Foundation Basics

A homes foundation is in direct contact with the ground and joins the buildings structure with the underlying zone of soil or rock. The foundations job is to transfer the structures load to the underlying soil or rock, without excessive settlement or movement.Movement of the soil beneath a home can severely damage the foundation. That’s why the soil beneath your home is often referred to as your second foundation.

Slab-on-grade foundations are shallow foundations that are most often constructed of reinforced concrete. Slab-on-grade foundations can be built quickly and are relatively inexpensive to build. The function of a slab-on-grade foundation is not to resist or limit the amount of heave that might occur beneath a slab foundation, but to move up and down with the shrink and heave. Slab-on-grade houses do not have basements.

Shallow foundations are susceptible to seasonal movement from rainfall, drought, freeze/thaw cycles, and temperature changes and transpiration of moisture thru the roots of large plants and trees.

Pier and beam foundations, as the name suggests, is a concrete footing and pier which supports wood beams and floor joists. These foundations usually have crawl spaces underneath the home.

Basements are most often constructed in northern climates where freeze thaw conditions occur and the footing depth must extend beneath the frost line- often four or more feet below the surface. In many of these cases, builders will go ahead and excavate for a basement and build basement walls that provide the support for the house. The bottom of the basement is typically below the depth over which the majority of the soil’s shrink or swell due to climate occurs. Basements can suffer basement floor heave and lateral wall movement, however.

Deep foundations reach depths that are not normally affected by seasonal environmental changes and are considered to be out of the zone of influence. There are a multitude of other types of both shallow and deep foundations.

Causes of Foundation Problems

Water is the main enemy in expansive soils problems. There is either too much water, causing the soil to swell, or not enough, causing the soil to shrink.If all soils beneath a foundation swells uniformly or shrinks uniformly it is unlikely to cause a problem. But when only part of the foundation heaves or settles, differential movement causes cracks and other damage.

Most differential movement is caused by differences in soils moisture. After construction, soil beneath part of the foundation becomes wetter or drier than the rest of the soil.

Here is why this happens:

When there is a gain in soil moisture

This is the most serious threat since the swelling potential of expansive soils is much greater than the shrinkage potential. Moisture gain can come from plumbing leaks, subsurface water like wet weather or a high water table, or surface water. Surface water is improper drainage of landscape water or rainwater.

Poor drainage can be a major contributor to soil moisture gains. Roof runoff should be directed away from the house through the use of gutters. Gutter downspouts should not be permitted to discharge the water next to the foundation. Surface drainage next to the foundation should slope away from the house approximately ¼ ” per foot.

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.

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.

A shallow foundation will be more impacted by soil and climate considerations than a deep foundation. (see Foundation Basics above for more information about shallow foundations).

Soil Types

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

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

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.

Other Resources

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: soils.usda.gov

Foundation Repair Network is a resource for both homeowners and engineers (and others in the building community) to get solid information about foundation repair.

www.foundationrepairnetwork.com

The Concrete Network is dedicated to showing site visitors what is possible with concrete and how to find a contractor to get the job done.
www.concretenetwork.com