Alain Corfdir

Annular shear of cohesionless granular materials: from the inertial to quasistatic regime.

Using discrete simulations, we investigate the behavior of a model granular material within an annular shear cell. Specifically, two-dimensional assemblies of disks are placed between two circular walls, the inner one rotating with prescribed angular velocity, while the outer one may expand or shrink and maintains a constant radial pressure. Focusing on steady state flows, we delineate in parameter space the range of applicability of the recently introduced constitutive laws for sheared granular materials (based on the inertial number). We discuss the two origins of the stronger strain rates observed near the inner boundary, the vicinity of the wall and the heteregeneous stress field in a Couette cell. Above a certain velocity, an inertial region develops near the inner wall, to which the known constitutive laws apply, with suitable corrections due to wall slip, for small enough stress gradients. Away from the inner wall, slow, apparently unbounded creep takes place in the nominally solid material, although its density and shear to normal stress ratio are on the jammed side of the critical values. In addition to rheological characterizations, our simulations provide microscopic information on the contact network and velocity fluctuations that is potentially useful to assess theoretical approaches.

A CYLINDER SHEAR APPARATUS

Studies of the interface between soil and an element of structure are actively carried out, considering the importance of the behavior of such interfaces in civil engineering applications such as deep foundations, retaining structures, and soil reinforcement. In the past a wide variety of different testing devices have been used, such as the modified direct or simple shear apparatus, pull-out devices, and annular shear apparatuses. Here a new shear device, the cylinder shear apparatus, is presented that has been designed to enhance our shear testing possibilities. This new shear device allows, as ring shear devices, significant shear displacements with an improved homogeneity of the interface. It allows also the control of the normal confining stress, stress control in a direction parallel to the interface, and visualization of the interface kinematics. The cylinder shear apparatus gives also the possibility of performing tests with natural granular material and/or with Schneebelis rollers, with dry, saturated, drained and undrained samples and continuous pore fluid pressure monitoring.

MRI investigation of granular interface rheology using a new cylinder shear apparatus

The rheology of granular materials near an interface is investigated through proton magnetic resonance imaging. A new cylinder shear apparatus has been inserted in the magnetic resonance imaging device, which allows the control of the radial confining pressure exerted by the outer wall on the grains and the measurement of the torque on the inner shearing cylinder. A multi-layer velocimetry sequence has been developed for the simultaneous measurement of velocity profiles in different sample zones, while the measurement of the solid fraction profile is based on static imaging of the sample. This study describes the influence of the roughness of the shearing interface and of the transverse confining walls on the granular interface rheology.

Mathematical model of flexure in a saturated geological layer: application to faulting

Abstract We model the flexure of an isotropic, poroelastic layer. We study two sets of boundary conditions: prescribed displacement and prescribed bending moment. These two kinds of boundary conditions are equivalent for an elastic, not for a poroelastic medium. The model is simple enough to yield analytic solutions. Late deformation takes place under constant bending moment, but stress relaxation is obtained under constant curvature. The magnitude of these phenomena depends on Poissons ratio. Faulting proves to be sensitive to the loading conditions (prescribed curvature or bending moment). For the Tresca strength criterion, faulting is most critical in the state reached immediately after loading at prescribed curvature, but is time-independent if the bending moment is prescribed. For the Mohr–Coulomb criterion at prescribed bending moment, the asymptotic state is most critical. At prescribed curvature, the conclusion depends on Poissons ratio.

Calculating the critical buckling force in compressed bottom flanges of steel–concrete composite bridges

French practice for assessing the risk of distortional buckling of composite bridges is based on the Engesser or Lazard methods which assimilate the phenomenon to in-plane buckling of the compressed flanges. A flange is modelled as a uniformly-compressed rectangular-section ‘beam’ of constant depth with periodic (Lazard) or continuous (Engesser) elastic restraints. The purpose of the study is to consider more realistic assumptions in order to optimize flange dimensions, bearing in mind the fact that the Eurocodes are more severe with respect to instability phenomena than the current French regulations. In particular, the study addresses the case of variable compressive force with variable flange thickness. The results are presented as designs chart for design use.

Validity conditions of the direct boundary integral equation for exterior problems of plane elasticity

Abstract Writing the boundary integral equation for an exterior problem of plane elasticity has been subordinate, so far, to hypotheses on the asymptotical behaviour of solutions at infinity. The sufficient conditions met in the literature are too restrictive and do not notably cover the case when the loading has a non-zero resultant force. This difficulty can be removed by considering the problem in displacements relatively to one point located at a finite distance from the loading. Finally, this auxiliary problem allows the widening of the conditions of validity of the usual formulation of the direct integral method. To cite this article: A. Corfdir, G. Bonnet, C. R. Mecanique 335 (2007).

La méthode de Kranz d'hier à aujourd'hui: une revue critique

La methode de Kranz concerne les murs et parois ancres et doit permettre de verifier la longueur des ancrages. Cette methode deja ancienne (1939) visait a satisfaire les besoins apparus quelque temps auparavant avec le developpement des ancrages et des ecrans. Au cours de presque soixante-dix ans, il s’est produit une evolution des techniques d’ancrages qui sont passees de la plaque d’ancrage au scellement. En parallele, diverses adaptations de la methode originale de Kranz ont ete proposees et utilisees. Des experiences ont aussi ete realisees pour tester la validite de la demarche proposee par Kranz. Finalement, on etudie le bien-fonde de methodes derivees de Kranz au vu des pratiques actuelles et on evoque quelques alternatives possibles.

Condition de glissement sous les ouvrages poids portuaires Étude bibliographique

Un grand nombre d’ouvrages portuaires resistent aux efforts horizontaux qui leur sont appliques par la mobilisation d’un frottement sur leur base (digues verticales, quais poids, quais en caissons...). La bonne connaissance des conditions de glissement de ces ouvrages permet d’en optimiser la masse et donc, de maniere directe, le cout. Les pratiques actuelles restent tres sommaires et simplistes dans l’appreciation de la mobilisation du frottement sous les ouvrages poids. La presente etude consiste en une revue de la condition de glissement sous les ouvrages poids portuaires. Ont ete examines guides et reglements, ouvrages techniques, articles scientifiques, rapports de recherche, ainsi que certains comptes rendus d’essais. Dans le cas des digues verticales, les travaux se sont averes nombreux et confirment l’importance du role de la condition de glissement. Nous avons aborde de nombreux aspects comme les principes de verifications des ouvrages, l’etude des accidents, le coefficient de frottement tel qu’il ressort des recommandations, des essais a grande echelle et des essais de laboratoire, l’effet de dispositions constructives sous la semelle, l’effet du vieillissement et des sollicitations cycliques, le role joue par le developpement des pressions interstitielles. Nous presentons aussi les modes de dimensionnement alternatifs qui ont ete proposes autour de la notion de limitation de la distance de glissement des caissons.L’etude complete est disponible sur le site Internet du Cetmef : www.cetmef.equipement.gouv.fr, Rubrique : Les Publications - Ouvrages & equipements.

Double Time Scale Analysis of the Consolidation of a Two-Strata Poroelastic Layer with High Permeability Contrast

The consolidation of a saturated porous medium submitted to a vertical load is a classical problem, and has been given a complete analytical solution within the framework of linear poroelasticity. For a layer constituted by several homogeneous strata, the problem can be dealt with by means of analytical as well as numerical methods, provided that the contrast between the properties of the strata remain moderate. In this paper, we consider a layer constituted by two strata and discuss the situation in which the contrast between the properties of the strata becomes very large and fluid conduction exhibits very different characteristic times in each stratum. The method used consists in introducing a double time scale, and gives an analytical solution, which makes it possible to discuss the influence of the ratio between strata thicknesses on the overall consolidation time of the layer.