Analysis of post-failure slope movements within the framework of hazard and risk analysis

The paper first recalls briefly a methodological framework to assess landslide hazard and risk analysis in terms of predisposition, triggering and revealing factors. This framework, that reflects the mechanisms involved in the landslide, is based on the Geotechnical Characterisation of slope movements proposed by Vaunat et al. (1994) and Leroueil et al. (1996). The Geotechnical Characterisation can be schematized by a 3-D matrix having the following axes: types of movement; types of material; and the four stages of movement: pre-failure, failure, post-failure and reactivation. For each relevant element of this 3-D matrix, there is a characterisation sheet including: the controlling laws and parameters, the predisposition factors, the triggering or aggravating factors, the revealing factors and the consequences of the movement. The paper focuses afterwards on the post-failure stage, which generally is the most destructive, and on the mobility index. It is shown that this later index can be described as the product of sub-indices associated with failure, brittleness of the material, ability of the soil to develop pore pressures, geometry of the moving soil mass and characteristics of the terrain. It is also shown how these aspects can be incorporated into the Geotechnical characterisation of slope movements. This seems to provide a rational basis for examining slope movements at the post-failure stage and assessing associated risks.