Inception of Debris Avalanches: A Material Point Method Modelling

Abstract

Rainfall-induced landslides of the flow type in granular soils are among the most complex natural hazards due to the variety of mechanisms which regulate the failure and propagation stages. Among these, debris avalanches are characterised by distinct mechanisms which control the lateral spreading and the increase in soil volume involved during the propagation. Two different stages can be individuated for debris avalanches, i.e. the failure stage and the avalanche formation stage: the former includes all the triggering mechanisms which cause the soil to fail; the latter is associated to the increase of the unstable volume. Regarding these issues, in the literature, either field evidence or qualitative interpretations can be found while few experimental laboratory tests and rare examples of geomechanical modelling are available for technical and/or scientific purposes.

In this paper a contribution is provided about the advanced numerical modelling of the inception of such hazardous debris avalanches. Particularly, the case of the impact of a failed soil mass on stable deposits is considered. This means that a small translational slide occurs; the failed mass causes the soil liquefaction of further material by impact loading; the landslide volume increases inside triangular-shaped areas during the so-called “avalanche formation”, and also soil erosion along the landslide propagation path plays an important role.

To this aim, an innovative numerical technique known as the Material Point Method (MPM) is used. It can be considered as a modification of the well-known Finite Element Method (FEM) particularly suited for large deformations. The continuum body is schematized by a set of Lagrangian points, called Material Points (MPs). Large deformations are modelled by MPs moving through a background mesh, which also covers the domain where the material is expected to move. The MPs carry all physical properties of the continuum such as stress, strain, density, momentum, material parameters and other state parameters, whereas the background mesh is used to solve the governing equations without storing any permanent information. Such advanced approach allows combining a hydro-mechanical coupled approach, any of the well-known soil constitutive models proposed over the years in soil mechanics and a large-displacement formulation.

The numerical analyses are performed adopting 2D geometrical configurations taken from field evidences and previous researches. Triangular 3-noded computational meshes are used, characterized by elements of about 1 m. The interaction between the impacting mass, and then of the propagating flow with the in-situ stable soil is examined, providing important insights about the behaviour of such type of landslide. The results achieved so far are encouraging and show that MPM can properly simulate the inception of debris avalanches and even their complex mechanisms during the impact and the interaction with in-situ stable zones.