Management of natural resources through automatic cartographic inventory

The author has identified the following significant results. Significant results of the ARNICA program (February - December 1973) were: (1) The quantitative processing of ERTS-1 data was developed along two lines: the study of geological structures and lineaments of Spanish Catalonia, and the phytogeographical study of the forest region of the Landes of Gascony (France). In both cases it is shown that the ERTS-1 imagery can be used in establishing zonings of equal quantitative interpretation value. (2) In keeping with the operational transfer program proposed in previous reports between exploration of the imagery and charting of the object, a precise data processing method was developed, concerning more particularly the selection of digital equidensity samples computer display and rigorous referencing

Stress-strain behavior and geometrical properties of packings of elongated particles

We present a numerical analysis of the effect of particle elongation on the quasistatic behavior of sheared granular media by means of the Contact Dynamics method. The particle shapes are rounded-cap rectangles characterized by their elongation. The macroscopic and microstructural properties of several packings subjected to biaxial compression are analyzed as a function of particle elongation. We find that the shear strength is an increasing linear function of elongation. Performing an additive decomposition of the stress tensor based on a harmonic approximation of the angular dependence of branch vectors, contact normals and forces, we show that the increasing mobilization of friction force and the associated anisotropy are key effects of particle elongation. These effects are correlated with partial nematic ordering of the particles which tend to be oriented perpendicular to the major principal stress direction and form side-to-side contacts. However, the force transmission is found to be mainly guided by cap-to-side contacts, which represent the largest fraction of contacts for the most elongated particles. Another interesting finding is that, in contrast to shear strength, the solid fraction first increases with particle elongation, but declines as the particles become more elongated. It is also remarkable that the coordination number does not follow this trend so that the packings of more elongated particles are looser but more strongly connected.Comment: Submited to Physical Review

Force transmission in a packing of pentagonal particles

We perform a detailed analysis of the contact force network in a dense confined packing of pentagonal particles simulated by means of the contact dynamics method. The effect of particle shape is evidenced by comparing the data from pentagon packing and from a packing with identical characteristics except for the circular shape of the particles. A counterintuitive finding of this work is that, under steady shearing, the pentagon packing develops a lower structural anisotropy than the disk packing. We show that this weakness is compensated by a higher force anisotropy, leading to enhanced shear strength of the pentagon packing. We revisit "strong" and "weak" force networks in the pentagon packing, but our simulation data provide also evidence for a large class of "very weak" forces carried mainly by vertex-to-edge contacts. The strong force chains are mostly composed of edge-to-edge contacts with a marked zig-zag aspect and a decreasing exponential probability distribution as in a disk packing

Rheophysics of dense granular materials : Discrete simulation of plane shear flows

We study the steady plane shear flow of a dense assembly of frictional, inelastic disks using discrete simulation and prescribing the pressure and the shear rate. We show that, in the limit of rigid grains, the shear state is determined by a single dimensionless number, called inertial number I, which describes the ratio of inertial to pressure forces. Small values of I correspond to the quasi-static regime of soil mechanics, while large values of I correspond to the collisional regime of the kinetic theory. Those shear states are homogeneous, and become intermittent in the quasi-static regime. When I increases in the intermediate regime, we measure an approximately linear decrease of the solid fraction from the maximum packing value, and an approximately linear increase of the effective friction coefficient from the static internal friction value. From those dilatancy and friction laws, we deduce the constitutive law for dense granular flows, with a plastic Coulomb term and a viscous Bagnold term. We also show that the relative velocity fluctuations follow a scaling law as a function of I. The mechanical characteristics of the grains (restitution, friction and elasticity) have a very small influence in this intermediate regime. Then, we explain how the friction law is related to the angular distribution of contact forces, and why the local frictional forces have a small contribution to the macroscopic friction. At the end, as an example of heterogeneous stress distribution, we describe the shear localization when gravity is added.Comment: 24 pages, 19 figure

Agroforesterie et services écosystémiques en zone tropicale

Respectueux de l’environnement et garantissant une sécurité alimentaire soutenue par la diversification des productions et des revenus qu’ils procurent, les systèmes agroforestiers apparaissent comme un modèle prometteur d’agriculture durable dans les pays du Sud les plus vulnérables aux changements globaux. Cependant, ces systèmes agroforestiers ne peuvent être optimisés qu’à condition de mieux comprendre et de mieux maîtriser les facteurs de leurs productions. L’ouvrage présente un ensemble de connaissances récentes sur les mécanismes biophysiques et socio-économiques qui sous-tendent le fonctionnement et la dynamique des systèmes agroforestiers. Il concerne, d’une part les systèmes agroforestiers à base de cultures pérennes, telles que cacaoyers et caféiers, de régions tropicales humides en Amérique du Sud, en Afrique de l’Est et du Centre, d’autre part les parcs arborés et arbustifs à base de cultures vivrières, principalement de céréales, de la région semi-aride subsaharienne d’Afrique de l’Ouest. Il synthétise les dernières avancées acquises grâce à plusieurs projets associant le Cirad, l’IRD et leurs partenaires du Sud qui ont été conduits entre 2012 et 2016 dans ces régions. L’ensemble de ces projets s’articulent autour des dynamiques des systèmes agroforestiers et des compromis entre les services de production et les autres services socio-écosystémiques que ces systèmes fournissent

Les micromécanismes de déformations plastiques dans un sol granulaire

L’analyse de la distribution orientée des forces de contact dans un milieu granulaire simple a permis d'identifier et de préciser deux mécanismes d’écrouissage, l'un lié à révolution des valeurs moyennes des forces de contact, l'autre à l’évolution de la largeur des distributions de ces forces. Ces résultats sont replacés dans le cadre d'une analyse élastoplastique. Nous montrons que le modèle ainsi défini permet de traduire l'essentiel des comportements observés expérimentalement dans les sols granulaires

Analyses du comportement des milieux granulaires basées sur leur nature discontinue

Cette étude tente de faire le lien entre les phénomènes discrets se produisant dans un milieu granulaire et le comportement global du matériau considéré comme continu.L'auteur présente tout d'abord une étude bibliographique sommaire sur ce sujet.Dans une deuxième partie, il propose une analyse basée sur un modèle analogique numérique qui permet :

Modélisation de l'interaction entre géomatériaux et structures à câbles : un exemple implicite de contrôle structurel

This paper considers recent contributions concerning metallic cable structures designed to stop rockfalls. These structures are composed of metallic nets made with unusual materials : Anti-Sub-Marin (ASM) rings. Because of the high level of stresses which are likely to be generated during the impact between a boulder and a protective work, it is useful to design dissipative springs : constitutive and structural ones. A proper mechanical framework to analyse the behaviour of cable structures hitted by boulders is built. This involves a complete description of the different dissipative springs to be proposed. This approach has allowed a specific computational software to be developed in order to simulate mechanical behaviour of cable structures, and thus to design and to optimize these works in interaction with geomaterials mass-movements.Cet article présente les contributions récentes en matière de dimensionnement des structures souples de protection contre les chutes de pierres. Ces structures sont habituellement constituées de nappes de filets constituées d'anneaux métalliques Anti-Sous-Marin (ASM). Le niveau important d'efforts auxquels la structure est susceptible d'être sollicitée justifie la mise en place de sources de dissipation d'énergie : de manière constitutive, au cours de la déformation élasto-plastique de la nappe de filet, mais aussi structurelle à l'aide d'organes spécifiques. Le cadre mécanique rigoureux de l'analyse est présenté ; celui-ci intègre le développement du modèle de comportement mécanique de la nappe de filet. Cette approche a permis le développement d'un code de calcul, dont l'objet est de dimensionner et d'optimiser les structures souples de protection soumises à l'interaction d'écoulements gravitaires de géomatériaux