Experiences in Identification and Treatment of Dispersive Clays in Mississippi Dams

Twenty floodwater retarding dams in Mississippi have been treated to protect them from failure due to tunnel erosion. All of these sites developed severe tunneling characteristics of dispersed soils because of extreme rains in 1973, causing rapid acceleration of the probably latent soil condition.

The Soil Conservation Service in Mississippi initiated a detailed sampling and testing program for every site, not only to design treatment, but also to provide a basis for followup evaluation of performance. All three laboratory tests—dispersion, pinhole, and chemical—correlated well. Field identificaton tests did not correlate as well to actual site conditions or laboratory tests. Field tests were crumb, ultraviolet light, and turbidity.

The primary treatment used was hydrated lime applied in a 15-in. blanket following reworking of the fill to eliminate tunnels. Lime applications varied from 0.5 to 3 percent. Two structures were treated with sand-gravel blanket.

The pH values for lime-treated soils stayed high following application, restricting root penetration and vegetative growth.

Laboratory data coupled with field testing and performance observations provide engineers with a unique opportunity for studies that will evaluate and improve identification and treatment of dispersive clays. Some comparisons are already available and are presented in this paper.