Geophysical investigation of a large landslide in glaciolacustrine clays in the Trièves area (French Alps)

Slope movements in clay deposits are spread all over the world and result from complex deformation processes, including internal strains in the landslide body and slipping along rupture surfaces. Such mass movements are likely to generate changes in the geophysical parameters characterizing the ground, which can be used to map the landslide body. In the last decade, geophysical techniques have been increasingly used for landslide investigation purposes. However, the success of any geophysical technique is overall controlled by the existence of a geophysical contrast differentiating the body to be mapped. For landslides affecting thick clay materials (from soft clay to shale or marl), electrical and seismic techniques have been mainly applied in the past. In this study, we attempt to physically characterize the deformation within a large slide (Avignonet) affecting laminated clays which were deposited in a glacially dammed lake during the Würm period. Clay deposits, which cover an area of 300 km 2 south of Grenoble (French Alps) and have a maximum thickness of 200 m, overlay compact alluvial layers and marly limestone of Mesozoic age. Piezometric data at Avignonet show that the water table is very shallow, implying that the slide developed in saturated clay. Several seismic and electrical profiles were performed in order to tentatively correlate the variations of P-wave (Vp) velocity, S-wave velocity (Vs) and electrical resistivity with geotechnical data and morphological observations. In such saturated and fine material, it turned out that only the S-wave velocity exhibits significant variations with the displacement rates and the morphological features. Vs values at shallow depth were found to be inversely correlated with displacement rates measured by GPS, with a division by at least a factor of 2 between the zones unaffected and strongly deformed by the landslide. These results suggest that Vs mapping could provide valuable information on the deformation state of the clay material and that the evolution of Vs with time could be used as an indicator for characterizing the landslide activity in the subsurface, including the evolution into a flow.