Soils that shrink or swell are found throughout the United States. Soils with shrink/swell potential create difficult stability problems for buildings constructed on these soils. As the ground surface water content increases, the soil swells and expands upward. As the ground surface water content decreases, the soil shrinks and the ground surface recedes and pulls away from the foundation. These conditions particularly create problems in homes with shallow foundations. Read about Foundation Basics.
There are no perfect soil conditions for building houses. When a developer searches for land to build on, he selects land for various reasons, which may include availability, price, proximity to schools, and proximity to industrial areas. The main reason for selecting a parcel to develop is that there is a high demand of people who want to purchase homes in that area and a profit can be made from building and selling those homes.
The best way to find out if the soil beneath your house is expansive is to have a report done by a geo-technical engineer. In many housing developments a Soils Report is prepared, this requirement varies depending on the region or country.
A geo-technical engineer will drill soil brings 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 geo-technical 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 clay soil will swell/ expand when wet, and contract/recede when dry. If the foundation system is in the active zone (a shallow foundation), the foundation will expand/contract as moisture conditions change in the active zone.
• Select Fill/ Loam:
Select fill is normally defined as a sandy loam that is hardly effected by moisture variations. A building pad that is properly built with select fill/loam will support the foundation. Problems could arise if erosion occurs that effects 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 filter down under the foundation. Sand can also recede into a crack created by drying sub-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 in accordance with each soil type. 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 of the house to the other. 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.
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