing drainage systems that move moisture through the soil as quickly as possible and by avoiding landscaping that directs runoff water toward the problem areas. You should also avoid planting trees and shrubs close to building foundations; landscaping may exacerbate soil swelling near the building as the plantings are irrigated, and when the plants take up water, they will hasten soil shrinkage.