Interacciones entre la gaviota patiamarilla Larus michahellis y la gaviota de Audouin Larus audouinii en la isla de Dragonera

Interacciones entre la gaviota patiamarilla Larus michahellis y la gaviota de Audouin Larus audouinii en la isla de Dragonera. La gaviota patiamatilla Larus michahellis es una espècie superabundante en el Mediterràneo occidental, lo que supone un grave impacto sobre la biodiversidad. En la isla de Dragonera, unas pequeñas colonias de gaviotas de Audouin Larus audouinii nidifican en la proximidad de una macro-colonia de gaviotas patiamatillas. La siguiente nota reporta los resultados de un estudio piloto sobre las interacciones entre ambas especies. El objetivo es determinar en que medida las gaviotas patiamatillas tienen un impacto negativo sobre el éxito reproductor de las colonias de gaviotas de Audouin.Interaccions entre la gavina vulgar Larus michahellis i la gavina de bec vermell Larus audouinii a l'illa de sa Dragonera. La gavina vulgar Larus michahellis és una espècie superabundant en el Mediterrani occidental, la qual cosa suposa un greu impacte sobre la biodiversitat. A l'illa de sa Dragonera, unes petites colònies de gavines de bec vermell Larus audouinii nidifiquen en la proximitat d'una macro-colònia de gavines vulgars. La següent nota reporta els resultats d'un estudi pilot sobre les interaccions entre ambdues espècies. L'objectiu és determinar en quina mesura les gavines vulgars tenen un impacte negatiu sobre l'èxit reproductor de les colònies de gavines de bec vermell.Interaction between Yellow-legged Gull Larus michabellis and Andouin 's Gull Larus audouinii on the island of Dragonera. Yellow-legged Gull Larus michahellis is a superabundant species in the West Occidental Mediterranean. This supposes dramatic impacts on global biodiversity. In Dragonera island, small colonies of Audouin's Gull breed close to a macro-colony of Yellow-legged Gulls. The following note reports the results of a pilot study on interactions between the two gull species. The goal is to determine how Yellowlegged Gulls have a negative impact on the reproductive success of the Audouin's Gulls

Stiffness pathologies in discrete granular systems: bifurcation, neutral equilibrium, and instability in the presence of kinematic constraints

The paper develops the stiffness relationship between the movements and forces among a system of discrete interacting grains. The approach is similar to that used in structural analysis, but the stiffness matrix of granular material is inherently non-symmetric because of the geometrics of particle interactions and of the frictional behavior of the contacts. Internal geometric constraints are imposed by the particles' shapes, in particular, by the surface curvatures of the particles at their points of contact. Moreover, the stiffness relationship is incrementally non-linear, and even small assemblies require the analysis of multiple stiffness branches, with each branch region being a pointed convex cone in displacement-space. These aspects of the particle-level stiffness relationship gives rise to three types of micro-scale failure: neutral equilibrium, bifurcation and path instability, and instability of equilibrium. These three pathologies are defined in the context of four types of displacement constraints, which can be readily analyzed with certain generalized inverses. That is, instability and non-uniqueness are investigated in the presence of kinematic constraints. Bifurcation paths can be either stable or unstable, as determined with the Hill-Bazant-Petryk criterion. Examples of simple granular systems of three, sixteen, and sixty four disks are analyzed. With each system, multiple contacts were assumed to be at the friction limit. Even with these small systems, micro-scale failure is expressed in many different forms, with some systems having hundreds of micro-scale failure modes. The examples suggest that micro-scale failure is pervasive within granular materials, with particle arrangements being in a nearly continual state of instability

Marsilea strigosa Willd. en dos localidades de la provincia de Córdoba

Marsilea strigosa Willd. es un helecho semi-acuático raro y muy localizado en la Península Ibérica (Paivia 1986), cuyo hábitat principal lo constituyen los estanques temporales mediterráneos. El descubrimiento de dos poblaciones cordobesas nos ha parecido digno de difundir. Se identifi caron varios especímenes con obras de referencia (Paivia 1986, Prelli 2001

Material stability analysis based on the local and global elasto-plastic tangent operators

International audienceThe present paper investigates bifurcation in geomaterials with the help of the second-order work criterion. The approach applies mainly to non associated materials such as soils. The analysis usually performed at the material point level is extended to quasi-static boundary value problems, by considering the finite element stiffness matrix. The first part of the paper reminds some results obtained at the material point level. The bifurcation domain is presented in the 3D principal stress space as well as 3D cones of unstable loading directions for an incrementally nonlinear constitutive model. In the second part, the analysis is extended to boundary value problems in quasi-static conditions. Non-linear finite element computations are performed and the global tangent stiffness matrix is analyzed. For several examples the eigenvector associated with the first vanishing eigenvalue of the symmetrical part of the stiffness matrix gives an accurate estimation of the failure mode even for non homogeneous boundary value problems

Modélisation des glissements de terrains par un critère d'instabilité matérielle

Cet article présente une étude du critère d'instabilité matérielle de Hill, qui est bien approprié pour décrire tous les modes de rupture des sols. La première partie est dédiée à l'étude analytique et numérique de ce critère au travers des relations constitutives octo et incrémentalement non linéaires de Darve. Les limites des domaines de bifurcation 3D ainsi que les cônes 3D de directions instables sont données pour ces deux lois. Puis dans une seconde partie, on montre que ce critère, implémenté dans un code aux éléments finis avec couplage hydromécanique, permet de décrire et de reproduire convenablement les mécanismes de rupture et les surfaces de glissement du glissement de terrain de Pettacciato, qui s'est produit après de fortes pluies sous une pente de seulement 6° en moyenne

P-positive definite matrices and stability of non conservative systems

International audienceThe bifurcation problem of constrained non-conservative systems with non symmetric stiffness matrices is investigated. It leads to study the subset $D_{p,n}$ of $ℳn(ℝ)$ of the so called $p$-positive definite matrices ($1 ≤ p ≤ n$). The main result ($D_{1,n} ⊂ D_{p,n}$) is proved, the reciprocal result is investigated and the consequences on the stability of elastic nonconservative systems are highlighted

Approche en contrainte effective pour le couplage hydromécanique dans les milieux poreux non saturés

Nous présentons ici un modèle hydromécanique couplé pour les milieux poreux non saturés. La partie mécanique est décrite par une loi élastoplastique non associée(Barnichon 1998). La partie hydrique est décrite par le modèle de Van Genuchten, pour les courbes limites de rétention d'eau, et celui de Mualem 1974 pour les courbes de balayages. De plus, une relation permet d'exprimer la pression d'entrée d'air en fonction de la porosité. Enfin le couplage complet se fait avec la contrainte effective de Bishop en utilisant la relation de Modaressi(1994)pour l'évolution du paramètre de succion Xi

Influence des cycles hydriques de la dessiccation et de l'humidification sur le comportement hydromécanique des géomatériaux non saturés

Ce travail de recherche porte sur le comportement des milieux poreux (triphasiques), plus particulièrement les sols non saturés sous sollicitations hydro-mécaniques. Un modèle constitutif élastoplastique couplé est développé. Ce modèle original est formulé selon les principes suivants: une loi constitutive est développée pour décrire le comportement de chaque phase (squelette solide, liquide, et gaz). Ensuite, des relations de couplage sont ajoutées entre chacune des phases. Pour le comportement du squelette solide, une loi élastoplastique non associée est adoptée, avec deux surfaces de charges, en cisaillement et en compression. La partie hydrique est décrite par une formulation qui permet de prendre en compte l effet d hystérésis. Ce modèle a été enrichi par une relation de couplage hydromécanique qui permet d exprimer la pression d entrée d air en fonction de la porosité. Ensuite, le couplage complet se fait avec la contrainte effective de Bishop en utilisant une nouvelle définition du paramètre de succion grâce à laquelle, les différents phénomènes présents dans la réponse des milieux poreux sous différentes sollicitations peuvent être reproduits. Ce modèle est validé par une confrontation à des données expérimentales issues de la littérature sur différents types de sol (sable, limon, ). Le modèle est implanté dans le code aux éléments finis Cast3M. L analyse de problèmes particuliers, tels que la mise en œuvre d un cas test d un sol d assise soumis à un cycle pluvial, ainsi que l étude de la stabilité d une pente, permette de montrer la capacité du modèle à reproduire le comportement des milieux poreux non saturés.This work focuses on the behaviour of porous triphasic media, particularly on unsaturated soils subjected to hydromechanical loading. A coupled elastoplastic constitutive model has been developed. This original model is formulated according to the following principles: (1) a constitutive law describing the behaviour of different phases (solid skeleton, liquid and gas). (2) coupling relationships between each phase. For the behaviour of the solid skeleton, a non associated elastoplastic constitutive law is adopted, with two loading surfaces: shear surface and compression cap surface. The hydric part is discribed using a formulation which allows to take into account the hysteresis effect. This model has been extended using a hydromechanical coupling relation between the air entry value and the porosity. Then the coupling is completed with the Bishop effective stress, using a new definition for the suction parameter . Using this formulation, the various phenomena present in the porous media behaviour under different loading can be reproduced. The developed model has been validated through a comparison with experimental data on different types of soil (sand, silt, ). This model is implemented in the french finite element code Cast3M. The analysis of specific problems, such as (1) the study of shallow foundation subjected to cyclic rain event, as well as (2) the study of slope stability, show the model capacity to reproduce the behaviour of unsaturated porous media.VILLEURBANNE-DOC'INSA-Bib. elec. (692669901) / SudocSudocFranceF

Relation between the collapse of force-chains and grain loops evolution in granular materials

This study focuses on the mesoscale mechanism of the two dimensional granular materials, involving 2 structures: the force-chains and the grain loops [3]. These two structures are the essential elements that contribute to the behavior of the specimen, sharing a very close relation. By using the discrete element method, a dense 2D specimen of a spherical particles assembly is investigated. The purpose is to point out the connection between the stability of the force-chains and the attached grain loops around the force-chains during their life span. On the other hand, the second order work [1][2] is also taken into account as an instability criterion. In this way, the evolution of the second order work of grain loops is tracked over the loading path, in order to point out a relation between force chains buckling and vanishing of second-order work. It is shown that during the life span of force-chains, the grain loops of 3 particles (loop-3) and of 6 particles (loop-6) show a consistent trend: the number of attached loop-3 decreases over time, and on the contrary, the number of loop-6 increases. The evolution suggests that there is a transition phase from loop-3 to loop-6, from the moment the force chains are created, till the moment they collapse. Moreover, the evolution of the second-order works of attached loop-3 and loop-6 shows that, respecting to time, the second-order work of attached loop-3 is greater than the attached loop-6. This result ensures that the loop-3 is more stable than the loop-6

Relation between the collapse of force-chains and grain loops evolution in granular materials

This study focuses on the mesoscale mechanism of the two dimensional granular materials, involving 2 structures: the force-chains and the grain loops