The residual shear strength mobilised by slow active landslides

Slow translational landslides are widespread phenomena in Italy, especially in stiff clay and clay shale deposits. After the triggering phase, which is generally characterized by moderate to rapid velocity, the landslide turns into a slow slope movement driven by fluctuations of the water table, which lead to consistent variations of the displacement rate. Such a behaviour is generally assumed to be governed by soil viscosity; in particular, the increase of the mobilized friction angle with the displacement rate is attributed to rate effects, which however in many cases seem to be overestimated. The paper focuses on some aspects of the problem that are often neglected.