Predicting soil—water characteristic curves of compacted plastic soils from measured pore-size distributions

The unsaturated hydraulic conductivity function, k, is often predicted from the soil–water characteristic curve (SWCC). Most methods implicitly or explicitly derive a pore-size distribution (PSD) from the SWCC, which is then used to calculate k at any suction. Two important factors ignored by most methods are the change in the PSD during the SWCC test, and the influence of pore geometry on the SWCC. In this paper the SWCCs and evolution of the PSDs of four fine-grained compacted soils are measured. The SWCCs are measured from 0 to 3000 kPa by the axis-translation technique, and the PSDs are obtained by mercury intrusion porosimetry. A method is developed to predict the SWCCs from the measured PSD distribution data. Processes modelled include the effect of pore geometry on shrinkage during the SWCC tests, the isolation of water by the invading non-wetting phase (air), and differences in the relative accessibility of pores from the surface of the mercury intrusion and SWCC test samples. Different methods to model the above phenomena are developed and compared. The best predictions of gravimetric water content lie between one and two standard deviations of the measured SWCC data.