An elastoplastic model for unsaturated expansive soils based on shakedown concept

It is important to model the behaviour of unsaturated expansive soils subjected to hydromechanical loadings, because these wetting and drying cycles alter significantly their hydromechanical behaviour which may cause a huge differential settlement on the foundations of individual buildings, pavements, dams, etc. From experimental observations, these expansive soils can generally reach a final equilibrium state at the end of the suction cycles where the soil behaviour can be supposed elastic. In this context, this paper presents an analytical method based on shakedown concept for the hydromechanical behaviour of expansive soils. The required parameters of the shakedown-based model are calibrated by the experimental results obtained for bentonite/sand mixtures subjected to cyclic suction loadings in an oedometric test. The comparison between the experimental results and the modeling demonstrates the capacity of the proposed shakedown-based model to simulate the hydromechanical behaviour of unsaturated expansive soils

Discrete element model for brittle materials

We adopt the discrete element method (DEM) to study the fracture behavior of brittle materials. We propose an approach which relates crack initiation to crack growth. The material consists of a set of particles in contact, which allows us to derive an expression for the stress intensity factor as a function of the contact forces and displacements. A classical failure criterion, based on the material’s toughness, is then adopted for the analysis of crack propagation, represented by the loss of cohesion forces between particles. Afterwards, we apply our discrete criterion to uncracked materials under homogenous stress conditions, obtaining a Rankine like behavior. The work results in a simple discrete model which is totally compatible to continuum mechanics, where no calibration tests are required, in contrast to most of discrete approaches

A discrete element approach in fracture mechanics of brittle materials

In this study, we use the discrete element method (DEM) to model the fracture behavior of brittle materials in 2D. The material consists of a set of particles in contact with a close-packed structure. It allows us to derive an expression for the stress intensity factor as a function of the contact forces near the crack tip. A classical failure criterion, based on the material’s toughness, is then adopted in the analysis of mixed mode crack propagation, represented by the contact loss between particles. We compare our model to classical solutions of tensile crack (mode I) and shear crack (mode II)

Fast multipole method applied to Symmetric Galerkin boundary element method for 3D elasticity and fracture problems

International audienceThe solution of three-dimensional elastostatic problems using the Symmetric Galerkin Boundary Element Method (SGBEM) gives rise to fully-populated (albeit symmetric) matrix equations, entailing high solution times for large models. This article is concerned with the formulation and implementation of a multi-level fast multipole SGBEM (FM-SGBEM) for elastic solid with cracks. Arbitrary geometries and boundary conditions may be considered. Numerical results on test problems involving a cube, single or multiple cracks in an unbounded medium, and a cracked cylindrical solid are presented. BEM models involving up to $10^{6}$ BEM unknowns are considered, and the desirable predicted trends of the elastostatic FM-SGBEM, such as a $O(N)$ complexity per iteration, are verified

Unsaturated resilient behavior of a natural compacted sand

Granular materials are generally used in unbound layers of road pavement structures. The mechanical behavior of these materials is widely studied with repeated load triaxial tests (RLTT) in which the elastic response is defined as the resilient behavior. Usually observed under total stress conditions, the effect of pore pressure changes during loading are not usually included in design. Further, the unbound layers frequently exist under partially saturated conditions. The influence of the unsaturated state, i.e., the suction, on the mechanical behavior, of unbound granular materials for roads has not been sufficiently studied and is generally not taken into account in models used for these materials. This article presents an experimental study of the repeated load response of a compacted clayey natural sand, and describes a model for the response which includes the effects of soil suction. The response of the proposed model formulated in terms of effective stress is compared with a similar model formulated in terms of total stress. The results from both the effective stress model and the total stress model are compared with the measured volumetric and deviatoric response. It is suggested that since the model parameters for the effective stress formulation are relatively constant for all values of suction (water content), the resilient response can be best captured by an effective stress model

Molecular structure evolution of asphaltite-modified bitumens during ageing; Comparisons with equivalent petroleum bitumens

This work focuses on the molecular structure evolution of asphaltite-modified paving bitumens during ageing. In order to quantify the effect of ageing on the molecular weight distribution (MWD) of bitumens, a new parameter, called hereafter the ageing molecular-distribution shift (AMDS), is introduced. The molecular evolutions of asphaltite-modified bitumens during aging are compared with the molecular evolutions of pure petroleum bitumens of equivalent grade. The results based on AMDS confirm previous research showing that the asphaltite attenuates the ageing and, compared to hard petroleum bitumens produced in refinery, the asphaltite-modified bitumens present a better ageing performance. The AMDS parameter reveals appropriate for the evaluation of evolutions due to ageing

Modélisation Discrète en Mécanique de la Rupture

Le comportement en élasticité linéaire isotrope d'un matériau continu peut être modélisé, dans un cadre discret, par une assemblée des particules de même diamètre formant un empilement compact. En partant de ce même principe, on modélise la fissuration et la rupture à traction d'un matériau fragile par des approches discrètes (DEM). Pour cela, on dérive une expression pour le facteur d'intensité des contraintes en fonction des forces des contacts entre les particules. La convergence de la méthode est vérifiée par la confrontation des résultats numériques avec des résultats de la littérature

Modélisation de l'orniérage des chaussées à faible trafic

L'orniérage, dû aux déformations permanentes des matériaux non traités (assise en grave non traitée), est l'un des principaux modes de dégradations des chaussées souples. De nos jours, des lois d'élasticité linéaire sont utilisées pour le dimensionnement. Une méthode d'analyse limite pour la prédiction des déformations permanentes des graves non traitées soumises à un chargement cyclique a été développée et présentée dans le cadre de ce travail. Cette méthode, basée sur le concept de l'état limite (shakedown theory), est développée par Zarka (1979) pour les structures métalliques soumises à un chargement cyclique. L'état de contraintes initiales, la teneur en eau et l'anisotropie initiale des graves non traitées sont pris en compte dans la méthode proposée. Une procédure, basée sur les résultats d'essais triaxiaux à chargements répétés, a été développée pour la détermination des paramètres du modèle proposé. Les résultats de modélisation éléments finis, du comportement à long terme d'une structure de chaussée, obtenus avec le modèle sont présentés et comparés aux résultats issus de l'expérience du manège de fatigue du LCPC

Comportement hydromécanique des matériaux granulaires compactés non-saturés

Ce travail présente l'effet de la non-saturation sur le comportement réversible mécanique des matériaux granulaires de chaussées à court terme. On commencera par caractériser le comportement hydrique des différents matériaux. A partir des matériaux précédents, on caractérisera à la boite de cisaillement direct (à chargements statiques) et à l'appareil triaxial (à chargements répétés) les comportements réversibles. Ce travail nous permettra finalement d'injecter la succion dans les modèles classiques d'élasticité non linéaire pour le comportement mécanique des sols granulaires saturés

Seasonal thermal energy storage in shallow geothermal systems: thermal equilibrium stage

This paper is dedicated to the study of seasonal heat storage in shallow geothermal installations in unsaturated soils for which hydrothermal properties such as degree of saturation and thermal conductivity vary with time throughout the profile. In the model, a semi-analytical model which estimates time-spatial thermal conductivity is coupled with a 2D cylindrical heat transfer modeling using finite difference method. The variation of temperature was obtained after 3 heating and cooling cycles for the different types of loads with maximum thermal load of qmax = 15 W.m−1 with variable angular frequency (8 months of heating and 4 months of cooling).and constant angular frequency (6 months of heating and 6 months of cooling) to estimate the necessary number of cycles to reach the thermal equilibrium stage. The results show that we approach a thermal equilibrium stage where the same variation of temperature can be observed in soils after several heating and cooling cycles. Based on these simulations, the necessary number of cycles can be related to the total applied energy on the system and the minimum number of cycles is for a system with the total applied energy of 1.9qmax