A Closer Look at Expansive Soil
Advisory Report - April 2001
A Closer Look at Expansive Soil
A key component in the stability of a house, which is often overlooked by homeowners and home buyers, is the soil on which the house is built. The stability of the soil and the design of the home's foundation play an integral role in the performance of the structure. If soils move by expanding, contracting or sliding, structural damage can occur to the home.
Expansive Clay Soils
If the moisture content 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.
Evidence of this movement can be identified by cracking of the slab or foundation, cracking in the exterior or interior wall covering, uneven floors 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.
This type of movement also occurs in structures with basements and crawlspaces, where the expansive soils apply additional horizontal pressure to foundation walls. An increase in moisture in the soils adjacent to the foundation wall will cause the soils to expand and increase lateral pressure applied to the foundation wall. If the foundation wall does not have sufficient strength, minor (or major) cracking, bowing or movement of the wall may occur.
Another effect, usually associate with claystone soil, is the movement of soils on unstable slopes. Expansive claystone soil can be found as a layer under a more rigid top layer of soils. The claystone layer of soils can become unstable as the moisture content increases, allowing the claystone and the top layers of soil to move. 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 affect 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 condition. Plans for long-term maintenance can also be developed.
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 by managing the water or moisture content of the soil to maintain a uniform and constant moisture level. 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.
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