Application of geophysical measurements for assessing the role of fissures in water infiltration within a clay landslide (Trièves area, French Alps)

Soumis à Hydrological Processes, accepté définitivement ; en attente de publication.

International audience

English. Rainfall and its consequences are usually considered as the major factor trig- gering slope instabilities within clay deposits. The link between rainfall, water inltration and landslide activity may be complex and comprehensive sets of data are still necessary to understand how water inltrates in cohesive material. On unstable slopes made of ne-grained sediments, gravitational deformation generates supercial shear ruptures and tension ssures whose continuous opening could be amplied by shrinkage during dry periods. This ssure network at the surface can be very dense and strongly controls the water inltration process. However, it re- mains dicult to assess the ssure evolution at depth and its eventual relationship with shallow slip surfaces. This work presents the results of a geophysical study which aimed to characterize the ssures that are observed along the unstable clay slopes of the Trieves area (French Alps). The site was rst investigated combining geomorphological analysis, drilling, borehole logging, geotechnical tests and geo- physical experiments. This investigation evidenced the presence of at least three rupture surfaces (at 5m, 10 to 15m and 42m) with a water ow at the second one. In a second step, permanent instruments were installed (piezometers and soil water content (SWC) probes). Results indicate high water inltration velocities, down to the water table located at 2.5m depth, that are likely to be linked with sub-vertical preferential paths, such as ssures. Geophysical parameters (electrical resistivity, S-wave velocities, Rayleigh waves attenuation) were monitored. Geophysical results suggest that ssures are permanently open, that they serve as preferential inltra- tion paths, and that they can reach, at least, a depth of 2m. The methods employed in this research put forward that the system of imbricated ssures drains water from the surface down to the shallow slip surfaces at 5m and 10-5m. This work highlights the role of ssures network in water inltration in the numerous clay landslides of the Trieves area.