Electrical Resistivity Method for Water Content Characterisation of Unsaturated Clay Soil

Abstract

This thesis presents an automated multi-electrode resistivity system which was developed for the water content characterisation of unsaturated clay soil. The system controls 64 electrodes in a fully automated procedure, offering continuous real-time data acquisition, which is one of the recent advances in resistivity instrumentation. The system was tested using a wide range of high precision reference resistors and different soils, and validated using commercial standard instruments. The results indicated a high precision, accuracy and resolution of the outputs, with a measurement error of 0.19% (maximum 0.80%) in a four-electrode method and 0.21% in an automated acquisition mode.
Water content characteristics of mechanically compacted BIONICS clay soil was extensively investigated, with particular focus on the effect of wetting, drying and cracking on soil properties. It was found that the electrical resistivity of soil is sensitive to water content and compaction conditions. Experimental relationships that relate soil resistivity, volumetric water content and degree of saturation were developed. These relationships are useful to estimate the in situ water content. The resistivity behaviour of clay soils subjected to drying and wetting procedures was discussed. Soil water content and microstructure changes are key controlling parameters for resistivity behaviour. Numerical and experimental techniques were used to characterise cracking in clay soils. The results showed that cracks have anomalous high resistivity values that can be distinguished from the background, and changing cracking depth, length, width and orientation causes significant changes in soil resistivity. As the cracks form barriers that disturb the flow of electrical current, the depth and length of the crack have the major influence on soil resistivity.
It was concluded that the degree of saturation or volumetric water content is a more reliable parameter than the gravimetric water content to calibrate in situ resistivity data against water content and soil resistivity can be used as a useful indicator for monitoring water content changes in clay soils subjected to drying and wetting cycles.