Ali Daouadji

Instability in Granular Materials: Experimental Evidence of Diffuse Mode of Failure for Loose Sands

The influence of three loading paths on the collapse of loose sand is analyzed with a particular attention paid to the onset of collapse and the mode of failure exhibited. Experimental results on conventional undrained triaxial compression tests, constant shear drained tests, as well as quasi-constant shear undrained path are presented, compared, and analyzed. It is now recognized that some collapses can occur before the Mohr-Coulomb plastic limit criterion is reached, and our recent results obtained with the new arrangement built up highlight that these collapses occur under a diffuse mode of failure. An extensive experimental series of tests shows that the first negative value of the second-order work computed using experimental data corresponds to the loss of controllability. Moreover, it is shown that the stress ratios at collapse and the corresponding mobilized angles of friction are very close for all types of tests. For similar void ratios, the onset of collapse is thus largely independent of the loading path under drained and undrained conditions but depends on a stress state to bring the material inside the unstable domain and also on the current direction of the stress increment. Indeed, it appears that the orientations of the stress increments at collapse for all tests are the same, what explains, according to the second-order work criterion, that collapse occurs at the same stress ratio. A potentially unstable domain, depending on the stress increment direction, can thus be defined.

Modeling Granular Materials: Century-Long Research across Scales

AbstractGranular materials are the most recurrent form of solid-state matter on Earth. They challenge researchers and engineers in various fields not only because they occur with a broad variety of...

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.

Experimental analyses of the behaviour of saturated granular materials during axisymmetric proportional strain paths

Instability of granular materials is usually studied using undrained triaxial compression tests or constant shear drained tests. These tests are usually performed on fully undrained or fully drained samples respectively. In this paper, experimental investigations of the behaviour of loose saturated sand under proportional strain paths performed by imposing a partial drainage condition are presented. Partially drained conditions observed in situ are due to potential pore pressure variations. Different drainage condition changes can lead to different stress–strain responses, and it is shown that the strength mobilisation for expansive drainage conditions is lower than what is usually obtained for undrained conditions. The stability analyses based on experimental results have shown that the second order work criterion, rewritten using the relevant control parameters, is a good tool to investigate the onset of instability. L’étude des instabilités dans les milieux granulaires est souvent menée en utilisant des résultats d’essais triaxiaux de compression non-drainés ou des essais à contrainte déviatoire constante. Ces essais sont réalisés respectivement en condition totalement non-drainée ou totalement drainée. Nous présentons dans cet article une étude expérimentale réalisée sur sable d’Hostun lâche saturé sous chargement proportionnel en déformation en imposant un drainage partiel. Ce drainage partiel est dû à une augmentation de pression interstitielle et reproduit des conditions in situ. Ces différentes conditions de drainage conduisent à des réponses contraintes-déformations différentes et l’on montre que la résistance mobilisée dans le cas d’un essai à dilatance imposée est inférieure à celle classiquement obtenue durant les essais non-drainés. L’analyse de stabilité réalisée en utilisant les résultats expérimentaux montre clairement que le critère du travail de second ordre, réécrit en utilisant les variables de contrôles adéquates, est un indicateur pertinent pour l’étude du déclenchement de l’instabilité.

Investigating instability in granular materials by means of a microstructural model

ABSTRACT Under certain loading conditions, instability in loose sand can develop at a shear stress level much lower than the critical state failure line. To analyse these types of problems, we adopt the micromechanical approach developed earlier for the modelling of granular material behaviour. The stress-strain relationship for a granular assembly is determined by integrating the behaviour of the inter-particle contacts in all orientations. The constitutive model is applied to simulate undrained triaxial and constant-q tests on loose sand. Numerical simulations are compared to experimental results in order to evaluate the models ability to predict the different modes of instability of the granular assembly. With the model we have also analysed instability at inter-particle level to observe how it is connected to instability at the assembly level.

Dynamic properties of dense sand-rubber mixtures with small particles size ratio

In this study, the small strain shear modulus Gmax and the damping ratio Dmin of sand–rubber mixtures with a small particles size ratio Sr (D50rubber/D50sand = .36) are characterised by shear wave propagation using bender elements inserted in a modified triaxial cell. The influence of addition of less than 50% rubber volume fractions Rf in a range of confining pressures varying from 50 to 500 kPa was investigated. The results showed that, for all confining pressures, the shear modulus decreases with increasing rubber volume fractions, while the damping ratio linearly increases. A quasi-conservation of shear modulus up to a volume fraction Rf = .2 and sharp decrease beyond this content is observed. The shear modulus decay and the damping enhancement with addition of rubber fractions are mitigated by increasing confining pressures. An optimal increase of the damping ratio without significant loss of rigidity and a minimal sensitivity to confining pressure for mixtures containing about 10% rubber is highlighted. The shear modulus and damping ratio are power function of the mean effective stress within the range studied.

Experimental and numerical analyses of failure in very loose sands

ABSTRACT It is now recognized that some collapses can occur before the Mohr-Coulomb plastic limit criterion is reached, and our recent results highlight that these collapses occur under a diffuse mode of failure. We present and discuss experimental data on constant shear drained tests. A non-associated elastoplastic multimechanism model is briefly presented and numerical results are compared with our data obtained constant shear drained tests. These simulations are in agreement with experiments. The second order work is computed and the loss of stability, corresponding to the vanishing of the second order work, is very close to the experimental collapse point.

Phase Transformation States of Loose and Dense Granular Materials under Proportional Strain Loading

AbstractDepending on the rate of external loading, the permeability, and the boundary conditions, different drainage conditions are expected during the loading of sands. These drainage conditions are thus neither fully drained nor fully undrained as usually assumed for saturated soils, which implies simultaneous changes in pore volume and in pore water pressure. For a given material, the transition between contractive behavior (increase of the pore water pressure) and dilative behavior (decrease of the pore water pressure) defines the phase transformation state. It is found that the position of this phase transformation state, which is known to be dependent on the relative density, depends on the rate of dilatancy (e˙v/e˙1) imposed on the sample. Constraining a loose sample to dilate leads to an unstable state earlier than observed during undrained tests. Even dense samples, which do not exhibit a nonlocalized unstable state during undrained tests, are prone to collapse if the rate to dilatancy is high en...

Instability in Loose Sand: Experimental Results and Numerical Simulations with a Microstructural Model

Under certain loading conditions, loose sand can develop instability at a shear stress level much lower than the critical state failure line. To analyze these types of problems, we have adopted the micromechanics model developed by Chang and Hicher for modelling granular material behaviour. The stress-strain relationship for a granular assembly is determined by integrating the behaviour of the inter-particle contacts in all orientations. The constitutive model is applied to simulate undrained triaxial, constant-q and proportional strain tests on loose Hostun sand. Experimental results are used to evaluate how well the model can capture the modes of instability at the assembly level. The notion of control variables is discussed according to these different loading conditions.

Behavior of Granular Materials Affected by Grain Breakage

Abstract: In geotechnical and particularly in dam engineering, observations in the field and, in some cases, dramatic failures of structures have often incited research and increased knowledge. For instance, the first rockfill dams were elevated without any prior knowledge of the mechanical capacities of the rockfill mass. As natural and local resources are usually preferred for economic and environmental issues, their non-optimal properties can lead to potential severe damage to the hydraulic structure. Similarly, a basic principle of construction of rockfill dams or embankments consists in sluicing rockfill after deposition and before compaction: this recommendation has been adopted for a long time. However, the beneficial effects of such a practice have been understood only in light of recent studies on unsaturated soil and multi-scale analyses.