Providing a ground image, electrical and seismic tomography tests are increasingly used for investigating landslide sites. After a slope failure, these techniques help to understand the geological structure of the site and to design support or remedial treatment. On the other hand, these non-destructive and relatively low-cost methods also contribute to assess the landslide risk in potential unstable areas. Two case histories are presented in this paper. The first one is an instantaneous rock failure of a few thousands of cubic meters which affected a railway slope, causing a spectacular train derailment without injury (Hemroulle et al., 1999). This slide occurred along a fault plane after heavy rain falls in a rock massif consisting of a succession of shale and sandstone layers which were folded and fractured during the Variscan orogeny. Five seismic tomography profiles were performed perpendicular to the slope with a spacing of a few tens of meters. The aim of this survey was to check the rock quality and to detect potential instability of the slope along the railway. Twenty-four geophones with a natural frequency of 10 Hz were placed along the slope with a spacing ranging between 2 m and 5 m. Waves were generated by hammer drops or explosions. For each profile, a few hundreds of travel times were inverted using the SIRT method (Krajewski et al., 1989).


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