All soils compress to some extent, and if the footing is not resting on bedrock, the foundation is subject to settlement. When the settlement is slight or uniform, or was anticipated in the design, it is of little concern. However, when the foundation settles unevenly (differential settlement), there is generally some cause for concern. Differential settlement introduces stresses that might seriously weaken the building. It often causes cracks in the foundation walls, unsightly cracks in the finished walls and ceilings, sloping floors and windows, and doors that bind. When the differential settlement is excessive, it can result in a structural Footing failure of a portion of the foundation wall, a condition that can be quite costly to correct.
The principal cause of foundation settlement is a reduction of the volume of the voids in the soil supporting the foundation. The voids are the spaces between the soil particles and contain air and water. Sandy soil contains large granular particles with a relatively small volume of voids; clay soil contains fine-grained particles with a large volume of voids. The settlement of foundations built on a sandy soil tends to be quick and slight, whereas the settlement of foundations built on a clay soil tends to be greater and might occur over many years.
Usually you do not have to worry about foundation settlement in older homes because by the time you are considering buying the house, the settlement has already occurred, along with any accompanying problems. And what you see is what you get. However, if what you see is a house out of plumb, with sagging sections, it might have structural problems, and as a precautionary measure, you should have the house evaluated by a professional.
Some soils are considered poor for build-ing-construction purposes. Homes built on a highly organic soil are vulnerable to excessive settlement when the soil dries out. Organic soil acts like a sponge, and as the soil dries, the organic matter shrinks. Depending on the amount of organic matter in the soil, the condition can cause differential foundation settlement.
Some soils on steep hillsides are subject to severe slippage. When the amount of buildable soil above an impervious subsoil layer is shallow, if the soil becomes saturated (because of an extended heavy rain), the entire upper layer of the hillside can slip to the bottom, along with any houses built on the side of the hill.
Some soils with a high clay content can swell or shrink up to 50 percent between wet and dry conditions. These soils are considered unstable, and unless special provisions are made during construction, the condition can cause excessive differential settlement cracks in the foundation.
If you have any questions about the soil the house is built on, you can usually get them answered at the local office of the U.S. Department of Agriculture, Soil Conservation Service, or at the office of the local county soil and water conservation district.