The influence of geology on the expansive potential of soil profiles

Abstract

A survey of 446 brick-veneer, single-family homes in College Station, Beaumont, San Antonio, and Waco, Texas shows that the geology of the home site greatly influences the expansive potential of a soil profile. Effective plasticity index is used as a measure of a soil's shrink-swell potential; however, the assumptions upon which the effective plasticity index is based represent a simplistic approach to the problem. In College Station, the geological conditions provided subsurface wetting fronts which greatly enhanced the shrink-swell activity at low values of effective plasticity index. In Beaumont, San Antonio, and Waco, the depth of the active zone is important in controlling the expansive potential of the soil profile. The water table fixes the active zone at about 3 ft in Beaumont. Soils below this depth do not experience changes in moisture content and thus do not contribute to soil expansion. In the San Antonio study area, the active zone corresponds to the thickness of an expansive clay soil overlying a non-expansive limestone. At the Waco site, a relationship between expansive potential and plasticity index shows a positive but nonlinear relationship. It is suggested that effective plasticity index alone does not adequately define the expansive potential of a soil profile and that a thorough consideration of soil stratigraphy and depth of the active zone is necessary to predict the expansive potential of a soil profile. (Author)