Effects of clay fraction and pore water conductivity on electrical conductivity of sand-kaolinite mixed soils

The interpretation of the electrical conductivity (σ_mix) of natural sand deposits, which contain varying amounts of fine-grained soils, is very complicated due to the presence of surface conduction. Therefore, the aim of this study is to evaluate/estimate the electrical conductivity of sand-kaolinite mixed soils with varying weight fraction of clay particles (CF=0–100%) at varying pore water conductivities (σ_w=0.013–3.356 S/m). The electrical conductivity formulas, including the original Archie's equation and three frequently used equations for clayey sands (i.e. the Waxman-Smits model, bicomponent model, and modified Archie's equation), were reviewed, and a number of electrical conductivity measurements on sand-kaolinite mixtures were performed in this study using a modified oedometer cell equipped with a 4 electrodes conductivity probe. The results of this study demonstrate that the measured σ_mix of tested sand-kaolinite mixed soils increases considerably with an increase in CF at low σ_w, while it does not increase at high σ_w, reflecting both an increase in surface conduction with an increase in CF and the insignificance of surface conduction at high σ_w. The comparison between the experimental results of this study and the three previous σ_mix estimating formulas for clayey sands demonstrates that the modified Archie's equation proposed by Glover et al. (2000) gives the most dependable estimate of σ_mix of tested sand-clay mixed soils. Finally, the matrix conductivity in the modified Archie's equation is empirically expressed as a function of the cation exchange capacity of soils in this study.