Soil is an essential component in the construction and stability of a house that is often overlooked by homeowners and home buyers. Since the house is built on soil, structural damage to a house can occur if the soil expands, contracts or slides.

Expansive clay soils
Throughout the United States, particularly in Texas, California, Virginia and Colorado (though not exclusively in these locations), expansive or reactive clay soils are known to cause adverse effects on residential structures. Expansive soil expands and contracts due to changes in the moisture content of the soil, causing structural problems through differential movement of the structure. If the moisture content and or soil type differs at various locations under the foundation, localized or non-uniform movement may occur in the structure. This isolated movement of sections of the structure can cause damage to the foundation and framing, evidenced by cracking of the slab or foundation, cracking in the exterior or interior wall covering (indicating movement of the framing,) uneven floors and/or misaligned doors and windows. This type of movement is usually associated with slab on grade construction that is common in the previously mentioned regions of the country. However, this type of movement also occurs in structures with basements and crawlspaces. The following image of a moderately reactive soil sample illustrates its effect when moisture is introduced.

Image A, initial view; Image B, after a small amount of water has been introduced; Image C, 48 hour time lapse.

A second effect of expansive soils is additional horizontal pressure applied to foundation walls found in basements and crawlspaces. Increased moisture in the soils adjacent to the foundation wall will cause the soils to expand and increase the lateral pressure applied to the foundation wall. If the foundation wall does not have sufficient strength, minor cracking, bowing or movement of the wall may occur. Serious structural damage to, or failure of, the wall may also occur.

A third effect associated with claystone soil (a type of expansive soil) is the movement of soils on unstable slopes. Expansive claystone soil, found as a layer under a more rigid top layer of soils, become unstable as the moisture content increases, allowing the claystone and the top layers of soils to move. If the soil is located on a slope, the top layer of soil can creep (slow movement) down hill or even cause a landslide (sudden and dramatic movement). Consequently, a house with an inadequate foundation built on unstable slopes can be subject to creeping of the structure down slope, or to failure of the structure in a landslide.

Possible Solutions
Pre-construction solutions: Prior to building the structure, a soil test of the site should be performed to ensure the soils are stable or to determine the approximate effect the soils will have on the structure. This will assist in determining if the soils are capable of properly supporting the structure. In addition, information on the soils can ensure that the foundation is designed to withstand the effects of the existing soil conditions, and assist in the development of plans for long-term maintenance.

Post-construction solutions: For structures already in existence, several possible solutions to counter the effects of expansive soils are available. Common preventative solutions include proper soil maintenance such as maintaining a uniform and constant moisture level in the soil. This may involve introducing moisture into the soils continually and uniformly to prevent shrinking; and/or preventing excessive or isolated saturation of the soil through proper drainage and grading techniques that prevent swelling. For structures affected by expansive soils, further movement can be prevented by providing additional strength and support to the foundation. This may include various methods of underpinning (to prevent vertical movement and/or sliding) and/or reinforcing of the foundation walls (to withstand lateral pressure).