Evolution of gravity-driven rock slope failure and associated fracturing: Geological analysis and numerical modelling

International audienceAn accurate analysis of fracture and cleavage anisotropies along two landslides in the Argentera massif (French Southern Alps) was performed in order to relate the fracturing mode to the rock slope failure state. The mature La Clapière landslide and the incipient Isola Landslide were investigated. In both cases, the gneissic rock schistosity was found sub-horizontal in the vicinity of the landslides at shallow depths, while it was sub-vertical elsewhere at the massif's scale. In addition to the well known regional tectonic fracture sets, we identified in both cases a new family of vertical valley parallel (VP) fractures. The VP fractures were only observed on the lower part of slopes affected by landslides where schistosity was found sub-horizontal. The VP fractures are clearly related to the decrease in schistosity dip and correspond to 'fold extrado'-like joints. The proximity of the schistosity folds and the associated VP joints to the landslides suggests that they are gravity-driven. The presented 2-D finite-difference models of the slope destabilisation in the La Clapière site shade light on possible mechanisms of the indicated gravity-induced processes. A progressive degradation of the material strength was imposed in these models to simulate the weathering effect which led to inelastic deformation/damage at the lower part of slope. This was followed by the formation of dense sub-vertical deformation bands and then by the initiation of a landslide in the area corresponding to the actual position of the La Clapière landslide. It is suggested that the gravity-induced damage in the lower part of slope increases the permeability and thereby accelerates the weathering that causes a more rapid strength reduction. The latter results in the tilting/folding of pre-existing fabrics and the related VP fracturing. The La Clapière landslide is initiated in the upper part of this zone