There are just a few regions of the country where the soil consists of expansive clays such as smectite, bentonite, montmorillonite, beidellite, vermiculite, attapulgite, nontronite, illite and chlorite or certain sulfate salts that will expand with changes in temperature. Although every region in the United States has experienced the problem, the worst areas include parts of eight states. Perhaps the urban areas most affected by problems of expansive clay soils are here eastern and central Texas including Fort Worth, Dallas, Austin, San Antonio and Houston.
The problem is that expansive clay is very responsive to seasonal moisture and temperature changes. Texas has a variable climate with monthly average temperatures ranging over 54 degrees from warm to cool extremes and monthly precipitation ranging from less than 2 inches to nearly four-and-a-half inches. This kind of variability causes large shifts and pressures in expansive clay soils which makes the building foundations unstable and vulnerable to damage. During the wet seasons in early Spring, the clay absorbs moisture and expands, then during the drier season in late Summer, the clay quickly contracts as it dries.
Uneven drying is one problem. Because of the highly expansive nature of the soil, trees and other large plants can make the foundations of homes settle unevenly. The roots of trees consume the moisture from the soil causing the soil to shrink much faster during dry weather. Moderate to large trees consume 50 to 75 gallons of water from the soil daily. The soil where the moisture is lost more rapidly will sink lower than the surrounding soil causing uneven-ness in the foundation supports.
Uneven moisture accumulation is another problem. Wet spots caused by dripping faucets, leaky drains, air conditioning condensate, leaky water pipes and other sources can make the foundation sink into the areas where the clay is kept wet while the soil dries and shrinks in other areas causing the foundation to shift and move unpredictably. Standing water or water running along the side of a foundation during rain can cause soil that would have supported the foundation to be dissolved and washed away into the rocks.
- If the soil under the rectangular slab expands because of increased moisture or other factors in the clay, the expansion will be uneven. The corners and edges of the slab will lift more than the center since there is less confinement. Because concrete is such a rigid material, uneven lifting may cause substantial cracking in the concrete slab.
- If the foundation is supported by shallow piers, constructed during beginning of the rainy season, the piers will be supported by friction with the soil. After a number of rain storms, the soil swells, lifting the house and piers. Then in the dry season, the groundwater table falls and the soil dries and contracts. Tension cracks may grow around the piers of the foundation. When the adhesion between the pier and the soil decreases, the foundation pier may sink.
- Sewer and water pipes sunk too shallow into the expansive clay layer of the ground are also subject to stresses caused by lifting and falling. Once a pipe breaks or cracks, the leakage will exacerbate the pressure on the house foundation.
Planning for Construction in Expansive Clay Soil:
- Homes should be designed to minimize moisture content changes with extensive drainage and to be insulated from soil volume changes.
- When homes are built, they should be constructed in a way that does not change the condition of the soil.
- Steps should be taken to maintain a constant moisture level in the soil after construction. The soil should be watered regularly during extended dry periods.
- If trees are planted near the foundation, their roots should be cut and capped to limit their consumption of water from the soil.
- Foundations and plumbing should be put in deep, below the expansive clay layer.
There are methods of lifting sunken parts of a foundation using mechanical jacks or “slabjacking” the edges with special grout. Sometimes differential watering or drying can adequately level affected parts of the foundation. Sometimes rebuilding parts of the slab or piers below the expansive clay layer is a feasible, though expensive solution.
Perma-Pier Foundation Repair has extensive experience with foundation repair in Texas expansive clay soils. Contact us for more information.