Modelling of interaction between a snow mantle and a flexible structure using a discrete element method

International audienceThe search of improvement of protective techniques against natural phenomena such as snow avalanches continues to use classic methods for calculating flexible structures. This paper deals with a new method to design avalanche protection nets. This method is based on a coupled analysis of both net structure and snow mantle by using a Discrete Element Method. This has led to the development of computational software so that avalanche nets can be easily designed. This tool gives the evolution of the forces acting in several parts of the work as a function of the snow situation

Divergence and flutter instabilities of some constrained two-degree-of-freedom systems

International audienceIt is now well ascertained that a variety of instability modes can appear before the conventional plastic limit condition is met. In this paper, both flutter and divergence instability modes are investigated. First, the mechanical meaning of these instability modes is reviewed, and the criterion for detecting their occurrence is established. Based on an illustration example, the competition between the occurrences of each of these instability modes is analyzed, showing that the prevalence of a given mode is strongly related to both the loading conditions and the stiffness properties of the material system in hand

Stability of non-conservative elastic structures under additional kinematics constraints

International audienceIn this paper, the specific effect of additional constraints on the stability of undamped non-conservative elastic systems is studied. The stability of constrained elastic system is compared to the stability of the unconstrained system, through the incorporation of Lagrange multipliers. It is theoretically shown that the second-order work criterion, dealing with the symmetric part of the stiffness matrix corresponds to an optimization criterion with respect to the kinematics constraints. More specifically, the vanishing of the second-order work criterion corresponds to the critical kinematics constraint, which can be interpreted as an instability direction when the material stability analysis is considered (typically in the field of soil mechanics). The approach is illustrated for a two-degrees-of-freedom generalised Ziegler's column subjected to different constraints. We show that a particular kinematics constraint can stabilize or destabilize a non-conservative system. However, for all kinematics constraints, there necessarily exists a constraint which destabilizes the non-conservative system. The constraint associated to the lowest critical load is associated with the second-order criterion. Excluding flutter instabilities, the second-order work criterion is not only a lower bound of the stability boundary of the free system, but also the boundary of the stability domain, for all mixed perturbations based on proportional kinematics conditions

Instabilities of a sand layer subjected to an upward water flow by a 2D coupled discrete element - Lattice Boltzmann hydromechanical model

This work deals with the numerical simulation of the instabilities occurring in a sand layer subjected to an upward water flow. A coupled Discrete Elements - Lattice Boltzmann hydromechanical model is used for this end. After a brief presentation of the numerical model, simulations of an upward fluid flow through granular deposits are performed for two cases namely under controlled hydraulic gradients and under controlled volumetric flow rates. In the first case i.e. under controlled hydraulic gradient, the simulations show that the quicksand condition is actually reached for a hydraulic gradient very close to the critical hydraulic gradient calculated from the global analysis of classical soil mechanics. The simulations point out moreover that the quicksand phenomenon could be produced locally under slightly lower gradients. In the second case i.e. under controlled volumetric flow rates, the simulations show that there are three levels of flow ; low flow rates that allow infiltration without any destabilization, medium flow rates that cause expansion of the deposit to increase its permeability and high flow rates which may cause the formation continuous tunnel between the upstream and the downstream sides as well as sand boils. It is shown also that under the controlled flow rate condition the hydraulic gradient remains in all cases less than the average critical hydraulic gradient

On the stability of nonconservative elastic systems under mixed perturbations

International audienceThis paper shows that the loading mode strongly influences the stability of discrete non-conservative elastic systems. The stability of the constrained system is compared to the stability of the unconstrained system, through the incorporation of Lagrange multipliers. Initially, the approach is illustrated for a two-degrees-of-freedom generalized Ziegler's column. Then, it is applied to a two-degrees-of-freedom model representing a soil constrained with isochoric loading. The isochoric instability load is not necessarily greater than the instability load of the free problem. Excluding flutter instabilities, it is shown that the second-order work criterion is not only a lower bound of the stability boundary of the free system, but also the boundary of the stability domain, in presence of mixed perturbations based on proportional kinematic conditions.Cet article étudie l'influence du mode de chargement sur la stabilité de systèmes élastiques discrets non conservatifs. La stabilité du système contraint est comparée à celle du système libre, par l'introduction de multiplicateurs de Lagrange. L'approche est illustrée avec le pendule généralisé de Ziegler. Elle est ensuite appliquée à un modèle à deux degrés de liberté représentant un sol contraint par un chargement isochore. On montre que le chargement isochore affecte sensiblement la frontière de stabilité pour le problème conservatif et pour le problème non conservatif. En dehors des instabilités par flottement, le critère de travail du second-ordre constitue une borne inférieure de la frontière de stabilité du système libre ainsi que la frontière du domaine de stabilité du système sous chargements mixtes proportionnels en déplacement

Approche multi-échelle de la rupture

Dans de nombreuses applications du génie civil, la détection précoce d'un état de rupture constitue un enjeu fondamental. Dans le contexte de la géomécanique, une classe fondamentale de rupture pour un système, contrôlé par des paramètres bien définis, correspond à la création d'énergie cinétique sans évolution des paramètres de contrôle. Il est alors montré que de telles bifurcations peuvent être détectées par l'annulation du travail du second ordre, à l'échelle macroscopique, défini à partir du champ de variables contraintes-déformations tensorielles. En outre, tenant compte de la nature souvent discrète des géomatériaux, on établit que le travail du second ordre macroscopique, évalué à l'échelle d'un assemblage granulaire, correspond à la somme de tous les travaux du second-ordre microscopiques, évalués au droit de chaque contact de l'assemblage à partir des grandeurs discrètes. Cette équivalence micro-macro fondamentale donne lieu à une interprétation micro-structurelle de l'annulation du travail du second ordre au sein d'un assemblage granulaire

Caractérisation numérique d'une loi stochastique d'impact d'un bloc rocheux sur un éboulis

Les logiciels de trajectographie constituent un outil indispensable pour la détermination des zones soumises à un risque de chute de bloc en zones montagneuses. Une amélioration fondamentale à apporter à ces codes de calcul est la caractérisation fine du rebond du bloc sur le sol. Dans cette optique, une modélisation numérique de l'interaction entre le bloc et le sol par la Méthode des Elements Discrets est développée de façon à étudier l'impact d'un bloc rocheux sur un éboulis. Le traitement statistique des résultats issus des simulations numériques permet de définir une loi d'impact stochastique valable quels que soient le point d'impact et les conditions cinématiques initiales du bloc

Культурные последствия глобализации

Mitigation strategies against natural risks in mountainous areas often include protection works against torrential flooding, avalanches or falling rocks. These structures, which are submitted to specific phenomena as well as they are built in a difficult geotechnical context, must be designed in compliance with specific requirements. Torrent control dams based on an active protection concept are often built to limit the risks of bank slip in the upper part of the catchment area. Usually, design of such structures is based on a two-dimensional approach, allowing force reactions applied by the soil to the foundations of the structure to be assessed. Furthermore, the thrust of the bank is seldom taken into account, even if the loading is likely to be the cause of a lot of observed damages. This paper exposes a methodology whose purpose is to improve the modelling of interaction between soil and structure. After an original analytical approach is presented, the main basis of numerical simulation (Finite Element Code) are defined.Les barrages de consolidation visent à limiter l'érosion du chenal d'écoulement des torrents et sont souvent implantés dans des sites où les berges sont fortement instables. Ces zones constituent en effet d'importantes zones d'alimentation en matériaux solides. Les méthodes actuelles de dimensionnement de ces ouvrages restent basées sur une approche bidimensionnelle d'estimation des actions et des réactions du sol en fondation. De même, les actions dues aux poussées des berges instables, qui entraînent de nombreuses pathologies sur les ouvrages, restent mal connues et ne sont pas prises en compte dans les justifications de la stabilité externe et interne des ouvrages. Cet article présente la problématique et la démarche d'étude en vue d'améliorer la connaissance des interactions entre les barrages et le sol au niveau des fondations et des berges. Dans cette perspective, une approche analytique est proposée, puis les bases d'une simulation numérique introduisant la méthode des Eléments Finis sont définies