Shear‐wave velocity as an indicator for rheological changes in clay materials: Lessons from laboratory experiments

Clay‐rich geological formations are responsible for many landslides, the dynamics of which are still poorly understood and intensely debated. Analysis of landslide motion shows that slow clayey slope movements can suddenly accelerate and fluidize as a result of sudden loading or heavy rainfall. This solid‐fluid transition, which involves disorganization of the particle network, is accompanied by a loss in rigidity that could potentially be monitored by S‐wave velocity ( Vs ) variations. To investigate this hypothesis, two types of laboratory experiments were performed on clay samples originating from an area affected by numerous landslides (Trièves, French Alps). First, creep and oscillatory rheometric tests revealed the thixotropic behavior of the clay with a highly pronounced viscosity bifurcation at a critical stress τ c . In relation with this reduction in apparent viscosity, a significant drop in Vs is also observed over τ c . Second, at zero stress, acoustic surface wave propagation experiments showed a rapid linear Vs decrease with the gravimetric water content ( w ) in the plastic domain, and a much lower decay in the liquid domain. The geotechnically‐defined liquid limit then appears as a break in the Vs ‐ w curve. For water contents in the liquid domain in particular, both experiments gave consistent results. These laboratory results demonstrate that rheological changes in clay can be revealed through Vs variations, offering the possibility of monitoring solid‐to‐fluid transitions in the field.