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

The interpretation of the electrical conductivity (σmix) 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 (σw=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 σmix of tested sand-kaolinite mixed soils increases considerably with an increase in CF at low σw, while it does not increase at high σw, reflecting both an increase in surface conduction with an increase in CF and the insignificance of surface conduction at high σw. The comparison between the experimental results of this study and the three previous σmix estimating formulas for clayey sands demonstrates that the modified Archie's equation proposed by Glover et al. (2000) gives the most dependable estimate of σmix 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.