John Eichenberger

Experimental and numerical analysis of an unsaturated volcanic ash deposit for the establishment of an early warning system in a quarry in Costa Rica

The paper presents the work carried out for the analysis of the hydro-mechanical behaviour of a volcanic ash in unsaturated conditions for the establishment of a landslide eary warning system (EWS) in a Costa Rican quarry. Rapid mass movements involving ash material occurred in the greater quarry area during heavy rainfall events. Laboratory testing and numerical analyses were combined to provide a better understanding of the triggering mechanism and to provide guidelines for the EWS. Along with conventional laboratory analyses, the experimental program involved tests carried out under suction control to assess the volumetric response in unsaturated conditions of the material and to highlight potential collapse-upon wetting phenomena. The behaviour of the material was analysed in a modern framework for soils in unsaturated conditions developed at the EPFL. Numerical simulations were performed with a coupled hydro-mechanical finite elements code in which the mentioned constitutive model is implemented. The analyses allowed a better understanding of the landslide triggering mechanism and provided pore water pressure values which are associated to the failure mechanism and can be used as threshold values for the EWS.

Techniques for the modelling of the process systems in slow and fast-moving landslides

This chapter reviews some of the current strategies for landslide modelling. Main physical processes in landslides are first recalled. Numerical tools are then introduced for the analysis of the behaviour of slow- and fast-moving landslides. Representative case studies are introduced through the chapter to highlight how different modelling strategies can be used depending on the physical processes that the modeller wants to take into account.

Modeling the Onset of Shallow Landslides in Partially Saturated Slopes Subjected to Rain Infiltration

Heavy rainfall can lead to shallow slips on slopes which are often initially in a state of partial water saturation. A multiphysics approach and advanced constitutive modeling are necessary to take into account the physical key processes in partially saturated soils during rainfall events, such as water flow through the solid matrix, soil water retention behavior and the effects of matric suction on the mechanical behavior. The elasto-plastic constitutive model ACMEG-s that includes an elasto-plastic water retention model with wetting-drying hysteresis is used in the framework of finite element modeling for the assessment of destabilizing, transient processes in a steep slope during rain infiltration. It is shown that at the onset of failure, wetting pore collapse and plastic shear strains occur in the lower part of the slope and develop upwards towards the slope surface to delimit a probable failure mechanism.

One-Dimensional Transient Analysis of Rainfall Infiltration in Unsaturated Volcanic Ash

The paper presents a one-dimensional hydro-mechanical analysis of rainfall infiltration in a loose volcanic ash and the utilisation of a factor of safety for the implementation of an early-warning system. Three different rainy seasons with different rainfall patterns were analysed . The analysis aims to understand the influence of the antecedent rainfall on the wetting front, the pore-water pressures and the factor of safety. The analysis was carried out in the context of a Master project of the first author at the Laboratory for Soil Mechanics of EPFL.