Soil Conditions

Soils with the potential to shrink or swell are found throughout the United States. Soils with this shrink/swell potential create difficult performance problems for buildings constructed on these soils. As the soil water content increases, the soil swells and heaves upward. As the soil water content decreases, the soil shrinks and the ground surface recedes and pulls away from the foundation. These problems are of particular concern in homes with shallow foundations. See Foundation Basics.

Homes are normally not built in areas where the soil conditions are perfect. The developer selects land for various reasons, which may include availability, cost, proximity to industrial areas, and proximity to schools. The main reason for selecting a parcel to develop, of course, is that there is strong demand from people who want to buy homes in that area, and a profit can be made from selling the homes.

The best way to find out if the soil beneath your house is expansive is to ask a geotechnical engineer. In many housing developments a soils report will have been prepared, but this requirement varies depending on the region of the country. A geotechnical engineer will make soil borings on your site, take samples, and test them for expansiveness at various depths, then provide a written report on the findings.

What is the "active zone"?

From the ground surface downward, there is a depth over which expansive soils experience a change in moisture conditions as the climate (or seasons) change. This results in the soils shrinking or heaving. This zone is an average of 18 feet deep. A shallow foundation will be more impacted by soil and climate considerations than a deep foundation (see Foundation Basics for more on shallow foundations).

Soil types

Expansive clay soils

Expansive clays will swell and heave when wet and contract and consolidate when dry. If the foundation system is in the active zone (a shallow foundation), the foundation will move as moisture conditions change.

Select fill / loam

Select fill is normally defined as a sandy loam that shows little change with moisture variations. A building pad properly built with select fill/loam will support the foundation. Problems could occur if erosion changes the bearing capacity of the soil.

Sand

Sand will not change as moisture conditions change. However, sand can erode if drainage around the lot allows water to work its way under the foundation. Sand can also fall into a crack created by drying soils and cause the foundation to drift (move horizontally).

Rock

Rock can erode and expand slightly only if it's a low density of shale. In some slope conditions, fractures or faults in the rock can allow sliding and failure if not properly pinned with tie-back anchors.

When a structure is supported by various soil conditions, the house may move differentially. As an example, if one half of the foundation sits upon expansive clay and the other half bears on select fill and/or rock, the amount of seasonal movement will vary between the two halves. If the foundation system is not properly designed, the differential movement may cause damage to the foundation and structure. Many times building pads will be cut and/or filled so the bearing soil is all of the same type.

Other resources

The USDA Natural Resources Conservation Service (formerly the Soil Conservation Service) has been publishing soil surveys for more than 100 years. A soil survey contains maps and a description of each major soil in the survey area. Your local USDA Service Center can point you to the survey for your area.

Find your local USDA Service Center →

It pays to ask

Building on land with poor or expansive soil, without adequate safeguards, is more common than most homeowners expect, and disputes over undisclosed soil conditions have led to costly litigation between homeowners and builders in multiple well-documented cases. If a soils report exists for your lot, ask for it before you assume the ground beneath your home is stable.