Study of the electrical resistivity of compacted kaolin based on water potential

In clayey soils the electrical current flows through the liquid phase (free water) and the surfaces of the electrically charged clay minerals, where water is adsorbed. The distinction between the two paths cannot be done by the usual way of considering only the degree of saturation (volume of water) or the gravimetric water content (mass of water). In this paper this distinction is made by relating water potential with electrical resistivity for fixed degree of saturation. This is because the relationship between water potential and degree of saturation, named as water retention curve (WRC), reflects the presence of the water in the free and in the adsorbed states. For this purpose, the electrical resistivity of kaolin compacted with different void ratios was measured in samples having different water potentials applied using vapour equilibrium technique. Experimental data were interpreted considering the continuity of the liquid phase and the influence of soil structure. The equation used to define the WRC was considered in the interpretation of soil electrical resistivity for different degrees of saturation: the electrical resistivity is clearly dependent on porosity below the air entry value, while it depends on the degree of saturation in the transition zone, before reaching the residual state of saturation. For suctions above this residual state, water is assumed to be only adsorbed to the surfaces of the clay minerals and electrical current flows only through the bridges between these charged surfaces The good adjustment of the experimental results achieved when the shape of the WRC was considered instead of using the usual expressions found in the literature allowed to validate this approach.