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Seismic metamaterials have been the subject of much recent interest, with notable conceptual advances and the demonstration of possible full-scale effectiveness in earthquake protection. Here, through a fine analysis of the dispersion properties and behavior of resonators introduced in a laboratory scale setup, we propose a strategy for the design of an efficient metaseismic barrier. When positioned onto an unconsolidated granular layer, rigid masses of nontrivial shape become elastic resonators by coupling with the relatively soft propagation medium. Additionally, they affect the mechanical boundary conditions and subsurface medium properties through the added weight. We demonstrate that all these ingredients can be rationally combined to improve the shielding effect and control the seismic waves.

This paper is devoted to the investigation of the density sorting of grains using water jigging. Experiments achieved in a laboratory scale water jig for two initial binary bed configurations are studied and compared to numerical results. The vertical composition of the deposit is estimated after different number of water pulses to represent the sorting evolution in time. Simulations are based on a multiscale model in which the fluid is solved at a larger scale than the grain diameter using the finite element method, while the grains are considered as rigid bodies and solved using the nonsmooth contact dynamics. Key parameters are numerically varied and observed to determine the sensitivity of the process.

We employ kinetic theory, extended to incorporate the influence of velocity correlations, friction and particle stiffness, and a model for rate-independent, elastic components of the stresses at volume fractions larger than a critical value, in an attempt to reproduce the results of discrete-element numerical simulations of steady, fully developed, dissipative, collisional shearing flows over and within inclined, erodible, fragile beds. The flows take place between vertical, frictional sidewalls at different separations with sufficient total particle flux so that differently inclined, erodible beds result. Numerical solutions of the spanwise-averaged differential equations of the theory and the associated boundary conditions are seen to be capable of reproducing profiles of stresses, solid volume fraction, average velocity and the strength of the particle velocity fluctuations, both in the rapid collisional flow above the bed and in the slower creeping flow within the bed. The indication is that extended kinetic theory has the unique ability to faithfully describe steady, inhomogeneous, granular shearing flows, ranging from dilute to extremely dense, using balances of momentum and energy and employing boundary conditions that are associated with the balances, with a small number of physically determined, microscopic parameters.

Motivated by the understanding of shape effects in granular materials, we numerically investigate the macroscopic and microstructural properties of anisotropic dense assemblies of frictionless polydisperse rigid pentagons in shear flow, and compare them with similar systems of disks. Once subjected to large cumulative shear strains their rheology and microstructure are investigated in uniform steady states, depending on inertial number I, which ranges from the quasistatic limit ([Formula: see text]) to 0.2. In the quasistatic limit both systems are devoid of Reynolds dilatancy, i.e., flow at their random close packing density. Both macroscopic friction angle [Formula: see text], an increasing function of I , and solid fraction [Formula: see text], a decreasing function of I, are larger with pentagons than with disks at small I, but the differences decline for larger I and, remarkably, nearly vanish for [Formula: see text]. Under growing I , the depletion of contact networks is considerably slower with pentagons, in which increasingly anisotropic, but still well-connected force-transmitting structures are maintained throughout the studied range. Whereas contact anisotropy and force anisotropy contribute nearly equally to the shear strength in disk assemblies, the latter effect dominates with pentagons at small I, while the former takes over for I of the order of 10-2. The size of clusters of grains in side-to-side contact, typically comprising more than 10 pentagons in the quasistatic limit, very gradually decreases for growing I.

Three-dimensional discrete numerical simulation is used to investigate the properties of close-packed frictionless granular assemblies as a function of particle polydispersity and shape. Unlike some experimental results, simulations show that disordered packings of pinacoids (eight-face convex polyhedron) achieve higher solid fraction values than amorphous packings of spherical or rounded particles, thus fulfilling the analogue of Ulam's conjecture stated by Jiao and co-workers for random packings [Y. Jiao and S. Torquato, Phys. Rev. E $\textbf{84}$, $041309$ ($2011$)]. This seeming discrepancy between experimental and numerical results is believed to lie with difficulties in overcoming interparticle friction through experimental densification processes. Moreover, solid fraction is shown to increase further with bidispersity and peak when the volume proportion of small particles reaches $30\%$. Contrarywise, substituting up to $50\%$ of flat pinacoids for isometric ones yields solid fraction decrease, especially when flat particles are also elongated. Nevertheless, particle shape seems to play a minor role on packing solid fraction compared to polydispersity. Additional investigations focused on the packing microstructure confirm that pinacoid packings fulfill the isostatic conjecture and that they are free of order except beyond $30$ to $50\%$ of flat or flat \& elongated polyhedra in the packing. This order increase progressively takes the form of a nematic phase caused by the reorientation of flat or flat \& elongated particles to minimize the packing potential energy. Simultaneously, this reorientation seems to increase the solid fraction value slightly above the maximum achieved by monodisperse isometric pinacoids, as well as the coordination number. Finally, partial substitution of elongated pinacoids for isometric ones has limited effect on packing solid fraction or order., 12 figures, 12 pages

Powders and Grains - 8th International Conference on Micromechanics of Granular Media, Montpellier, FRA, 03-/07/2017 - 07/07/2017; International audience; Using a recently developed 2D numerical modelling that combines Discrete Element (DEM) and Lattice Boltzmann methods (LBM), we simulate the destabilisation by an hydraulic gradient of a cohesive granular soil clogging the top of an underground conduit. We aim to perform a multi-scale study that relates the grain scale behaviour to the macroscopic erosion process. In particular, we study the influence of the flow conditions and the inter-particle contact forces intensity on the erosion kinetic.

Enwrapping liquid droplets with hydrophobic particles allows the manufacture of so-called "liquid marbles" [Aussillous and Quéré Nature (London) 411, 924 (2001); NATUAS0028-083610.1038/35082026Mahadevan Nature (London)411, 895 (2001)NATUAS0028-083610.1038/35082164]. The recent intensive research devoted to liquid marbles is justified by their very unusual physical and chemical properties and by their potential for various applications, from microreactors to water storage, including water pollution sensors [Bormashenko Curr. Opin. Colloid Interface Sci. 16, 266 (2011)COCSFL1359-029410.1016/j.cocis.2010.12.002]. Here we demonstrate that this concept can be successfully applied for encapsulating and protecting small gas pockets within an air environment. Similarly to their liquid counterparts, those new soft-matter objects, that we call "gas marbles," can sustain external forces. We show that gas marbles are surprisingly tenfold stronger than liquid marbles and, more importantly, they can sustain both positive and negative pressure differences. This magnified strength is shown to originate from the strong cohesive nature of the shell. Those interesting properties could be exploited for imprisoning valuable or polluted gases or for designing new aerated materials.

The present paper sheds new light on ongoing drawbacks of three classical algorithms dedicated to initial contact detection between overlapping convex polyhedra, (i) Cundall’s common plane, (ii) Nezami’s fast common plane and (iii) Gilbert-Johnson-Keerthi’s algorithm (GJK). Solutions to these drawbacks are suggested and implemented into revised versions of those three algorithms, which are further benchmarked for accuracy and speed using nine overlapping contact situations. The benchmarking results show that the revised version of GJK, called GJK - TD , and Nezami (revised) return values of the contact normal components and overlap depth which are identical to machine precision, whereas Cundall (revised) results differ beyond the ninth decimal place. Furthermore, for a given contact situation, GJK-TD returns those values within a few tens of microseconds on average, whereas Nezami (revised) and Cundall (revised) are respectively 6 and 65 times more computationally intense. It is believed that the robustness and efficiency of GJK-TD will boost its use into DEM simulations, all the more that this versatile algorithm may easily be customized to detect contact between convex polyhedra and spheroid particles.

A new averaging method linking discrete to continuum variables of granular materials is developed and used to derive average balance equations. Its novelty lies in the choice of the decomposition between mean values and fluctuations of properties which takes into account the effect of gradients. Thanks to a local homogeneity hypothesis, whose validity is discussed, simplified balance equations are obtained. This original approach solves the problem of dependence of some variables on the size of the averaging domain obtained in previous approaches which can lead to huge relative errors (several hundred percentages). It also clearly separates affine and nonaffine fields in the balance equations. The resulting energy cascade picture is discussed, with a particular focus on unidirectional steady and fully developed flows for which it appears that the contact terms are dissipated locally unlike the kinetic terms which contribute to a nonlocal balance. Application of the method is demonstrated in the determination of the macroscopic properties such as volume fraction, velocity, stress, and energy of a simple shear flow, where the discrete results are generated by means of discrete particle simulation., Comment: Accepted forpublication in Physical Review E

Small quantities of liquid in a granular material control the flow dynamics as well as the triggering and jamming phases. In order to study this problem, some experimental collapse tests conducted in a rectangular box were reproduced with a 1:1 scale numerical model using the Discrete Element Method. In simulations the effect of the capillary bridges has been investigated implementing a mid-range attractive force between particles based on the minimum energy approach. Also a bonding-debonding mechanism was incorporated in the algorithm and the volume of each sessile drop on the particle surface was considered during its motion. The influence of some variables was investigated with respect to the final slope profiles and the runout lengths: the initial liquid content, the particle size, the solid density, the liquid surface tension, and the liquid-solid contact angle. Also the crucial effect of the confinement walls on the collapse phenomenon was investigated: wet particles adhere to the lateral walls providing a higher flow resistance in comparison to the same material in dry conditions. It was observed that particles with largest path-lengths are localized near the movable wall at a middle-height of the initial column sample. Other particles at the surface moves in a rigid way especially if they were wet and with a low solid density. The “fidelity” of each particle with respect to the nearest neighbours was evaluated allowing to recognize the emergence of clusters of particles and rigid parts, to extract the failure surface and to localize where debonding mechanisms concentrate in the wet case.

International audience; A strain-rate dependent rheology for granular materials with a constant cohesion, is proposed and applied in the case of a flow down an inclined channel. The results obtained show that a plug flow zone appears below the free surface and show that low cohesion may affect drastically the flow behaviour when the inclination of the channel is close to the repose angle. These results give also a basic understanding of h stop , the depth of the remaining granular layer on an inclined channel, treating dilatancy like a cohesion-like behaviour.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE [ADD1_IRSTEA]Hydrosystèmes et risques naturels; International audience; The mechanical behaviour of granular materials is widely governed by microstructure reorganizations. This constant evolution of the microscale geometry is often taken into account directly at the macroscale within the plasticity context, and more particularly non-associated plasticity. In this paper we propose to revisit the non-associated plastic behaviour of granular materials with respect to material instability assessment in terms of the loss of positiveness of the second-order work. It is shown that large incremental plastic strain is a necessary condition for the existence of mechanical instability. The ability of a material to develop substantial plastic incremental strain is then related to the existence of rattlers, i.e., free particles that get jammed into force chains when existing contact networks fail to adapt to incremental loadings. This link between rattlers, plastic deformation and the vanishing of the second-order work is explicitly derived i) from the micro-formulation of the second-order work and ii) directly from discrete element simulations.