Shear-wave-velocity drop prior to clayey mass movement in laboratory flume experiments

•We model initiation of clay landslides in laboratory flume experiments.•We observe precursory drops in acoustic wave velocities prior to movement onset.•Rheometrical tests reveal a time-dependent rheological response of the clay.•Precursors and transient rheological response could be related.

Clay slopes are susceptible to suddenly liquefy into rapidly accelerating landslides, thereby threatening people and facilities in mountainous areas. Because the shear-wave velocity (Vs) characterizes the medium stiffness, this parameter can potentially be used to investigate the rheological behavior of clay materials before and during the solid-to-fluid transition associated to such landslide failures. Previous rheometrical studies performed on clay samples coming from Trièves landslides (French Alps) have established that this material behaves as a yield stress fluid with a marked viscosity bifurcation. When the applied stress reaches a critical level, the viscosity decreases abruptly, along with Vs which tends to zero in the fully fluidized material. Here, we monitor the Rayleigh wave velocity (VR) variations in a saturated clay layer placed in a flume and progressively brought to failure by tilting the device. Experiments performed on clay samples with different water contents show a significant relative drop in VR values (and hence in Vs) before the onset of the mass movement. Additional rheometrical analyses point out that this precursory drop in Vs is presumably due to a complex transient rheological response of the clay. These new results confirm that Vs variations constitute a good indicator for monitoring clay slope stability.