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

Abstract

Slope movements in clay deposits are world widespread 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 velocity, S-wave velocity and electrical resistivity with geotechnical data and morphological observations. In such saturated and fine material, it turned out that only the S-wave velocity (Vs) 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 at depth, including the evolution into a flow.

Landslide characterization using P- and S-wave seismic refraction tomography — The importance of elastic moduli

“…This reduced 193 the acquisition time as only one source type was required, and also ensured the same source 194 location and signature for both P-and S-wave SRT. This is in contrast to many other studies that are 195 using distinct P-and SH-wave data acquisitions (e.g., Jongmans et al, 2009;Turesson, 2007). 196…”

Combining Seismic Noise Techniques for Landslide Characterization

“…Likewise, due to wireless data transmission, these units can serve as valuable low-cost tools for monitoring environmental phenomena and are particularly suited for landslide monitoring, since they allow repeated images of the S-wave velocity model. With regards to this, Jongmans et al (2009) have shown that S-wave velocity is very sensitive to clay consolidation (and the corresponding decrease in porosity) and should tend to zero in the event of liquefaction. Also, a significant drop in v s of around 10 % was measured at the base of a small active landslide several days before its failure (Mainsant et al 2012).…”

“…Finally, Jongmans et al (2009) showed a clear correlation between surficial displacement rates (related to clay deformation) and S-wave velocity changes (obtained by active seismic refraction) on a landslide in the French Alps.…”

“…Although a clear relationship between geophysical parameters and geological/geotechnical properties is often still unclear, these methods now appear as major tools for investigating and monitoring unstable slope masses. Therefore, for characterizing intrinsic properties like geological layering, material stiffness and material porosity, several different parameters like electrical resistivity and seismic body-wave velocities are of great interest (Grandjean 2006, Jongmans et al 2009). …”

Characterization of groundwater dynamics in landslides in varved clays

“…At least 15 % of the area covered by the varved clays is considered to be unstable (Jongmans et al, 2009). Displacement velocities range from several millimeters to several centimeters per year, with the risk that the velocities may suddenly increase and lead to catastrophic failure.…”