ABSTRACT
If soil is observed to move along a slope AB the moving area is called a landslide (Fig.1). The definition is independent from measured displacements (d), computed velocities, the evolution of velocities with time or the detected presence of slide planes (S1, S2). A landslide therefore may either move slowly for indefinite periods, may accelerate or decelerate, or may end up in failure. The notion of movement is sufficient because movements are both a nuisance, and pose a threat. A nuisance, since movements create fissures disrupt drainage paths and provoke damage to private and public property, and a threat since slide planes tend to be smoothed and soil masses tend to get weakened by the movements. Hence, in principle, the resistance to failure decreases.
