There are various types of soil that serve as part of a building's foundation. Different types can be combined together to form a more stable structure as many soil types fall into the expansive soil category. These soils shrink and well according to the water level, which can have negative effects on a building's foundation. The U.S. Department of Agriculture provides a map of the U.S. that lets builders and home owners know the type of soil foundations will rest on.
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Expansive soil swells when water content increases, and recedes when the soil water content decreases. This is particularly problematic for buildings with shallow foundations because the soil will pull away from the structure as it dries out. This type of soil is also referred to as "expansive clays," "shrink-swell soils" and "heavable soils," among other terms. The shrinking or heaving occurs in the active zone, which is an average of 18 feet deep. The water content changes are less problematic for the foundation the deeper it is.
Sandy loam is a stable soil that is not heavily affected by changes in moisture or temperature. It is also called select fill. This type of foundation soil can easily support a slab foundation. However, it is susceptible to erosion. If the sandy loam begins to erode under the foundation, builders may have to turn to slabjacking to repair the damage and stop further erosion. Slabjacking involves pumping the holes created by the erosion with grout--a water, Portland cement, Bentonite or flyash and sand mixture, plus additives to prevent shrinkage.
Like sandy loam/select fill, sand is very stable in terms of moisture but is subject to erosion. It is also susceptible to drifting. Basically, the sand can shift beneath the foundation and seep into cracks underground. This will cause the structure above to move horizontally and drift.