The Websoil Survey website is essentially the digital version of the USDA Soil Survey that had been initiated many years ago. The Survey had publications for individual counties. The soils information data has been available for some time, but the ready access provided on-line has certainly enhanced it’s value.
The City of Amarillo’s Building Safety Department has taken advantage of this on-line access by incorporating the soil description provided in the Panhandle Residential Foundation Manual. For the most part, the Websoil Survey is useful in determining the soil type for any particular site. The described soil type can be utilized for the residential foundation design configuration. However, soil type data is not provided by the Survey in certain specific locations in the Panhandle region. Typically, these areas where no soil data is available are the areas where the soil type is usually lakebed type soil which is a fat clay. These fat clays are the most expansive clays found in the region.
Therefore, in areas where the soil data are most critical for building purposes, no information is provided. Under these circumstances, the Panhandle Residential Foundation Manual directs the builder to use the worst case more substantial foundation system provided by the Manual.
The Websoil Survey does generally provide useful soil type information. In some cases, it gives soil types at varying depths. However, the data is rather generic in nature, and cannot account for recent artificial changes, development modifications, or site specific variations that are probably not noted in the Websoil Survey.
Comments provided by the previous blog, “Initial Site Building Preparations”, where awareness of the history of any site conditions should be the order of the day.
The Panhandle Residential Foundation Manual’s specific design criteria are primarily based on the “effective plasticity index” of a building site. Reference to page 4-1, (Methodology for Determining Effective PI) of the Manual is noted.
The “effective plasticity index” is determined by an analysis of the weighted plasticity index (PI) values of the layers of the soils taken down to the depth of fifteen feet below the finished soil surface. This kind of information on a particular site is of considerably more value that assumed Websoil Survey provided data.
In fact, obtaining actual on-site data may prove to be more economical than reliance on on-line information. The site may be found to contain superior soils than those presumed by the on-line data, hence lower cost foundation.
I have encountered building sites, most particularly in terrain of significant elevation variations, where soil conditions vary within the site. Under these circumstances, it would be wise to examine the soils at the various changing locations.
It must be recalled, though, that at least the “effective plasticity index” must be determined. A surface soil sample is not sufficient. Other considerations such as site drainage, soil moisture contents, soil stiffness, or water table considerations may be called for.
In the long run, a careful understanding of the building site’s soil conditions are essential, whether determined by Websoil Survey or actual on site determinations.