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 a strong demand of 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 and take samples so these soil samples can be tested for expansiveness. These samples will show how expansive the soil is and at what depths. The geotechnical engineer will provide a written report on his 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’ deep.
A shallow foundation will be more impacted by soil and climate considerations than a deep foundation (see Foundation Basics for more information about shallow foundations).
Here is an overview of soil types:
Expansive Clay Soils
Expansive clays will swell/ heave when wet and contract/consolidate when dry. If the foundation system is in the active zone (a shallow foundation), the foundation will move as moisture conditions change in the active zone.
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 occurs that changes the bearing capacity of the soil.
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 in a crack created by drying soils and cause the foundation to drift (move horizontally).
Rock can erode and expand slightly only if it is a low density of shale. In some slope conditions, fractures/ 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 from one half to the other half. 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.
The USDA Natural Resources Conservation Service, formerly the Soil Conservation Service, has been publishing soil surveys for 100 years. A soil survey contains maps and a description of each major soil in the survey area.
USDA Service Centers are designed to be a single location where customers can access the services provided by the Farm Service Agency, Natural Resources Conservation Service, and the Rural Development agencies. This web site will provide the address of a USDA Service Center and other Agency offices in your area along with information on how to contact them.
You can find the center nearest you by clicking here:
Having a home built? You may anticipate a few things going wrong, but you'd expect your builder to erect your house on solid ground, right? Don't be so sure.
Read this excerpt from Ten Things Your Home Builder Won't Tell You, by Terrance Noland:
Population growth and urban sprawl mean there's not much residential land left in many areas. "What's left is not very good," says Daniel G. Knowler, a senior engagement manager at Navigant Consulting, which specializes in construction disputes. A lot of homes are being built on expansive soil — earth that swells when it rains — without adequate safeguards. In mid-1994, shortly after John Duffy and his family moved into their $234,000 home in Highlands Ranch, Colo., long cracks started showing up in the walls, and the porch started pulling away from the house. After badgering his builder for the soil report, Duffy learned his lot was a hot spot for potential swell. Writer Homes, the builder, was ordered to pay Duffy $544,000. John Palmeri, Writer's attorney, says the company offered to fix the Duffys' house, but "they were bent on going to court."
Swelling soil isn't the only problem. In March 1998, four hillside homes built on the site of an ancient landslide in Laguna Niguel, Calif., toppled after the unstable soil gave way. Early in 1999, Capital Pacific Homes (which had bought the builder, J.M. Peters), the lot developer, the grading company and the engineering company that checked the soil agreed to pay about $35 million to the homeowners, the homeowners association and the people whose condos at the bottom of the slope were also destroyed, according to Andrew Kurz, the association's attorney. Capital Pacific declined to comment.