B 1 Analysis of the mechanical response of a sand at grain-level with DEM-MTL N. Belheine, F.V. Donzé & F. Darve Laboratoire Sols, Solides, Structures et risques, Grenoble, France noura.belheine@hmg.inpg.fr Frederic.Donze@hmg.inpg.fr Felix.Darve@inpg.fr ABSTRACT Particle rotations are known to have a dominant influence on the behavior of granular materials, especially when the particles are circular or spherical. The definition of the relative displacement can be used to decompose the motion into separate modes: incremental translational and rotational movements. The definition of the rotational motion allows us to identify four modes. Sliding and rolling are the two principal local deformation mechanisms. This paper presents the results of a series of numerical simulations conducted to examine the effect of the rolling and sliding mechanisms on the macroscopic response. The finding results aggress well with experimental results. Comparisons are presented between models with freely rolling (without Moment Transfer Law) and with incorporating strongly rolling resistance (MTL). It was observed that the number of rolling contacts is more than the sliding contacts, and rolling becomes more pronounced for the free rolling case. Thus the DEM with MTL attenuate the particle rotation and has a strong effect upon the relative proportion of sliding and rolling between particles, and consequently upon the macroscopic strength of the granular assembly. INTRODUCTION In granular soils, each particle can move against neighboring particles by sliding and/or rolling at contact points. The classical microscopic theories of strength and dilatancy of granular media, were emphasizing the influence of frictional sliding (Newland & Alley 1957, Rowe 1962, Horne 1965, 1969, Oda 1972, Shodja et al 2003), and neglected the effects of particle rolling. In fact, there exist some experimental results contracting this. Skinner (1969) for example, observed that as the inter- particle friction increases, rolling become dominant. This was later confirmed by Oda et al (1982) by showing experimentally- during biaxial compression tests on photo-elastically sensitive oval cross- sectional rods-, the effective presence of particle rolling as a dominant microscopic deformation mechanism. They also concluded that rolling produces a softening effect and that particle rolling partially negate the strengthening effect of inter-granular friction (Kuhn et al 2002, 2004). Oda et al (1982) observations also agree with the results obtained by Bardet and Proubet (Bardet et al 1994), who numerically examined particle rotation within the shear band that forms during the biaxial compression of two dimensional assemblies of circular cross-sectional grains. Because of the great difficulty in measuring particle rotation in the geo-laboratory even with such advanced techniques as the stereo-photogrammetric technique (Jiang et al 2005, Shodja et al 2003) and particle image velocimetry, mechanisms of rolling and sliding contacts and their relation with dilatancy have not been comprehensively investigated, and many aspects of these phenomena and the contributing parameters remain unresolved. Since the pioneering work of Cundall and strack, the DEM has been widely accepted to analyze the mechanical characteristics of granular material (Akke et al 2004). The main objective of this paper is to study the contact mechanisms that include the particle rolling and its effects on the macroscopic response of granular materials using ″ Discrete Element Method with Moment Transfer Law ″( DEM with MTL) (Plassiard et al 2007, Belheine et al 2008).