Unsaturated soil mechanics: Critical review of physical foundations

Most constitutive models for unsaturated soils are based on identification of soil-water suction with the capillary component of the matrix potential, ignoring the contribution of adsorption to this potential. Identification of potential (energy per unit volume) with stress (or suction), is questioned, since these quantities have different physical significance despite their common dimensions. It is suggested that the identification of matrix potential with (ua − uw) results from neglecting the adsorption potential, and adopting an unrealistic pore space model. This identification was probably motivated by the laboratory axis translation technique, but it is not valid under normal field conditions where the air pressure is usually atmospheric, and soil water cannot develop high tension without cavitating. Axis translation alters soil behavior by preventing cavitation, thus casting doubt on the relevance of laboratory results obtained from these tests to actual field conditions. Specifically, in soils having large specific surface areas, there is a range of conditions, relevant to geotechnical engineering, in which capillary potential appears to account for only a small part of matrix potential, the major contribution resulting from water adsorption onto the soil particles. Consideration of a double porosity model and cavitation of water under the tension generated by capillary mechanism appear indispensable for the interpretation of unsaturated soil behavior.