Derivation of soil water retention curve incorporating electrochemical effects

Water retention of clayey soils with wide particle size distributions involves a combination of capillary and adsorbed layers effects that result into suction–saturation relations spanning over multiple decades of matric suction values. The present study provides a physics-based analysis to reproduce the water retention curve of such soils based solely on particle size distribution and porosity. The distribution of inter-particle pore sizes is inferred through a probabilistic treatment of the particle size distribution, which is then used, together with an assigned pore entry pressure, to estimate the inter-particle water volume at a given suction. The contribution to water content from adsorbed layers is also taken into account by considering the balance of electrochemical forces between water and clay material. The total water content is therefore found by summing up the contribution of inter-particle water, as well as adsorbed layers that form around clay particles and around the individual clay platelets. Comparisons with experimental results on nine different soil samples verify the capability of the model in accurately predicting the wide water retention curves without any prior calibration. Additional to capturing the essential features of the water retention curve with remarkable detail, the analytical model also provides insights into the relative contributions of capillary and adsorbed waters to the overall saturation at different suction regimes. Being based upon easily accessible information such as particle size distribution and void ratio, the model can therefore be considered as a substitute for costly and lengthy laboratory and in situ measurements of water retention curve.