Triggering and motion of landslides
Authors: Alonso Eduardo E. 
Published: August 2020

The paper analyses the dynamic behaviour of a class of landslides characterised by a well-defined failure surface where shear strains accumulate. The subject goes beyond the common concepts of safety factor and static analysis, and discusses procedures to identify the velocity and runout, once stability is lost. Three initial case histories serve to highlight the relevance of predicting the motion after failure. These cases and a few others discussed in the paper help to connect the theoretical developments with their relevance in practice. The following landslides receive attention in the paper: Pampaneira, Cortes, Aznalcóllar, Vallcebre, Selborne, Vajont and Canelles. Existing publications describe all of them in some detail. These landslides illustrate the following phenomena: creeping motion, first-time failures, rapid sliding and the transition from slow to very rapid motion. These phenomena are present in the concept and organisation of the paper. Simple geometries (planar, double block) facilitate the description of the basic physics but are also capable of delivering useful solutions and deep understanding. In a second stage, the simple sliding cases evolve into continuum analysis. The material point method (MPM) offered the possibilities of approaching arbitrary geometries and removed a main limiting assumption of simple cases, namely the ‘a priori’ knowledge of the failure mechanism and its subsequent propagation. Two well-documented cases of progressive failure in brittle, high-plasticity, overconsolidated clays (Aznalcóllar and Selborne) provided useful data to check the capabilities of the MPM analysis to predict correctly the internal development of shearing surfaces and the observed runout. The method also provides information regarding the transition from an essentially ‘static’ behaviour to an accelerated displacement. A sensitivity analysis, inspired by the Selborne case, resulted in an interesting relationship between runout and soil brittleness. Rate effects on friction angle explain the creeping behaviour of landslides. Changing velocities of Vallcebre slide under a time history of pore water pressures provided a validation exercise for a simple modelling approach. However, friction laws do not easily explain the sudden acceleration observed in landslides such as Vajont. Previous work to explain the rapid acceleration and fast motion observed in some landslides rely on heat-induced pore pressurisation of the sliding surface. Further work introducing a double wedge for Vajont landslide stressed the relevance of shear band thickness and, in particular, the soil permeability. A generalisation of the involved thermo-hydro-mechanical (THM) formulation, by way of MPM, met the difficulty of solving the inconsistent results deriving from the mesh-size control of the shear band thickness. The solution was to embed shear bands, with a reasonable thickness, in the material points describing the soil matrix. This approach successfully reproduced the known Vajont after-failure motion. The model provided estimations of the internal shearing of the sliding rock mass, the temperature increase of the sliding surface and its transient excess pore pressures. A final section describes the close relationship between the creeping motion of a landslide and its eventual evolution towards a rapid phenomenon. A simple planar slide provided a set of dimensionless parameters governing the interaction. Strain rate effects on friction explained the creeping part of the problem. The paper describes a generalised approach to arbitrary geometries by means of MPM formulation. Canelles landslide was useful to discuss the merits and limitations of predictions based on the absence or presence of creeping and THM physics of the formulation and their possible combinations.