Eric Vincens

Numerical modelling of backward front propagation in piping erosion by DEM-LBM coupling

Abstract A granular soil REV located on the upstream side of the erosion pipe front is modelled numerically, at the grain scale, by coupling the Discrete Element Method (DEM) with the Lattice Boltzmann Method (LBM) for the representation of the solid and fluid phases, respectively. The implementation of DEM follows a standard molecular dynamics approach and the interactions between grains are regulated by unilateral contacts and breakable bonds. A synopsis of the LBM scheme is provided, with focus on the implementation of non-slip conditions for moving boundaries and use of the Multiple Relaxation Time approach for improved numerical stability. The coupling scheme is described along with the criteria for setting the numerical parameters of the two methods. After a “dry” preparation procedure, the numerical REV is tested under fully saturated conditions and increasing pressure difference. Backward erosion is observed and a micromechanical inspection of the granular phase suggests that arching through force chains and the breakage of tensile bonds are prominent resistance and degradation mechanisms, respectively.

Undrained strength of clays derived from pressuremeter tests

The behaviour of saturated clays under undrained conditions around pressuremeter path is studied by means of a theoretical and a numerical analysis based on the generalised elastoplastic Prager’s model with the Von Mises Criterion which is suitable for these cases of soils and gives a realistic response when unloading is considered. The soil constitutive model is described depending only on three parameters. The model response on pressuremeter path and its identification from experimental data lead to the determination of the undrained cohesion and the initial Young modulus of clays. Comparisons between the undrained cohesions determined with the proposed method and those obtained by other means illustrate the applicability of the method. An application to predict the bearing capacity and the settlement of bored piles using parameters derived from the proposed approach is also presented with a comparison to the measured data. Le comportement des argiles saturées en conditions non drainées sur un chemin pressiométrique est étudié au moyen d’une analyse théorique et numérique basée sur le modèle élastoplastique du Prager généralisé avec prise compte du critère Von Mises. Ce critère est bien adapté aux sols considérés et donne une réponse réaliste en déchargement. Le modèle constitutif de sol proposé est défini seulement par trois paramètres. Cette approche permet la détermination de la cohésion non drainée et le module de Young initial des argiles saturées à partir d’essais pressiométriques. La comparaison entre les cohésions non drainées déterminées avec la méthode proposée et celles obtenues par d’autres moyens illustrent son applicabilité. Une application de l’approche proposée pour prévoir la capacité portante et le tassement d’un pieu foré soumis à une charge axiale centrée est également présentée avec une comparaison avec les mesures expérimentales.

The characteristic state

Simulations of cyclic biaxial tests on samples composed of 2D irregular polygonal particles were performed to investigate the properties of the characteristic state, transition state between contraction and dilation. Two aspect ratios of 2D angular particules and different states of initial fabric are considered. The stress ratio at characteristic state is found to be much more sensitive to initial anisotropy than to density changes. Fabric is the kind of anisotropy that seems to influence the most the characteristic state. Constant amplitude cycling does not erase the influence of the initial anisotropy of fabric which was not expected. Then, in the framework of usual soil modelling where the material is supposed to be in an initially isotropic state, the usual models for the characteristic state are only valid for constant cyclic strain amplitude loadings where the mean anisotropy throughout the cycles remains low. Des simulations d’essais biaxiaux cycliques ont été entreprises sur des échantillons constitués de particules polygonales irrégulières afin d’étudier les propriétés obtenues à l’état caractéristique, transition entre un comportement contractant et dilatant. Deux rapports d’allongement des particules sont étudiés tout comme différents états initiaux de structures. Le rapport de contraintes à l’état caractéristique est trouvé beaucoup plus sensible à l’anisotropie initiale qu’aux variations de densité. Seule l’anisotropie, dite de structure, semble agir sur cet état. Des cycles à déformations d’amplitude constante n’effacent pas l’influence de cette anisotropie initiale, ce qui n’était pas attendu. Aussi, dans un contexte classique où le matériau est initialement à l’état isotrope, les modèles décrivant l’état caractéristique ne sont-ils valides que pour des chargements cycliques symétriques où l’anisotropie moyenne reste faible.

Tassements et pouvoir endommageant d'un mouvement sismique

ABSTRACT The design of an index measuring the damaging power of a seismic record is generally undertaken within the frame of Structural Dynamics. Herein, we propose to undertake it within the frame of Soil Dynamics. The global settlement undergone by a soil profile is regarded as an indicator of the damage caused by a seismic motion. The ability of some classical damage index to predict the settlement is analysed. A predictor of expectable settlement is proposed. This predictor, which account for essential features of the soil profile, appears therefore as a new improved damage index.

Simplified Estimation of Some Main Characteristics of Pores and Constrictions in Granular Materials

The void space of granular materials can be considered as a collection of poly-sized pore bodies separated by narrow pore throats or constrictions. Pore network models have been developed to estimate the probable path length covered by fine particles flowing through a granular filter. For this calculation two pieces of information are required: the constriction size distribution and the mean void spacing between two constrictions which corresponds to the mean pore diameter. Different assumptions have been previously made in the literature to determine this void spacing. However, they all neglect the influence of the density and thus, the estimation of this quantity remains an open issue. This paper compares different definitions for the mean pore size based on statistical analyses performed on numerical samples composed of spheres by means of the Discrete Element Method. Different sphere packings with different gradings and density states were considered and a weighted Delaunay tessellation was applied to extract the main void characteristics. In a second part, a simple formula is proposed to quickly estimate the equivalent sieve opening size of the granular filter. This value is very close to the mode value of the constriction size distribution which is related to the most represented constriction size in a granular material.

A model for the computation of engineering earth structures to a seismic motion

ABSTRACT Under cyclic loading, vulnerable soil structures are prone to experience liquefaction. Actually, their typical contractive behaviour can lead, under a deviatoric loading and constant volume condition, to a pore pressure build up which may cause an important or total loss of the shear resistance. BRO model, which is able to model this phenomenon, differs from the other soil models devoted to seismic computation by a simple identification procedure for the model parameters. It makes it convenient for the use of engineers. BRO model used to present a lack of robustness, it has been improved and then tested on San Fernando dam during the same-named seismic event in 1971.

Evolution Of Internal Variables In Granular Materials Throughout Cyclic Loadings

The aim of this paper is to propose accurate analyses of the evolution of both global and local variables in granular media using DEM simulations throughout both monotonic and cyclic loadings. They provide a better understanding of the evolution of the internal state and then can be helpful to define better phenomenological models for such materials. In this work, the influence of the structural anisotropy and the density of the material are considered. The simulations clearly show that the critical state is independent of the initial state but dependent on the loading path. On the other hand, the characteristic state is dependent on both the initial state and the loading path. This latter finding is peculiarly important for a good modeling of the cyclic behavior of granular material.