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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. [Copyright &y& Elsevier]

Abstract: Studying long term-evolution of gravitational slope evolution is a key to understanding deep-seated landslide processes. This paper deals with three large Deep-Seated Landslides (DSLs) at a front of a subalpine meridional chain, on the “La Marbrière” slope near the town of Grasse (Alpes-Maritimes, France). The geological framework controlling the stability and morphology of the DSLs is associated with thick and tamped Triassic layers of mudstone with gypsum overlain by highly faulted Jurassic limestone. Gravitational deformation affects the entire slope, involving a movement of about 1.1×108 m3 of rock material. It creates large disturbances in landscape morphology, such as scarps, counter-slope scarps, trenches and other typical gravitational morpho-structures. Geomorphological mapping coupled with deep electrical resistivity tomography (ERT) reveals a strong correlation between these morpho-structures and inherited brittle tectonic features. This observation relies on spatial and geometrical relations (on the surface and at the depth of more than 150m, checked by ERT) between the most persistent fault and the gravitational morpho-structures. The specific distribution of the morpho-structures on the basis of their morphological typologies and variations in the stage of evolution of three DSLs provides an interpretation of their kinematics during the last 400ka. It appears that soft substratums combined with inherited persistent anisotropies are key factors in the development of the DSLs. Indeed, outflow of mudstone due to the lithostatic pressure imposed by individual limestone compartments has led to general slope subsidence. Then, a progressive toppling of a rock mass may have led to the catastrophic rock collapse along bedding planes. The evolution of the DSLs can be divided into three distinct stages represented by three zones: a young collapse stage (zone 1), a pre-collapse stage (zone 2) and an old mature stage (>400ka, zone 3). As the DSLs occur on the same slope and in the same geological context, this area offers interesting perspectives for understanding factors controlling the long-term gravitational evolution of slopes. [Copyright &y& Elsevier]