Swelling phenomena and effective stress in compacted expansive clays

Abstract

The central aim of this paper is to discuss the applicability of the effective stress principle as defined by Terzaghi (total stress minus pore-water pressure) to predict the behaviour of expansive clay aggregates. Phenomena occurring between individual clay minerals are reviewed first at the molecular level obtained in the colloid science research. In particular, it is noted that, for interparticle distances higher than approximately 1.5 nm, the pore-water pressure in the bulk equilibrium solution forms an additive component of the interparticle disjoining pressure. It is concluded that for these distances Terzaghi’s effective stress principle should be adequate to describe the clay behaviour. To support these developments, an extensive experimental database of nine different sodium and calcium bentonites available in the published literature was analysed. With the aid of double structure constitutive modelling, procedures were developed to extract information about the behaviour of clay aggregates from the experimental measurements. It was then shown that unconfined water retention curves, swelling pressure tests, swelling under constant load tests, and mechanical unloading tests are all uniquely related in terms of the dependency of dry density (or void ratio) of clay aggregate versus mean effective stress. By considering reversibility of aggregate behaviour and full saturation of aggregates, this implies that the effective stress principle is a valid way of predicting expansive clay aggregate volumetric deformation.