Évaluer l'impact du vieillissement des digues sur les mécanismes et scénarios de rupture

Digues Maritimes et Fluviales de Protection contre les Inondations : Digues 2019, AIX EN PROVENCE, FRANCE, 20-/03/2019 - 21/03/2019Les scénarios de rupture des digues sont basés sur des hypothèses d'enchaînement de dégradations ou de ruptures partielles de différentes parties des ouvrages. Ainsi, pour qu'une défaillance intervienne sur un tronçon de digue, il faut que certaines parties d'ouvrage arrivent à la limite de leur 'bon' fonctionnement et que leurs propriétés passent en-dessous d'un certain niveau de performance. L'évolution des propriétés et des performances n'est cependant pas toujours 'binaire' : l'ouvrage semble être dans un état correct puis, instantanément, arrive à la rupture. Cet article propose une discussion sur les mécanismes de vieillissement des matériaux utilisés notamment pour leur fonction d'étanchéité, à partir d'exemples concrets. Le vieillissement est un phénomène normal qui apparaît soit par un usage répété de l'ouvrage soit par des phénomènes normaux de dégradation et d'usure par le temps et les conditions extérieures. Ce vieillissement normal peut être identifié au travers de mécanismes de dégradation qui seraient classés par type de dégradation (météorologique, interaction avec le vivant, interaction avec des masses d'eau), par intensité (fort, moyen, faible) et par cinétique (très long, rapide, instantané). Ces mécanismes de dégradations seraient dépendants de la localisation géographique de l'ouvrage, de la nature des composants et de la qualité initiale de la réalisation de l'ouvrage

Numerical modeling of soil liquefaction at the structure scale : application to hydraulic structures

Les matériaux granulaires présentent un large spectre de propriétés mécaniques. Développer des modèles constitutifs permettant d'intégrer ces caractéristiques dans le cadre de simulations à l'échelle de l'ouvrage demeure un réel challenge scientifique. A cet égard, les approches multi-échelles constituent aujourd'hui une voie très prometteuse. Elles permettent de faire émerger des propriétés macroscopiques à partir de modèles micromécaniques calibrés à l'échelle microscopique.Parmi les modèles multi-échelles, le modèle H marque une avancée majeure pour la prise en compte des effets de la microstructure dans le comportement des matériaux granulaires. La structure du matériau granulaire est décrite par une distribution d'hexagones orientés dans l'espace. A partir d'opérations d'homogénéisation, les contraintes et les déformations incrémentales sont reliées à l'échelle de la distribution, donnant lieu à un modèle de comportement qui a la capacité à reproduire propriétés mécaniques essentielles des matériaux granulaires.Nous étudions dans un premier temps les propriétés mécaniques de l'assemblage hexagonal de grains, élément de base du modèle H, afin d'identifier les conditions menant à sa déstabilisation. Nous réalisons dans un second temps une étude de sensibilité du modèle constitutif vis-à-vis des paramètres micro-mécaniques et microstructurels. Enfin, nous démontrons les capacités opérationnelles du modèle à partir d'essais triaxiaux non drainés réalisés sur un sable lâche liquéfiable.Dans un second temps, le modèle H est implémenté en tant que loi constitutive dans un code de calcul aux différences finies. Des simulations d'essais biaxiaux non homogènes sont conduites afin d'explorer les capacités du modèle à reproduire les différents modes de rupture observés en laboratoire. L'utilisation du modèle H pour modéliser des essais biaxiaux drainés et non drainés met clairement en évidence l'influence de la microstructure sur la réponse mécanique des matériaux granulaires. Enfin, le modèle H est utilisé dans le cadre d'une simulation hydro-mécanique couplée à l'échelle de l'ouvrage pour modéliser le chargement d'une fondation superficielle et la rupture d'une digue soumise à une crue.Granular materials generally exhibit a broad spectrum of mechanical properties. Developing constitutive models to integrate these properties in the context of simulations at the structure scale remains a real scientific challenge. In this respect, multi-scale approaches offer very promising solutions as they allow the emergence of macroscopic properties from micromechanical models calibrated on a microscopic scale.Among the multiscale models, the H model marks a major step forward in taking into account the effects of the microstructure in the behavior of granular materials. The structure of the granular material is described by an assembly of hexagons, oriented in space. From homogenization operations, stresses and incremental strains are related to the scale of the assembly, giving rise to a constitutive model that has the ability to reproduce the essential mechanical properties of granular materials.We first study the mechanical properties of the hexagonal grain assembly in order to identify the conditions leading to the triggering of its instability. We then carry out a study of the sensitivity of the constitutive model with respect to micro-mechanical and microstructural parameters. Finally, we demonstrate the operational capacities of the model from triaxial undrained tests carried out on a liquefiable loose sand.In a second step, the H model is implemented as a constitutive law in a finite difference code. Simulations of non-homogeneous biaxial tests are carried out in order to explore the model's capacities to reproduce the different failure modes observed in the laboratory. The use of the H model to model drained and undrained biaxial tests highlights the influence of the microstructure on the mechanical response of granular materials. Finally, model H is used in numerical simulations at the structure scale to model the loading of a shallow foundation and the failure of a levee subjected to a flooding event

Modélisation numérique du mécanisme de liquéfaction des sols : application aux ouvrages hydrauliques

Granular materials generally exhibit a broad spectrum of mechanical properties. Developing constitutive models to integrate these properties in the context of simulations at the structure scale remains a real scientific challenge. In this respect, multi-scale approaches offer very promising solutions as they allow the emergence of macroscopic properties from micromechanical models calibrated on a microscopic scale.Among the multiscale models, the H model marks a major step forward in taking into account the effects of the microstructure in the behavior of granular materials. The structure of the granular material is described by an assembly of hexagons, oriented in space. From homogenization operations, stresses and incremental strains are related to the scale of the assembly, giving rise to a constitutive model that has the ability to reproduce the essential mechanical properties of granular materials.We first study the mechanical properties of the hexagonal grain assembly in order to identify the conditions leading to the triggering of its instability. We then carry out a study of the sensitivity of the constitutive model with respect to micro-mechanical and microstructural parameters. Finally, we demonstrate the operational capacities of the model from triaxial undrained tests carried out on a liquefiable loose sand.In a second step, the H model is implemented as a constitutive law in a finite difference code. Simulations of non-homogeneous biaxial tests are carried out in order to explore the model's capacities to reproduce the different failure modes observed in the laboratory. The use of the H model to model drained and undrained biaxial tests highlights the influence of the microstructure on the mechanical response of granular materials. Finally, model H is used in numerical simulations at the structure scale to model the loading of a shallow foundation and the failure of a levee subjected to a flooding event.Les matériaux granulaires présentent un large spectre de propriétés mécaniques. Développer des modèles constitutifs permettant d'intégrer ces caractéristiques dans le cadre de simulations à l'échelle de l'ouvrage demeure un réel challenge scientifique. A cet égard, les approches multi-échelles constituent aujourd'hui une voie très prometteuse. Elles permettent de faire émerger des propriétés macroscopiques à partir de modèles micromécaniques calibrés à l'échelle microscopique.Parmi les modèles multi-échelles, le modèle H marque une avancée majeure pour la prise en compte des effets de la microstructure dans le comportement des matériaux granulaires. La structure du matériau granulaire est décrite par une distribution d'hexagones orientés dans l'espace. A partir d'opérations d'homogénéisation, les contraintes et les déformations incrémentales sont reliées à l'échelle de la distribution, donnant lieu à un modèle de comportement qui a la capacité à reproduire propriétés mécaniques essentielles des matériaux granulaires.Nous étudions dans un premier temps les propriétés mécaniques de l'assemblage hexagonal de grains, élément de base du modèle H, afin d'identifier les conditions menant à sa déstabilisation. Nous réalisons dans un second temps une étude de sensibilité du modèle constitutif vis-à-vis des paramètres micro-mécaniques et microstructurels. Enfin, nous démontrons les capacités opérationnelles du modèle à partir d'essais triaxiaux non drainés réalisés sur un sable lâche liquéfiable.Dans un second temps, le modèle H est implémenté en tant que loi constitutive dans un code de calcul aux différences finies. Des simulations d'essais biaxiaux non homogènes sont conduites afin d'explorer les capacités du modèle à reproduire les différents modes de rupture observés en laboratoire. L'utilisation du modèle H pour modéliser des essais biaxiaux drainés et non drainés met clairement en évidence l'influence de la microstructure sur la réponse mécanique des matériaux granulaires. Enfin, le modèle H est utilisé dans le cadre d'une simulation hydro-mécanique couplée à l'échelle de l'ouvrage pour modéliser le chargement d'une fondation superficielle et la rupture d'une digue soumise à une crue

DEM models using direct and indirect shape descriptions for Toyoura sand along monotonous loading paths

International audienceTwo different DEM models are proposed for quantitatively simulating Toyoura sand macroscopic response along various monotonous loading paths and for a wide range of initial densities. The first model adopts spherical particles and compensates for the irregular shapes of Toyoura sand grains by adding an additional rolling resistance stiffness to the classical linear contact model. The second model follows a different strategy whereby rolling stiffness is abandoned in favour of more complex shapes in the form of a few different 3D polyhedrons defined from a 2D micrograph of Toyoura particles. After a preliminary analysis of the number of particles for optimal REV simulations, the two different modelling approaches are calibrated using triaxial compression in so-called drained conditions, adopting a common contact friction angle for the two models. Similar predictive abilities are then obtained along so-called undrained (constant volume) triaxial compression and extension paths. Although it leads to 9-times longer simulations, the polyhedral approach is easier to calibrate regarding the contact parameters. It also enables a more precise description of the microstructure in terms of particle shapes and initial fabric anisotropy, whose crucial role is evidenced in a parametric analysis

A discrete-based multi-scale modeling approach for the propagation of seismic waves in soils

International audienceA three-dimensional multi-scale discrete–continuum model (Finite Volume Method × Discrete Element Method,FVM × DEM) is developed for a discrete-based description of the mechanical behavior of granular soils inboundary value problems (BVPs). In such a scheme, the constitutive response of the material is derived throughdirect DEM computations on a representative volume element attached to each mesh element. The developedmulti-scale approach includes the inertial effect in the stress homogenization formulation and serves to studythe mechanism of propagation of seismic waves, in comparison with a more classical BVP simulation thatadopts an advanced bounding surface plasticity model ‘‘P2PSand’’. We start with a detailed and fair calibrationand validation of these two models against laboratory tests for Toyoura sand under monotonic and cyclicloading. Then, the performance of the two approaches is compared for the case of a seismic wave loadingpassing through a saturated soil column with different relative densities, revealing several differences betweenthe results of the two models

Mesoscopic scale instability in particulate materials

International audienceThis manuscript investigates some instability features in granular materials by considering an elementary grain arrangement on the intermediate scale. Although force chains have long been recognized as playing a basic role in the strength of granular specimens, the collaborative contribution of grain loops (grain arrangement) has been highlighted more recently. As a result, the stability of grain loops is expected to strongly govern the stability of the whole assembly. This paper shows that such elementary patterns can be destabilized even though the contact law between granules is elastic. This behavior stems from the nature of the kinematical model describing the geometrical interaction between neighboring grains

A protocol to assess the seismic criticality of existing small concrete dams

Dams are critical infrastructures whose failure would entail serious consequences for community safety. Although large dams represent the most dangerous items, small size dams may be critical, as a large number of these constructions are built in the proximity to inhabited zones. Earthquake is one of the hazards that may affect an existing dam. To assess safety and plan investments, dam owners need to create prioritisation sequences of interventions through a protocol considering technical, political and societal aspects. It is presented herein a specific procedure, based on a multi-criteria analysis with a rapid screening survey of the infrastructures, which addresses to classify the seismic criticality of dams considering the effects (in terms of loss of life and property) of a failure scenario subsequent to an earthquake. A protocol is used to define a prioritisation of interventions that mitigate the seismic risk. It has been applied to a group of 9 small concrete dams and 17 fixed weirs, built from 1920 to 1990 in Aosta Valley (Italy). As a result, about 30% of the existing dams, which are more vulnerable than fixed weirs, shows the highest level of seismic criticality, and needs to be retrofitted in accordance with the current standards

Influence of plant root system morphology and architectural traits on soil shear resistance

Background and aims : Vegetation can be used to stabilise slopes with regard to shallow landslides, but the optimal plant architecture for conferring resistance is not known. This study aims at identifying root morphological traits which confer the most resistance to soil during shearing.Methods : Three species used for slope stabilisation (Ricinus communis L., Jatropha curcas L. and Rhus chinensis Mill.) were grown for 10 months in large shear boxes filled with silty clay similar to that found in Yunnan, China. Direct shear tests were then performed and compared to fallow soil. Root systems were excavated and a large number of traits measured.Results : Shear strength and deformation energy were enhanced by the presence of roots. Regardless of confining pressure, R. communis conferred most resistance due to its taprooted system with many vertical roots. J. curcas possessed oblique and vertical roots which created fragile zones throughout the soil profile. The least efficient root system was R. chinensis which possessed many horizontal lateral roots. Soil mechanical properties were most influenced by (i) density of roots crossing the shear plane, (ii) branching density throughout the soil profile, (iii) total length of coarse roots above the shear plane and (iv) total volume of coarse roots and fine root density below the shear plane. During failure, fine, short and branched roots slipped through soil rather than breaking.Conclusion : Root morphological traits such as density, branching, length, volume, inclination and orientation influence significantly soil mechanical propertie

Numerical analysis of capillary bridges and coalescence in a triplet of spheres

International audienceThe behavior of a natural soil is known to change substantially in presence of water under unsaturated conditions, due to additional capillary forces. Water can be absorbed by hygroscopic soil particles (such as clay), or remains at the surface of solid grains (sand, silt) and forms either a discontinuous (pendular regime) or a continuous phase (funicular regime), depending on the water content of the soil. Capillary bridges exist solely between pairs of grains at small water contents, giving rise to simple capillary force expressions and straightforward subsequent modeling. For larger water contents, these generic capillary bridges progressively merge into more complex coalesced bridges involving several grains (i.e. at least three) and whose description remains little known. In the present study, a numerical approach based on surface energy minimization isproposed to compute capillary forces for assemblies of two or three grains. The methodology is frst validated for a standard capillary bridge between two grains by comparison both with previous experiments and with other alternative theoretical and numerical approaches. The method is next extended to a triplet of grains within a wide range of water content (or equivalently reduced water volume) during imbibition, to switch from uncoalesced to coalesced bridges. Eventually, the infuence of contact angle, surface tension and gravity on the capillary force, the volume of coalescence and the morphology of the bridge as well is investigated. The present study paves the way for the implementation of capillary efects in micromechanical models relying on mesostructures composed of a few grains