On the relationship between microstructure and electrical and hydraulic properties of sand-clay mixtures

A series of laboratory experiments, including measurements of electrical pr operties, permeability, and porosity, were performed on saturated sand-clay mixtures. Different mixtures and packing geometries of quartz sand and 0 t o 10% Na-montmorillonite clay were investigated using solutions of CaCl2 an d deionized water. Two main regions of electrical conduction exist: a regio n dominated by surface conduction and a region where the ionic strength of the saturating fluid controlled conduction. For low fluid conductivities, t he sample geometry was found to greatly affect the magnitude of the surface conductance. The influence of the microstructural properties on the electr ical properties was quantified by estimating formation factors, Lambda-para meters, and surface conductances. The surface conductances estimated using the theory of Johnson ct al. [1986] agreed well with measured values. We su ggest that high and low bounds on the expected surface and bulk conductance s in a natural system can be derived from the measurements on these artific ial geometries.