Integration of geoelectrical and geotechnical parameters of lateritic soils through laboratory and field studies

Abstract

All physical matters involve electricity. Soil has been considered as a specific electrolyte with free ions in the pore water and free electrons in the electrical double layer. This electrical nature of the soil has been studied, and applied to predict various soil parameters by various researchers. In this study, electrical resistivity (ER) of laterites and lateritic soils at controlled and natural field conditions are measured and compared with various engineering properties. Laboratory measurement of electrical resistivity is done using a soil resistivity box, a dc power supply and two high precision multimeters and the field ER measurements are done using signal stacking resistivity meter. The effect of various geotechnical parameters such as water content, dry density, porosity, degree of saturation, percentage of ions and degree of compaction, in controlling the electrical resistivities of lateritic soil samples are studied. Quantitative correlations are obtained between strength and electrical resistivity of lateritic soils, in regulated laboratory conditions. Vertical Electrical Soundings (VES) were conducted at 14 locations in NITK campus. Standard Penetration Tests (SPT) were also conducted up to 10 to 12m depth at the same locations where VES were conducted. True resistivities at different soil layers interpreted were correlated with the SPT blow counts at the same depth. Overall, there exists a good correlation between SPT and ER. A comparison is made on the laboratory and field electrical resistivity in lateritic formations for surface soil samples. Electrical behaviour of soil stabilised with cement/lime is also studied. Quantitative correlations are developed between electrical resistivity and strength parameters. The multiple regression models developed can be used to predict the 7th day unconfined compressive strength of the soil-cement/lime mix, in the freshly prepared state itself, so that if it doesn’t meet the performance criteria, it can be remixed with additional cement/lime and wastage of material can be prevented. A graphical method is introduced in this study which predicts the shrinkage limit (point of just saturation at maximum compaction) of the soil. The results of this research, propose that by properly managing the uncertainties and ubiquitous resistivity measurement errors, Electrical Resistivity tomography can be applied as a pre-investigation method in sites, preceding to direct testing methods like Standard Penetration Test to reduce labour, cost and time involved and to increase efficiency of the testing programme.