Caractérisation de versants argileux instables dans des conditions hydrogéologiques hétérogènes. Approche géophysique

摘要

This work aims to understand the relationships between, on the one hand, structural and lithological heterogeneities within a clayey landslide and, on the other hand, the observed differences in displacement velocities and water drainage down to shear surfaces. The study site is the Avignonet landslide (Trièves area, French Alps) made of quaternary glaciolacustrine clays. This landslide encompasses a surface of about 1.5 km² and is located in the immediate vicinity of the Harmalière landslide, which evolved into a mudflow in 1981. Several slip surfaces are known at different depths (5, 10-15 and 40-50 m). Within these saturated levels (except for the superficial fringe), mean displacement rates can reach 0.15,m/year. Geodetic data collected since the 60s shows that, even if located in similar geological and meteorological contexts, the two slides exhibit different kinematics. The combination of geophysical (ambient noise, resistivity and seismic tomographies, well logging), geotechnical (drillings, lab tests, sample analysis), geodetic and meteorological tools allow the identification of the main controlling factors, the effects of which vary as a function of the spatial scale. At a regional scale, the paleomorphological setting controls landslide kinematics. At the landslide scale, lithological heterogeneities allow pore pressure regulation and, consequently, variable kinematics. At the scale of an intensely deformed zone, fissures serve as preferential infiltration paths and allow water drainage down to the perched water table and shear surfaces down to 10 m depth.