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Shrink-swell soils are referred to by many names. "Expandable soils," "expansive clays," and "heavable soils" are some of the many names used for these materials.
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Soils are composed of a variety of materials, most of which do not expand in the presence of moisture. However, many clay minerals are expansive. When a soil contains a large amount of expansive minerals, it has the potential of significant expansion. When the soil contains very little expansive minerals, it has little expansive potential.
Goochland County has potential problem areas, however, with careful onsite investigation or analysis by structural engineers, foundations can be properly designed to address the potential movement due to shrink-swell soils. Prospective homeowners who are aware of these potential hazards can insist on a soil report and/or structural design from your contractor. You may also negotiate a warranty with your contractor against future problems.
The Department of Building Inspection cannot perform specific on-site evaluations. It does, however, provide information like the shrink-swell map and assists with interpreting and using the information. An engineered soil report by a qualified professional will address soil issues specifically related to your property.
An engineer’s soils report, also known as a geotechnical report, is a tool used to communicate site conditions as well as design and construction recommendations to building design and construction personnel. Specifically, soils reports provide detailed information on the interpretation and analysis of sub-surface soil, rock and water conditions. Based on this information the engineer makes precise design and construction recommendations for such things as footing, basement and retaining wall designs.
For all new single-family residences, duplexes, townhomes, residential attached structures (including but not limited to additions, screen porches, sunrooms, or any expansion to the footprint of the existing structure), detached structures exceeding 400 square feet in size and all commercial construction.
Shrink-swell soils are present throughout the world and are known in every U.S. state. Every year they cause billions of dollars in damage. The American Society of Civil Engineers estimates that 1/4 of all homes in the United States have some damage caused by shrink-swell soils.
Even though expansive soils cause enormous amounts of damage, most people have never heard of them. This is because their damage is done slowly and cannot be attributed to a specific event. The damage done by shrink-swell soils is then attributed to poor construction practices or a misconception that all buildings experience this type of damage as they age.
There are some ways to minimize problems associated with shrink-swell soils without resorting to redesigning the foundation. These techniques involve keeping expansive soils from either expanding or shrinking too much around the foundation of a house by maintaining a relatively constant moisture content. Here are a couple of ways to help maintain soil moisture content:
1. Remove large trees and bushes that grow within about 10 feet of the house. Large plants tend to dry out the soil especially during the summer months.
2. Utilize drip irrigation systems to water vegetation. This minimizes the use of large volumes of water at one time.
3. See that down spouts and roof gutters disperse run-off away from the foundation. If possible, direct roof water into closed pipes that empty onto the street or other suitable location away from the foundation. This is the single most important step that can be taken to help minimize the average expansive soil problem.
4. Slope grade away from the foundation a minimum of 6” in the first 10’ of grade (0.5%). This aids runoff and helps prevent water from puddling or seeping into the ground. This simple step can significantly decrease a problem.
Soil Survey of Goochland County, Virginia 1968-1975
The original policy was created in the late 1990’s and based from paper maps. Since that time, the maps have been digitized allowing staff to better analyze the data. Staff determined that the presence of shrink swell soil is more prevalent than contemplated in the original policy. To protect the citizens of Goochland County, a revised policy and map were created that identify most of the County as having the potential for shrink swell soil.
A design professional such as an architect or a structural engineer will be able to evaluate your house to determine if you should hire a contractor.
The 2015 Virginia Residential Code, (VRC) Section R401.4 states that the Building Official shall require soils tests to determine if shrink swell soils exist in areas that are likely to have shrink swell soils.