Slopes and Mass Movements

The study of slope movements is complicated because they can take very different configurations and mechanisms ranging from rock topple to mudflow, can involve a variety of materials ranging from hard rock to sensitive clay and loess, and can result from a variety of phenomena ranging from rapid snow melt or heavy rainfall to earthquake. Leroueil et al. (1996b) defined four different stages of slope movement (Fig. 14.1): (a) the pre-failure stage, including deformations associated with changes in stresses, viscous deformations, and strains and displacements associated with progressive failure; (b) the onset of failure characterized by the formation of a continuous shear surface through the entire soil or rock mass or by the separation of an unstable rock mass from the slope (rock fall or toppling); (c) the post-failure stage, which includes movement of the soil or rock mass involved in the landslide, from just after the onset of failure until it essentially stops; and (d) the reactivation stage, when the soil or rock mass slides along one or several pre-existing shear surfaces. This reactivation is generally occasional. Continuous movements with seasonal variations of the rate of movement (active landslides), which are the rule in some countries and geological environments, represent a peculiar situation that might be associated with reactivation since these movements occur along well-defined shear surfaces.