Instability conditions of marly hillslopes: towards landsliding or gullying? The case of the Barcelonnette Basin, South East France

Black marl hillslopes in the French Alps are strongly affected either by mass movements or by gully erosion due to their susceptibility to weathering processes. This paper presents experimental data (geomorphological, geotechnical, geomechanical and hydrological) on the parameters of the material of three well-known earthflows and of the two stratigraphic parts of the Callovo–Oxfordian black marl. The main objectives are to define the geomechanical behaviour of the various formations using different characterisations (grain size distribution, mineralogy, retention capacity, consolidation test, direct shear test and triaxial test) and to demonstrate how such soils can affect the stability of natural slopes. Particular care must be taken when interpreting the geomechanical tests on this evolving and very heterogeneous clay-rich material. There are strong relationships between the parent rock and the landslide materials. This paper presents a conceptual model of the development of strength over a period of time and investigates the parameters of the material for use in slope stability analysis and risk assessment.