Geoelectrical Characterization for Relating Electrical Resistivity and Geotechnical Properties of Soils

Abstract

This thesis proposes electrical resistivity measurement as a potential method to characterize different types of geomaterials under different environmental conditions. Electrical resistivity method is a promising non-destructive method for subsurface investigations compared to conventional geotechnical methods which pose difficulties in obtaining good quality undisturbed soil samples. This method provides a simple way to identify the variation of geotechnical properties as well as the presence of heterogeneity in the subsurface. Integration of electrical resistivity with geotechnical properties and understanding their interactions with each other allow characterizations of soils properties. The focus of the thesis is to relate the electrical behaviour of various types of soils to changes in their physical and mechanical properties. Controlled laboratory investigations are carried out on different types of soils such as fine sand, medium sand, coarse sand, bentonite, sand-bentonite mixes, mine tailing, red soil, kaolinite, black cotton soil and gravels. To study resistivity variation with physical properties, resistivity boxes are fabricated using acrylic sheet to facilitate four-electrode method of measurement. For investigating resistivity changes with engineering properties, the conventional triaxial setup is modified using two-electrode method, and the rigid wall falling head permeameter and oedometer are modulated using Wenner four array method. Prior to testing soil samples in fabricated apparatus, calibration studies are carried out using standard salt solution. The experimental investigations are compensated with model studies in an acrylic tank to detect the anisotropies and numerical tomography studies. The work in this thesis can be divided into four major parts. The first part of the thesis focuses on the assessment of sand-bentonite mix characteristics using the electrical resistivity method. Effects of particle size, surface conductivity, volumetric water content, dry density, void ratio, bentonite content, and temperature on electrical resistivity is investigated. In the second part of the thesis, an attempt is made to characterize mine tailings using electrical resistivity method through a series of laboratory experiments. Results from triaxial compression tests, permeability tests and electrical resistivity studies are coupled to establish correlations between electrical resistivity and engineering properties of tailings including permeability and shear strength. The third part of the thesis deals with the effectiveness of the electrical resistivity technique for geo-electrical characterization of compacted clays. A comprehensive study is carried out to investigate the variation in the electrical resistivity with change in mechanical characteristics of black cotton soil, bentonite, kaolinite clay, and red soil. Observations show that soils belonging to the same classification group can behave electrically different due to the differences in their mineralogical compositions. During consolidation, the changes in soil structure and expulsion of water voids alter the electrical pathways, which result in increased resistivity in case of red soil and reduced resistivity in case of black cotton soil. In the fourth part of the thesis, subsurface anisotropy is studied for sand, red soil, black cotton, and gravels using the square array method that allows measurement of resistivity along different azimuths. The electrical anisotropy of the clayey samples is studied with induced anisotropy in the form of cracks. Finally, azimuthal cross-square array resistivity soundings are carried out in field to investigate the features of anisotropy of the subsurface. Further, numerical investigations are carried out on simulated models of soil deposits to detect the inhomogeneities or anomalies in the subsurface using electrical resistivity distributions. Based on the studies carried out in this thesis, electrical resistivity technique is found to be very efficient for basic characterizations of all types of geomaterials, quick evaluation of their engineering properties with reasonable accuracy and anisotropy and fault detection in soil or rock layers. Results from this study have great significance for many applications like quick subsurface surveys of large areas, geotechnical characterizations of different materials including hazardous wastes, detection of leakage in landfills, detection of faults and fractures in rocks, identifying failure zones in slopes for early detection of landslides and detection of anisotropy in soil deposits.