Clayey Landslide Investigations Using Active and Passive VS Measurements

Abstract : Clay slopes frequently are affected by gravitational movements. Such movements generate complex patterns of deformation that have slip surfaces located at different depths and are likely to modify geophysical parameters of the ground. Geophysical experiments performed on the large clayey Avignonet landslide (Western Alps, France) have shown that shear-wave velocity (VS) is most sensitive to clay deconsolidation resulting from the slide. Values of VS at shallow depths exhibit an inverse correlation with the GPS-measured surface-displacement rates. Compared with measurements in stable zones, VS values in the most deformed areas of the slide can be reduced by a factor of two to three. Laboratory measurements on clay samples set in triaxial cells have shown that a strong decrease of VS values accompanies an increase in the void ratio, in a velocity range similar to that measured in situ. Although other factors (stress change, cementation, granularity) can modify VS values, these results justify the potential of VS imaging to map spatially the deformation induced by a landslide. Several active and passive techniques for measuring VS are tested and compared on the kilometer-size and 50-m-deep Avignonet landslide. The crosscorrelation technique, applied to seismic noise recorded by a large-aperture array and associated with shot records, turns out to be an effective tool for imaging the landslide in three dimensions. If permanent stations are installed, the same method also can be used to monitor the evolution of seismic velocity with time, as an indicator of landslide activity.