Nejib Hadda

Granular media failure along triaxial proportional strain paths

This paper revisits a famous phenomenon sometimes observed in granular soils: liquefaction. Liquefaction occurs when all stress components vanish. According to the loading conditions, and the initial void ratio of the soil specimen, it can be shown that a convenient stress quantity passes through a peak, and then decreases. It is usually thought that the deviatoric stress peak also corresponds to a failure state, since a sudden collapse of the specimen is expected from this point according to the loading control adopted. After a brief review of the theoretical background describing the occurrence of such phenomena (liquefaction and failure), the results of laboratory tests run along a triaxial proportional strain loading path are presented. Then, the main conclusions drawn are discussed and confirmed from numerical tests using a discrete element method. In both approaches, it is shown that the choice of the stress response variable is fundamental, in order to properly detect such a failure.

Strain Localization as a Function of Topological Changes in Mesoscopic Granular Structures

In this paper, the microphysical aspects leading to the occurrence of a localized failure were investigated by examining the initial emergence of ubiquitous potential slip planes and their link to the shear band development. A Discrete Element Model (DEM) representing a dense granular assembly was considered and subjected to a biaxial compression such that it undergoes localized failure into a persistent shear band. The paper includes both energetic and meso-structural analyses based on so-called minimal closed loops.

Micromechanical Analysis of Second Order Work in Granular Media

This paper examines the evolution of instabilities in granular materials from a microscopic point of view, using the discrete element method. The relationship between the unstable loading directions that result in negative values for the macroscopic expression of the second order work and the terms in the microscopic expression of the second order work was investigated. Good agreement was found between the microscopic and macroscopic expressions of the second order work for the two three-dimensional specimens studied, in both the elastic and plastic tensorial zones. The vanishing of the microscopic and macroscopic second order work is shown to coincide in both specimens.

Plastic Deformations in Granular Materials with Rotation of Principal Stress Axes

The paper examines the nature of plastic flow in granular materials under loading that involves rotation of principal stress axes. Examples in engineering practice involve offshore foundations under wave loading, earthquake loading, pavement and railway track foundations, among others. Micro-scale investigations through discrete element modelling (DEM) with a focus to non-coaxiality of incremental plastic strains to stress tensors are presented. Results can enter constitutive models for numerical applications through an amended plasticity flow rule.

To Which Extend the Failure Mode Originates from Microstructure

The theoretical study of instabilities in random heterogeneous media has proved to be an interesting pursuit in the development of a proper continuum framework for defining instability in geomaterials. Its influence transcends the scales down to the micro level and extends beyond applications that are being herein contemplated. In this present work, we are specifically interested in establishing a linkage between macroscopic and microscopic instabilities and their respective mathematical expressions through a theoretical analysis which bridges the two scales.

A multiscale description of failure in granular materials

This paper presents conditions of initiation and development of failure in granular materials through a twodimensional discrete element model. General condition for the effective development of failure and its physical characteristics are recalled. Then relation between failure and the second order work expressed in terms of microscopic variables is discussed. Eventually, correspondence between a localized mode of failure marked with shear band patterns and space distribution of negative values of microscopic second-order work is investigated.