The place of expansive clays in the framework of unsaturated soil mechanics

Abstract

This paper reviews and re-examines the place of expansive clay soils within the framework of unsaturated soil mechanics. Direct and indirect physical evidence is presented and discussed, which recognizes that natural expansive clays remain perpetually saturated (i.e. degree of saturation is one) as they gain or lose water over the suction range that is of interest to engineers, and that is likely to be encountered under normal field conditions. As a consequence, expansive soils do not have a unique saturated water content. Perpetual saturation is a consequence of the small particle size of smectitic clays, which in a structured clay soil, leads to very small pores and a high air entry value. The behaviour of structural soil elements is distinguished from the behaviour of the total soil mass, to consider expansive clay soils as composite soil materials on a macro scale, composed of saturated, structured soil peds, separated by air-filled cracks. It is considered that peds of expansive clay soils in natural environments evolve, under cycles of wetting and drying, to attain an equilibrium micro-structure which allows them to shrink and swell by significant amounts in a completely reversible way. As peds remain saturated during wetting and drying, volume change behaviour is simplified, because the volume of water gained or lost from the soil equates directly to the volume change of the soil peds, and to the change in crack volume in the soil mass. This behaviour is discussed in the context of the normal shrinkage concept as used in soil science. It is suggested that constitutive models formulated in a continuum mechanics approach may be unsuited to the modeling of desiccating expansive clay soils even though they are commonly used.