Measurement of geophysical parameters on clay samples at the solid–fluid transition
Fluidisation occurring in clay-rich landslides poses serious threats to populations and infrastructures and has been the subject of numerous studies to apprehend its rheological origin. In parallel, noninvasive geophysical techniques on landslides have known considerable development as a means to approach in-situ geotechnical parameters. This study investigates the influence of fluidisation on two geophysical parameters: the shear wave velocity, Vs, and the electrical resistivity, ρ. Both parameters are widely used in landslide monitoring as they are sensitive, respectively, to soil stiffness and water content, two key parameters for material fluidisation. Laboratory tests were carried out on soil samples collected in five flow-like landslides occurring in very different geological conditions. A plate–plate rheometer was used to provoke fluidisation, and Vs was measured during oscillatory tests. The rheometer was redesigned for resistivity measurements, incorporating circular electrodes in polyvinyl chloride plates. Results show that (i) all soils exhibit a dramatic drop in Vs at the fluidisation, and (ii) the resistivity does not significantly vary at the solid–fluid transition. These last results are analysed in terms of clay particles arrangement using the electrical laws of Archie and Waxman–Smits, and the impact on landslide geophysical monitoring is discussed.