We have clay in our soil in most of Oklahoma. Kaolinite and Illite are very common. We even have some very expansive Montmorillonite in some areas. Some locations have a high concentration of clay and other locations a different mixture of clay and fertile loam. Clay is always cycling between being moist (expanding) and holding water, to shrinking and becoming dry and cracked. For this reason, untreated clay is not a stable building surface. Clay has been used for centuries to create firm, smooth surfaces for buildings, dams, even ponds. However, clay has one “imperfection.” It possesses tiny electrical charges that attract water. While these charges are very small and pose no problem individually, when millions of them are combined into a large clay mass, their ability to attract water becomes significant. Clay masses attract moisture on a grand scale, attracting huge quantities of water. The vast numbers of small voids or capillaries act as passageways for the water. These capillaries exist throughout the clay mass, like infinite “veins” where water can travel. As the water is pulled into these tiny pathways, the soil swells to make room for the water. After clay has absorbed water once, the capillaries provide passageways for the water to flow in and out of the clay depending on factors such as rainfall, irrigation, temperature, etc. This causes the characteristic cracked look of dry clay, as it shrinks back to its normal size. Depending upon the depth of the clay, the swelling creates pressures that could easily be 5,000 to 9,000 lbs/SF against foundations, swimming pool walls, and paved surfaces. As a result, these pressures can easily break apart roads and lift a home or office building off its pier foundation. Our own testing of Tulsa soils revealed a clay/loam mixture with an expansion of at least 3.5% to 7% of volume. This may not sound like much, but if you consider one dimension of your building foundation can easily be 50 feet, your foundation could move horizontally by as much as 21 inches in pure clay! If your building sits on clay that is only 5 feet deep, your foundation or floor slab could move over 2 inches vertically!

As incredible as it seems, in times of drought, reduced water usage requirements, and even rationing by various municipalities, that is exactly what some experts are instructing homeowners to do, is keep the soil moist around the foundation of your home or building. Why do they ask you to do this? We’ve already learned that when clay gets wet, it swells. And when it dries out, it contracts or shrinks. So the logic in watering dictates that, if you keep the moisture constant, then the clay soils will not swell or shrink. In some locations, such as in Texas where there is a preponderance of clay in the soil, a new home or building will be constructed by injecting a lot of water into the clay before the foundation or slab is poured. The idea is that if you put the maximum amount of water in the clay that it can hold, then it cannot swell any more and once the water is under the slab, it does not evaporate. Makes sense, right? Unfortunately, it is not that simple.

Drying  of the soil can be simply caused by abnormal drought or wet conditions that are not normal for the climate or from trees and shrubs that have been planted too close to foundations. Trees can dry out the soil beneath a building or paved area in one of two ways.  The tree as it increases in size can actually expand its root system to areas beneath the building foundation or paved area.  This is one way. The second and less commonly understood way is very indirectly drawing water from below the foundation or paved area. According to many studies both in the U.S. and Europe, it has been found that a tree can have a zone of influence, whereby moisture is extracted from the soil, over an area 1-1/2 times its height. That's right, 1-1/2 times its height! This effect occurs due to capillary action and hydrostatic pressure in the soil. The tree will directly extract water from the soil directly in contact with the roots within the drip line. Once that moisture has been extracted, water will move in to the same area from surrounding areas of the soil because of the hydrostatic pressures and capillary action of the soil particles. This is what causes the zone of influence of trees to be so large. Although local wisdom recommends perimeter watering of a foundation during dry seasons, it has been our personal experience that no reasonable amount of perimeter watering will keep up with trees and other vegetation that is planted around or on the same lot as a house. It would make more sense watering the trees and shrubs rather than the perimeter of a foundation. If the trees have an adequate supply of water, then the home slab and foundation will be happier. Mature large trees can consume anywhere between 100-250 gallons of water per day per tree. So, if you want them to not draw moisture from under your home foundation and make it unstable, bring plenty of $. In Tulsa, OK the 2011 water rates are $2.35 per gallon of water. This means that during the drier months of June-September in Oklahoma, the replenishing the tree water could be costing you up to $15 per month per tree. There is a better solution to this problem. Make your foundation immune to the seasonal wet and dry cycles with Condor SS soil stabilization. Condor SS will not allow the trees and vegetation to extract moisture from under the foundation because there will not be any free excess moisture for them to extract. Not only will Condor SS minimize the moisture changes under your home or building, it will allow all of the rainfall to become available for the vegetation.