Evaluating effective stress conditions in soils using non-invasive electrical measurements – Laboratory studies

Abstract

Changes in the effective stress conditions in soils affect their strength with implications on the stability of near-surface earth materials. The ability to, therefore, characterize and monitor the gradual loss of soil strength due to a decrease in effective stress is essential to ensuring the safety and integrity of engineered structures and help in mitigating geohazards. A faster and cheaper alternative to quickly and non-invasively characterize and monitor changes in subsurface conditions is the utilization of geophysical techniques such as the electrical resistivity method. It can provide a means to effectively monitor and detect potentially unstable conditions (lower effective stress zones) in the earth subsurface.In this study, the effect of varying effective stress levels on the frequency-dependent resistivity measurements of sand-clay mixtures is evaluated. The effect of varying compressibility characteristics such as void ratio and coefficient of compressibility on the electrical parameters are also assessed. A bilinear relationship was found between the electrical parameters (resistivity and capacitance) and the compressibility parameters. The electrical parameters also revealed transitional effective stress or void ratio below which the soil material can barely sustain any mechanical loading. This transitional effective stress was found to be close to the liquid limit of the sample and represents the situation when the soil material begins to lose its strength and potentially become unstable. Being able to predict this transitional effective stress with electrical measurements may be cardinal and useful in our quest to use non-invasive methods in the assessment and monitoring of potential instability conditions in the earth subsurface.