Electrical resistivity tomography to understand clay behavior during seasonal water content variations

Problems with foundations highlight a lack of understanding surrounding factors which influence ground movements during wet–dry cycles (Vincent, 2009). To address this issue, geotechnical characterizations of a clayey formation can be used to identify significant variability in lithological facies, both vertically and horizontally over very short distances. Soil heterogeneities explain in this case a wide range of geotechnical parameters and weak correlations between them, assessed on soil behavior observed on site. This paper focuses on soil water deficits and benefits over time, related to soil composition in a heterogeneous clay formation through the use of electrical resistivity tomography (ERT). Electrical resistivity tomography and time domain reflectometry (TDR) were used simultaneously to measure resistivity and soil moisture at an experimental field site with the unique objective of qualifying soil moisture in this first stage of the study. The resistivity variations obtained from ERT were compared to local effective rainfall and soil moisture measurements from time domain reflectometry (TDR) down to a depth of 3 m. Results show the potential of qualifying soil water content variations over the seasons, and especially of detecting a rapid increase in humidity thanks to spatial soil heterogeneity at a decimetric scale. ERT proves to be a useful method of delineating soil facies based on their drying and humidification behavior.