University of Cambridge > > Engineering Department Geotechnical Research Seminars > Unsaturated soil mechanics - Critical review of physical foundations

Unsaturated soil mechanics - Critical review of physical foundations

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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. 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 suction as (ua-uw) results from neglecting the adsorption potential, and adopting an unrealistic pore space model. Experimental studies of unsaturated soils commonly use axis translation, wherein “suctions” are established by simultaneously increasing air and water pressures to the desired (ua – uw) value. This procedure ignores the fact that under normal field conditions, the soil air pressure is atmospheric, and the soil water cannot develop tension greater than about one atmosphere without cavitating. Axis translation alters soil behavior by preventing cavitation, thus casting doubt on the relevance of common laboratory results to actual field conditions. It is suggested that capillary potential accounts for only a small part of matrix potential, the major contribution resulting from water adsorption onto the soil particles, particularly in soils having large specific surface areas. Consideration of a double porosity model appears essential to the interpretation of unsaturated soil behaviour.

This talk is part of the Engineering Department Geotechnical Research Seminars series.

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