Geophysical and geotechnical investigation of three clay deposits in eastern Canada to assess their sensitivity based on electrical resistivity thresholds

Many studies have shown that sensitive clay areas which are prone to retrogressive landslides can be delineated using electrical resistivity tomography (ERT) along with drilling, soil sampling and laboratory testing, and piezocone penetration tests with an electrical resistivity module. Electrical resistivity of clays depends on the salinity of pore water which is also affecting their sensitivity. To assess the correlations between electrical resistivity and geotechnical properties of sensitive clays, three sites, at Brownsburg, Saint-François-de-la-Rivière-du-Sud, and Louiseville in Québec, Canada, in which retrogressive landslides occurred in the past, were selected to carry out geophysical and geotechnical investigation. Based on the electrical resistivity models obtained from the inversion of ERT data and sensitivity of clay samples measured in the laboratory, clays with remoulded shear strength lower than 1 kPa are characterized by electrical resistivity values greater than 10.9 and 12.0 Ω-m for Louiseville and Saint-François-de-la-Rivièredu-Sud study sites, respectively. These electrical resistivity thresholds are similar to the ones reported for other study sites in Canada and Scandinavia. However, for the Brownsburg clays, an electrical resistivity threshold of 2.8 Ω-m was found. This lower electrical resistivity threshold for the sensitive clays of Brownsburg than the ones of Louiseville and Saint-François-de-la-Rivière-du-Sud is due to the pore water salinity higher than 6.2 g/L, as observed in some sensitive clays in Sweden. Different factors such as stratigraphy specific to each study site and particularities in clay leaching can explain the differences in electrical resistivity thresholds found for the three study sites in eastern Canadian clays. These electrical resistivity thresholds were used to delineate clay areas with remoulded undrained shear strength lower than 1 kPa in the three study sites.