The authors have studied the mechanical properties of two-size disc mixtures, in relation to their geometrical structure. These packings present an orientational order: the spatial extent of this order, depending on the composition of the sample, has been measured in terms of a characteristic length xi . The stress-strain law in an oedometric compression is particularly sensitive to the disorder of contacts between the discs, but also clearly depends an orientational order.

In granular media under compression, the forces are transmitted in a very heterogeneous way as can be seen from photoelasticity experiments.

Nous présentons une synthèse de travaux menés par notre équipe sur des empilements 2d de cylindres soumis à une compression verticale œdométrique.

We measured the conductivity of 2nd and 3d packings of conducting grains under a uniaxial vertical pressure. The conductance as a function of stress is a power law with an exponent which is different from the microscopic Hertz exponent. We propose a simple theoretical explanation Mesure de la conductivite d'empilements bi- et tridimensionnels de grains conducteurs soumis a une pression uniaxiale verticale. La conductance en fonction de la contrainte suit une loi de puissance dont l'exposant est different de l'exposant microscopique prevu par la loi de Hertz. On propose une explication theorique simple

We present a critical analysis of experiments on 2d regular packings of cylinders submitted to a uniaxial vertical pressure, by studying finite size effects. The role of the disorder of contacts is emphasized. The influence of the walls, especially the frictions, is discussed; the ratio of the transmitted to the applied force has been measured. We have tested the influence of the size of the samples by increasing width, height or both simultaneously. Conclusions are partly supported by photoelasticity measurements. The role of a characteristic length larger than the grain size is put into evidence Analyse de la deformation uniaxiale d'empilements reguliers de cylindres a 2d. On souligne le role du desordre de contact, et l'influence des parois. Etude des effets de la taille des echantillons

We study uniaxial compression of 2d-ordered packings of parallel cylinders, which present weak geometrical defects responsible for a heterogeneity of the spatial distribution of intergranular stresses. The strongest stresses form links which generate an irregular network. By creating different types of vacancies, we show the importance of that network.

An experimental study of uniaxial compression of a mixture of 'hard' and 'soft' cylinders is made. The macroscopic strain-stress law strongly depends on the geometrical and compositional heterogeneities. Two changes in the behaviour are observed: one is weak, at the percolation threshold, and the other is strong, at the 'rigidity' threshold.