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International audience; This contribution tackles the issue of incipient conditions for initiation of erosion by a fluid flow at the surface of cohesive materials. To this end, a typical assessment procedure consists of subjecting a soil sample to progressive hydrodynamic stresses induced by a submerged impinging jet flow whose injection velocity is gradually increased. This paper presents the results of an extensive use of this protocol both in experiments and numerical simulations, the latter being based on a coupled DEM and LBM approach. Here we consider the specific case of weakly cemented soils, either made experimentally of glass beads bonded by solid bridges or modelled numerically by a solid bond rheology with a parabolic yield condition involving the micromechanical traction, shearing and bending of the bonds. The results show that, as expected, the hydrodynamic stress for erosion onset substantially increases with solid cohesion as compared to cohesionless cases but can, however, be satisfactorily predicted by a simple extension of the usual Shields criterion that only applies for cohesion-less granular sediments. This extension includes a cohesion number, the granular Bond number, with a simple definition based on tensile yield values.

We experimentally investigated cohesion of artificially bonded granular materials made of spherical glass beads cemented by solid paraffin bonds. By means of laboratory tests designed and carried out for investigation at different scales, we measured the tensile yield strength for solid bonds both at the inter-particle micro-scale and cemented samples at the meso-scale. A parametric study has been performed by varying some of the granular material properties (bead diameter, paraffin content as well as the dimension of the sample for the meso-scale tensile tests. We finally propose a discusion on: (i) the relationship between the microscopic and macroscopic cohesion forces relying on classical homogenisation laws ; (ii) the potential impact of size effects based on a simple phenomenological model.

International audience; Empirical predictions of discharge rates for dry non-cohesive grains are commonly based on the Beverloo law (Chem Eng Sci 15:260–269, 1961). The present work extends this practical configuration to submerged and cohesive cases to investigate the flow behaviour of granular media with applications in the geophysical process of sinkhole formation. The analysis of the hydrostatic collapse of soil in the presence of underground conduits is performed with a 2D GPU-parallelized simulation coupling the lattice Boltzmann method and the discrete element method to describe the fluid and the solid phases, respectively. The discharge rate of a large submerged granular sample is analysed by varying orifice sizes and inter-particle cohesion strengths. For the submerged cohesionless case, we first study the revisited Beverloo relationship that includes the terminal velocity of a single falling particle in the fluid, proposed in the experimental work of Wilson et al. (Pap Phys 1307: 2812, 2014). We consistently take into account the interstitial fluid with an effective orifice size smaller than in the dry case. Then, the additional contribution of grain cohesion is examined. Our main finding is that the empirical prediction remains valid provided that the orifice cut-off increases with cohesion. Finally, the evolution of fluid pressure during the discharge, at the vicinity of the orifice, is studied and favourably compared with the recent experimental study of Guo et al. (Granul Matter 19(3): 1–8, 2017). By considering the pressure drop around the orifice as a driven-term, we succeed in predicting the solid flow rate with a similar Beverloo approach.

International audience; This article deals with the relevance and practical feasibility of micromechanical simulations for their application to general geomechanical problems involving fluid-saturated granular assemblies, whether frictional or cohesive. A set of conceptual and numerical tools is here presented, advocating for a parallel computation using graphical processing units (GPUs) to treat large numbers of degrees of freedom with conventional desktop computers. The fluid phase is here simulated with a particle-resolved approach in the frame of the Lattice Botzmann Method (LBM) while the granular solid phase is modelled as a collection of discrete particles from a Molecular Dynamics DEM perspective. The range of possible material behaviours for the solid granular phase is intended here to cover a broad spectrum from purely frictional to viscous cohesive materials with either brittle or transient debonding features. Specific details of the implementation and some validation cases are put forward. Finally, some exemplary applications in the fields of soil erosion and geotechnical profile installation are provided along with a discussion on the parallel performance of the presented models. The results show that a micromechanical approach can be feasible and useful in practice, providing meaningful insights into complex engineering problems like the erosion kinetics of a soil under an impinging jet or the penetration resistance of a deep foundation in a layered soil profile.; Cet article traite de la pertinence et de la faisabilité pratique des simulations micromécaniques pour leur application aux problèmes géomécaniques généraux impliquant des milieux granulaires saturés en fluide, qu'ils soient frictionnels ou cohésifs. Un ensemble d'outils conceptuels et numériques est présenté ici, dans le cadre d'un calcul parallèle utilisant des unités de traitement graphique (GPU) pour traiter un grand nombre de degrés de liberté avec des ordinateurs de bureau conventionnels. La phase fluide est simulée avec la méthode de Lattice Boltzmann (LBM) tandis que la phase solide granulaire est modélisée comme une collection de particules discrètes par dynamique moléculaire (Molecular Dynamics DEM). L'éventail des comportements possibles des matériaux pour la phase granulaire solide vise ici à couvrir un large spectre allant des matériaux cohésifs purement frictionnels aux matériaux cohésifs visqueux présentant des caractéristiques d'endommagement visqueux. Des détails spécifiques sur la mise en oeuvre et quelques cas de validation sont présentés. Enfin, quelques applications illustratives dans les domaines de l'érosion des sols et de l'installation de profils géotechniques sont présentées ainsi qu'une discussion sur la performance des modèles présentés en terme de parallélisation. Les résultats montrent qu'une approche micromécanique peut être réalisable et utile dans la pratique, car elle permet de mieux comprendre des problèmes techniques complexes comme la cinétique d'érosion d'un sol sous l'effet d'un jet ou la résistance à la pénétration d'une fondation profonde dans un profil de sol en couches.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE [ADD1_IRSTEA]Hydrosystèmes et risques naturels; International audience; The ability of viscoplastic fluids to self-select their flow geometry through the formation of unyielded dead zones has important consequences for flows in wavy channels, flows over obstacles, etc. Yet, the mechanisms controlling the formation and dimensions of the dead zones remain poorly understood. We present a detailed cross-comparison of experimental and numerical results concerning channel flows of a viscoplastic fluid over a rectangular cavity filled by the same material. In all the configurations studied, which correspond to moderate values of the Bingham number, a continuous dead zone forms inside the cavity. Both numerical and experimental data reveal that, unlike at high Bingham numbers, the shear-rate profiles above the dead zone display an asymmetric shape. Accordingly, two different flow zones can be distinguished: a Poiseuille-like zone, in which the velocity profile is similar to that over a rigid wall, and a boundary layer ensuring the transition with the dead zone below. It is shown that the effective boundary condition felt by the Poiseuille-like layer is essentially controlled by incoming flow characteristics, such that the thickness of this layer does not obey simple relations with cavity length. Interestingly, however, the thickness of the boundary layer appears to follow a generalized Oldroyd’s scaling with cavity length.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE; International audience; The current study presents experiments for the initial stages of fluidization induced by a localized fluid injection. The process was studied by recording high-speed videos in a 2-D-region far from the boundaries using Planar Laser Induced Fluorescence and Refractive Index Matching. The experimental setup allowed for several parameters to be systematically studied including particle sizes, initial bed heights, and injection port diameters. The results show that the critical flow rate required for fluidization is primarily dependent on the initial height of the granular bed. However, this dependence is not a linear relation but progressively plateaus for larger heights. Conversely, the diameter of the chimney relates only slightly to the injection port diameter and significantly more to the diameter of the particles.

International audience; Groundwater is an aggravating factor in karstic sinkhole activity as it exacerbates infiltration, percolation, soil saturation and drainage. The exceptionally high number of ground collapses triggered during the major flood of spring 2016 in the Orléans region (France) clearly supports this assertion. In this article, we examine the role of flooding in sinkhole occurrence in cohesive soil layers covering karstified limestone rock. An innovative hydro-mechanical model is applied to simulated field scenarios. Our numerical simulations combine the Discrete Element Method (DEM) to model the solid phase with the Lattice Boltzmann Method (LBM) for the fluid phase. This coupled numerical method allows us to explore the micromechanical features of internal soil erosion in a flood situation. Three processes, consistent with field observations, are simulated and studied through phase diagrams: the formation of a stable cavity within the cover material, the upward propagation of a cavity leading to a dropout sinkhole, and the downward discharge of the granular media, called the subsidence sinkhole. In particular, we perform a parametric analysis of the dropout sinkhole that gives an estimate of the collapse width. In the first approximation, the characteristic length is shown to increase linearly with cover thickness, regardless of the other main parameters (soil cohesion, hydraulic head, system geometry). Finally, we present an exploratory experimental study, using sand with artificial cohesion that successfully reproduces different erosion regimes predicted by our numerical simulations.

The Loire River basin is regularly impacted by sinkholes because of its specific geological context, karstic limestone overlain by soft cover deposits. The intense rainfall event and the associated floods that occurred in this area in May and June 2016 triggered tens of sinkholes on a few square kilometers. At least 20 houses, one flood protection dyke of the Loire River and two highways were damaged by collapses. These events highlight not only the vulnerability of the area, especially in the case of a disastrous flood of the Loire River, but also the unexpected kinetics of the collapse process. Two different types of sinkholes occurred in the impacted areas: north of Orleans, collapses of the filling of vertical caves is suspected; in the Loire valley, flood accelerated the internal erosion of alluvium in the karstic active network, triggering cover collapses (spatial and temporal sinkhole frequency increased by an estimated factor of 16,000 to 24,000). In parallel, an innovative internal erosion numerical modeling approach, based on Discrete Element-DEM and Lattice Boltzmann methods-LBM, has been developed through a partnership between the French Geological Survey (BRGM) and the Environment and Agriculture National Research Institute (IRSTEA). The upward propagation of cavities within the cover were successfully simulated. The role of different parameters (soft cover cohesion, hydraulic head, system geometry, etc.) in the sinkhole occurrence were tested by a parametric analysis.

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.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]RIVAGE; International audience; This paper presents an experimental characterization of the flow of an elasto-viscoplastic fluid (Carbopol) over a dead zone constituted of the same material. The studied configuration consists of a closed rectangular channel with a cavity in its base. A curved solid-fluid interface forms into the cavity, separating a yielded flowing layer above from an unyielded dead zone below. The hydrodynamics of the flow is investigated by means of high-resolution optical velocimetry (PIV). We focus in particular on the velocity profiles and shear-rate evolution in a quasi-longitudinal flow domain located around cavity mid-length. Our measurements show a non-monotonous evolution of the shear rate, which increases from zero at the solid-liquid interface, passes through a peak (sometimes leveling off at its maximum value), and returns to zero in a plug zone sufficiently far above the cavity. Two main flow zones can be distinguished: a Poiseuille zone, in which velocity profiles are the same as in the flow over a rigid wall, and a boundary layer ensuring the transition with the dead zone. Hence, consistently with our previous work (Luu et al., Phys. Rev. E, 2015), the flow self-organizes to partially smooth out the bottom perturbation. The characteristic thicknesses of the flow zones are shown to evolve with cavity length and hydrodynamic properties of the incoming flow. The study also points out the influence of elastic effects, notably on the shape (asymmetry) of the solid-fluid interface.

We focus here on the major and always topical issue of soil erosion by fluid flows, and more specifically on the determination of both a critical threshold for erosion occurrence and a kinetics that specifies the rate of eroded matter entrainment. A synthetic state-of-the-art is first proposed with a critical view on the most commonly used methods and erosion models. It is then discussed an alternative strategy, promoting the use of model materials that allow systematic parametric investigations with the purpose of first identifying more precisely the local mechanisms responsible for soil particle erosion and second ultimately quantifying both critical onsets and kinetics, possibly through existing or novel empirical erosion laws. Finally, we present and discuss several examples following this methodology, implemented either by means of experiments or numerical simulations, and coupling erosion tests in several particular hydrodynamical configurations with wisely selected mechanical tests.

International audience; We present an experimental study investigating the transition zone between a liquid-like unyielded region and a solid-like yielded region in a yield-stress fluid. The configuration consists of a rectangular closed-channel flow disturbed by the presence of a step. Upstream of the step, a solid-liquid interface between a dead zone and a flow zone appears. In this study, we use a model fluid, namely polymer micro-gel Carbopol, which exhibits Herschel-Bulkley viscoplastic rheology. Exploiting the fluid transparency, the flow is monitored by particle image velocimetry (PIV) using an internal visu-alization technique. The main outcome of this study is to show that, except in a thin transition layer close to the solid-liquid interface, the flow behaves as an apparent Poiseuille flow with an apparent slip condition at the base. The slip frontier is found to be almost independent of the flow rate while the corresponding slip velocity increases with the flow rate.

One of the most noticeable collective motion of non-cohesive granular matter is clustering under certain conditions. In particular, when a quasi-two-dimensional monolayer of mono-disperse non-cohesive particles is vertically vibrated, a solid-liquid-like transition occurs when the driving amplitude exceeds a critical value. Here, the physical mechanism underlying particle clustering relies on the strong interactions mediated by grain collisions, rather than on grain-grain cohesive forces. In average, the solid cluster resembles a drop, with a striking circular shape. We experimentally investigate the coarse-grained solid-liquid interface fluctuations, which are characterized through the static and dynamic correlation functions in the Fourier space. These fluctuations turn out to be well described by the capillary wave theory, which allows us to measure the solid-liquid interface surface tension and mobility once the granular "thermal" kinetic energy is determined. Despite the system is strongly out of equilibrium and that the granular temperature is not uniform, there is energy equipartition at the solid-liquid interface, for a relatively large range of angular wave-numbers. Furthermore, both surface tension and mobility are consistent with a simple order of magnitude estimation considering the characteristic energy, length and time scales, which is very similar to what can be done for atomic systems. Please find the supplementary material of this article here: arXiv:1304.2646, Supplementary material is available at arXiv:1304.2646, 5 pages, 3 figures

International audience; We describe a new spreading regime during the drop impact of model yield-stress fluids (Carbopol microgel solutions) on rough hydrophobic surfaces, in a range of parameters where classical Newtonian drops usually splash. For large surface roughness and high impact velocity, we observe that the maximal inertial spreading diameter of the drops can be as much as twice larger than on smooth surfaces in the same conditions, corresponding to apparent basal friction reductions of more than 80%. We interpret this large drag reduction using a simple energy balance model and a dynamic slip length that depends on both the surface roughness and the drop's dynamics.

The erosion mechanisms driving the particle detachment of a soil under an impinging jet are here analysed from a micromechanical perspective combining the numerical Discrete Element and Lattice Boltzmann methods (DEM-LBM). Firstly, the local hydrodynamic conditions of both free and impinging jets are examined and briefly discussed, particularly on the relevance of the superficial irregularities of the granular assembly, which can dominate the local distribution of hydrodynamic variables over the impingement area. Profiles of both macromechanic and discrete (micromechanic) variables will be put forward. Then, the onset of erosion is reviewed from a topological point of view (locality of first grain motion) on the basis of a parametric study. Finally, the variation of the critical inflow velocity for the initiation of erosion in dependence on the particle size is also examined and briefly discussed.