A modified series-parallel electrical resistivity model of saturated sand/clay mixture

Electrical resistivity tomography, ERT, is a geophysical method that is used to image subsurface geomaterials for various geological, environmental and geotechnical engineering problems. The success of ERT depends on the availability of a robust electrical resistivity model of soils that is able to characterise the different type of soils using its electrical resistivity measurements. The aim of this paper is to present an electrical resistivity/conductivity model for isotropic saturated sand/clay mixture that considers the effects of sand content, and surface conduction of clay content. The proposed model is formulated using a modified series-parallel mixing model theory which uses phase-volumetric and surface conduction model parameters. The phase-volumetric parmeters describe the configuration of the different soil/water phases in a representative soil unit cell which includes solid soil particles, free water, diffuse double layer (DDL) water. The surface conduction model parameters are used to describe the volume fraction of DDL water in the unit cell, and its electrical conductivity compared to the electrical conductivity of free water. Simple test methods are used in this study to determine the proposed model parameters. Laboratory experimental programme was also conducted in this study to validate the proposed model. Two types of clays, namely, kaolin and bentonite, and one type of clean sand were used to constitute five different sand/clay and clay/clay mixtures. Dynamic compaction method using Standard Proctor tools was used to constitute the testing samples. The electrical conductivities of different reconstituted soil samples were measured and compared with the proposed model predictions, and a good agreement was achieved. The small differences between the model predictions and the experimental measurements could be attributed to the slight deviation of the laboratory test samples from the model assumptions, which require the soil sample to fulfil fully saturated and electrical isotropy conditions.