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

Déstabilisation gravitaire d'un matériau granulaire immergé

La stabilité gravitaire des ouvrages hydrauliques (digues fluviales, barrage en remblai, ...) est un problème complexifié par la présence d'infiltration d'eau au sein même de l'ouvrage et de ses fondations. Le territoire national compte près de 10000 km de digues, d'où un enjeu important en terme de risque d'inondation. Les déstabilisations gravitaires observées sur les ouvrages hydrauliques se distinguent par leur brièveté et par leur faible emprise spatiale. L'objectif de ce travail de thèse est d'étudier spécifiquement ces instabilités brèves et de faible déplacement cumulé le long de la pente dans le cas de matériaux granulaires saturés. Dans un premier temps, une campagne expérimentale a été effectuée sur la base d'un protocole de chargement progressif dans lequel un échantillon de sol est lentement incliné jusqu'à obtenir une déstabilisation massive. Il en ressort une phénoménologie complexe composée d'un nombre de précurseurs d'avalanche dépendant de la fraction volumique de l'échantillon. Une forte influence de la fraction volumique du sol sur le seuil de stabilité est également mise en avant. Une deuxième campagne expérimentale basée sur un protocole d'effondrement contrôlé, à inclinaison imposée, a été menée dans le but d'amplifier la cinématique de déstabilisation observée en chargement progressif. Mis à part les échantillons les plus denses qui produisent de simples avalanches de surface, on observe dans les autres cas un ou plusieurs événements successifs, d'allure circulaire, mobilisant des profondeurs très importantes contrairement aux avalanches de surface.. Outre l'inclinaison, on voit bien que, là encore, la fraction volumique est un paramètre clé dans le contrôle de la cinématique de déstabilisation. Des essais préliminaires laissent cependant entendre que, au-delà de la fraction volumique, c'est la microstructure qui semble être le vrai paramètre de contrôle à travers notamment la forme des grains et leur enchevêtrement.Gravitational stability of hydraulic structures (river dykes, embankment dam,...) is a problem complicated by the presence of water infiltration within the structure and its foundations. Our country has about 10,000 km of dikes and gravitational failure is a major challenge in terms of flood risk. In hydraulic structures, these destabilizations are characterized by their brevity and their small spatial extent. The objective of this thesis is to study specifically this type of brief instabilities in the case of a saturated granular material. A first series of experiments was carried out on the basis of a progressive loading protocol in which a soil sample is tilted slowly until a massive destabilization is triggered. It shows a phenomenology complex consisting of a number of precursory events prior final avalanching, dependent on the solid volume fraction of the sample. A strong impact of the solid volume fraction of the soil on the stability threshold is also highlighted. A second series of experiments based on a protocol of collapse controlled at imposed slope was conducted to amplify the kinematics of destabilization observed by progressive loading. Apart from the denser samples that produce simple surface avalanches, one or more successive events are observed in other cases. Unlike surface avalanches, these events mobilize grains much deeper, in an almost semi-circular area. In addition to slope angle, solid volume fraction is once again a key parameter in controlling the kinematics of destabilization. Preliminary tests suggest, however, that, beyond solid volume fraction, microstructure appears to be the true control parameter, particularly through grain shape and texture.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF