An experimental investigation on the independent isotropic stress variables for unsaturated soils

The isotropic stress state in an unsaturated soil is generally described by three measurable stress variables: the mean total stress, the pore air pressure and the pore water pressure. Constitutive models of unsaturated soils presented so far have been developed on the basic assumption that only two of these variables are actually independent. As a consequence, the experimental data based on the axis translation technique have been extrapolated to real conditions, where the pore air pressure is atmospheric and the pore water pressure is negative. However, experimental evidence to support this assumption is still limited. In an attempt to provide a better understanding of the response of unsaturated soils, a specific experimental programme has been planned and completed. To this aim a new apparatus to test unsaturated soils in a broad range of degree of saturation and negative pore water pressures was designed and constructed. Using this apparatus a total of eight tests was performed on saturated and unsaturated kaolin samples consolidated from slurry and air dried to degrees of saturation ranging from 0·5 to 0·8 with corresponding negative pore water pressures in the range –1400 to –1100 kPa. In this paper the apparatus used in the experimental programme is described and the results obtained so far are presented and discussed.