This work was carried out in the framework of a larger project devoted

to the micro-mechanical modeling of clayey soils by means of discrete element

simulations. Here, we study the rheology and microstructure of granular media

composed of platy particles. The particles are three-dimensional square plates,

approximated as spheropolyhedra. Several samples composed of particles of

different levels of platyness (related to the ratio of length to thickness) were

numerically prepared and sheared up to large deformations. Specifically, we

revisit and analyze in detail an ordering phenomenon described briefly in a

previous paper [1] and investigate its consequences in terms of the alignment of

particles and cluster formation. We find that particle alignment is strongly

enhanced by the degree of platyness and leads to the formation of face-connected

clusters. Due to dynamics, this spontaneous clustering is a robust characteristic of

granular systems composed of platy particles even in the absence of attraction

forces.

JF - 15th Pan-American Conference on Soil Mechanics and Geotechnical Engineering (PCSMGE) / 8th South American Congress on Rock Mechanics (SCRM) CY - NOV 15-18 2015 Buenos Aires, ARGENTINA VL - FROM FUNDAMENTALS TO APPLICATIONS IN GEOTECHNICS ER - TY - JOUR T1 - Particle alignment and clustering in sheared granular materials composed of platy particles JF - The European Physical Journal E Y1 - 2014 A1 - Boton, Mauricio A1 - Estrada, Nicolas A1 - Emilien Azéma A1 - Farhang Radjaï AB -

By means of molecular dynamics simulations, we investigate the texture and local ordering in sheared packings composed of cohesionless platy particles. The morphology of large packings of platy particles in quasistatic equilibrium is complex due to the combined effects of local nematic ordering of the particles and anisotropic orientations of contacts between particles. We find that particle alignment is strongly enhanced by the degree of platyness and leads to the formation of face-connected clusters of exponentially decaying size. Interestingly, due to dynamics in continuous shearing, this ordering phenomenon emerges even in systems composed of particles of very low platyness differing only slightly from spherical shape. The number of clusters is an increasing function of platyness. However, at high platyness the proportion of face-face interactions is too low to allow for their percolation throughout the system.

VL - 37 IS - 11 JO - Eur. Phys. J. E ER - TY - Generic T1 - Comparison of the effects of rolling resistance and angularity in sheared granular media T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Estrada, Nicolas A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Taboada, Alfredo ED - Yu, A ED - Dong, K ED - Yang, R KW - angularity KW - anisotropy KW - force distribution KW - Granular material KW - rolling resistance KW - Shear strength KW - solid fraction AB -In this paper, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. For this purpose, we use contact dynamics simulations. By means of a simple shear numerical device, we investigate the mechanical behavior of these materials in the steady state in terms of shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. We find that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 UR - https://hal.archives-ouvertes.fr/hal-00842799 ER - TY - JOUR T1 - Quasistatic rheology and microstructural description of sheared granular materials composed of platy particles JF - Physical Review E Y1 - 2013 A1 - Boton, Mauricio A1 - Emilien Azéma A1 - Estrada, Nicolas A1 - Farhang Radjaï A1 - Lizcano, Arcesio AB -This is the first paper of a series devoted to the micro-mechanical modeling of clayey soils, by means of discrete element simulations. We specifically focus here on the effect of the platy shape of particles by reducing the interactions between particles to mechanical contact forces (i.e., neither electrostatic repulsion nor van der Waals forces are taken into account). The particles are three-dimensional square plates, approximated as spheropolyhedra. Several samples composed of particles of different levels of platyness (related to the ratio of length to thickness) were numerically prepared and sheared up to large deformations. We analyzed the shear strength, packing fraction, orientation of the particles, connectivity, fabric of the interactions network, and interaction forces as functions of the platyness. We find that both the mechanical behavior and microstructure are strongly dependent on the degree of platyness. The principal underlying phenomenon is the alignment of particle faces along a particular direction. This ordering phenomenon, which emerges even for shapes that deviate only slightly from that of a sphere, enhances the ability of the packing to develop an anisotropic structure leading to large shear strength, especially as a consequence of the fabric and mobilization of friction forces. Moreover, the connectivity of the packings and their packing fraction also evolve with the platyness. In particular, the packing fraction evolves in a nonmonotonic fashion, as observed in other granular materials composed of elongated or angular particles.

VL - 87 IS - 3 JO - Phys. Rev. E ER - TY - Generic T1 - Shear strength and microstructure of 3D assemblies of platy particles T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Boton, Mauricio A1 - Emilien Azéma A1 - Estrada, Nicolas A1 - Farhang Radjaï A1 - Lizcano, Arcesio ED - Yu, A ED - Dong, K ED - Yang, R KW - Clays KW - DEM KW - Microstructure KW - Shear strength AB -As a first step towards particle-scale modeling of clayey soils, we investigate the mechanical behavior and microstructure of assemblies of three-dimensional rectangular platy particles by means of the discrete element method. Several samples composed of particles of different levels of platyness (ratio of width to thickness) were numerically prepared and sheared to large deformations. We analyze the shear strength, packing fraction, connectivity, contact and force anisotropies, and mobilization of friction forces as functions of platyness.We find that both the mechanical behavior and microstructure are strongly dependent on the degree of platyness. This happens, in particular, because of the alignment of particle faces along a particular direction. Additionally, as observed for other granular materials with complex shapes, the packing fraction passes by a peak value before decreasing for larger values of platyness.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 UR - https://hal.archives-ouvertes.fr/hal-00842787 ER - TY - Generic T1 - Shear strength, force distributions and friction mobilization in sheared packings composed of angular particles T2 - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) Y1 - 2013 A1 - Emilien Azéma A1 - Estrada, Nicolas A1 - Farhang Radjaï ED - Yu, A ED - Dong, K ED - Yang, R AB -In this paper, we explore the effect of particle shape angularity on the mechanical behavior of sheared granular packings. A first series of contact dynamics simulations is performed in 2D with regular polygons with an increasing number of sides ranging from 3 (triangles) to 60. Then, in order to approach “idealized” angular particles, a second series of simulations is performed in 3D with irregular polyhedra with the number of faces ranging from 8 (octahedron-like) to 596. A counterintuitive finding is that the shear strength increases with angularity up to a maximum value and saturates as the particles become more angular (below 6 sides in 2D and 46 faces in 3D). A micromechanical analysis of force and contact orientations, all enhanced by face-face and face-side contacts, reveals that this increase is due to an increase of both contact and force anisotropies, and the saturation for higher angularities is a consequence of a rapid fall-off of the contact and normal force anisotropies compensated by an increase of the tangential force anisotropy.

JF - 7th International Conference on Micromechanics of Granular Media (Powders and Grains) PB - AIP CY - JUL 08-12 2013 Sydney, AUSTRALIA VL - Book Series: AIP Conference Proceedings POWDERS AND GRAINS 2013 ER - TY - JOUR T1 - Nonlinear effects of particle shape angularity in sheared granular media JF - Physical Review E Y1 - 2012 A1 - Emilien Azéma A1 - Estrada, Nicolas A1 - Farhang Radjaï AB -We analyze the effects of particle shape angularity on the macroscopic shear behavior and texture of granular packings simulated by means of the contact dynamics method. The particles are regular polygons with an increasing number of sides ranging from 3 (triangles) to 60. The packings are analyzed in the steady shear state in terms of their shear strength, packing fraction, connectivity, and fabric and force anisotropies, as functions of the angularity. An interesting finding is that the shear strength increases with angularity up to a maximum value and saturates as the particles become more angular (below six sides). In contrast, the packing fraction declines towards a constant value, so that the packings of more angular particles are looser but have higher shear strength. We show that the increase of the shear strength at low angularity is due to an increase of both contact and force anisotropies, and the saturation of the shear strength for higher angularities is a consequence of a rapid fall-off of the contact and normal force anisotropies compensated by an increase of the tangential force anisotropy. This transition reflects clearly the rather special geometrical properties of these highly angular shapes, implying that the stability of the packing relies strongly on the side-side contacts and the mobilization of friction forces.

VL - 86 IS - 4 JO - Phys. Rev. E ER - TY - JOUR T1 - Identification of rolling resistance as a shape parameter in sheared granular media JF - Physical Review E Y1 - 2011 A1 - Estrada, Nicolas A1 - Emilien Azéma A1 - Farhang Radjaï A1 - Taboada, Alfredo AB -Using contact dynamics simulations, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. In simple shear conditions, we consider four aspects of the mechanical behavior of these systems in the steady state: shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. Our main finding is that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.

VL - 84 IS - 1 JO - Phys. Rev. E ER -