Luc Sibille

Analysis of failure occurrence from direct simulations

ABSTRACT Stress probes are simulated with the discrete element method (DEM). From these simulations we show that the numerical discrete model presents a non-associative flow rule. As the material is non-associate, sign of the second-order work is checked for stress states included within the plastic limit condition. Conditions of occurrence of failure when the second-order work vanishes are discussed.

Suffusion tests on cohesionless granular matter

ABSTRACT Under internal flow, hydraulic earth structures (dikes, levees, or dams) can incur a migration of particles possibly inducing a modification of hydraulic and mechanic characteristics. With the objective to characterize this phenomenon named internal erosion and its consequences on mechanical behaviour of granular materials, a large oedo-permeameter device has been developed. An axial load is applied on specimen together with a downward flow with a constant hydraulic gradient. During the testing time, the bench can measure the spatial change of density and interstitial pressure along the specimen. Axial deformation, injected flow and extracted mass are also measured during the testing time. Erosion of fine particles is characterised by an extraction (out of specimen) of these particles on downstream specimen part and also by a fine particles migration in the whole specimen. This suffusion induces a settlement and a localized increase of interstitial pressure. Afterwards a localized blowout appears and triggers large specimen deformations.

Multiscale characterisation of diffuse granular failure

We study the evolution of structure inside a deforming, cohesionless granular material undergoing failure in the absence of strain localisation – so-called diffuse failure. The spatio-temporal evolution of the basic building blocks for self-organisation (i.e. force chains and minimal contact cycles) reveals direct insights into the structural origins of failure. Irrespective of failure mode, self-organisation is governed by the cooperative behaviour of truss-like 3-cycles providing lateral support to column-like force chains. The 3-cycles, which are initially in scarce supply, form a minority subset of the minimal contact cycle bases. At large length-scales (i.e. sample size), these structures are randomly dispersed, and remain as such while their population progressively falls as loading proceeds. Bereft of redundant constraints from the 3-cycles, the force chains are initially just above the isostatic state, a condition that progressively worsens as the sample dilates. This diminishing capacity for redistribution of forces without incurring physical rearrangements of member particles renders the force chains highly prone to buckling. A multiscale analysis of the spatial patterns of force chain buckling reveals no clustering or localisation with respect to the macroscopic scale. Temporal patterns of birth-and-death of 3-cycles and 3-force chains provide unambiguous evidence that significant structural reorganisations among these building blocks drive rheological behaviour at all stages of the loading history. The near-total collapse of all structural building blocks and the spatially random distribution of force chain buckling and 3-cycles hint at a possible signature of diffuse failure.

Assessing the susceptibility of gap-graded soils to internal erosion: proposition of a new experimental methodology

Suffusion and global backward erosion are two of the main internal erosion processes in earth structures and their foundations which may increase their failure risk. For other processes of internal erosion, different classifications exist in order to evaluate the soil erodibility, whereas in the case of suffusion and global backward erosion, no susceptibility classification is available. The absence of suffusion susceptibility classification may be due to the complexity of this process, which appears as the result of the coupled processes: detachment–transport–filtration of a part of the finest fraction within the porous network. Twelve soils, covering a large range of erodibility are tested with a specific triaxial erodimeter. Different criteria based on particle size distribution are compared in order to identify the potential susceptibility to suffusion. For the susceptibility characterization, a new energy-based method is proposed. This method can be used for cohesionless soils and clayey sand, and a single classification is obtained for suffusion tests realized under flow rate-controlled conditions or by increasing the applied hydraulic gradient. For several tests performed on a mixture of kaolinite and sand, suffusion of clay is accompanied by a global backward erosion process. Characterization of the development of clayey sand backward erosion is also addressed by this method. Finally, a complete methodology is detailed for the suffusion and global backward erosion susceptibility characterization.

Effects of Hydraulic Loading History on Suffusion Susceptibility of Cohesionless Soils

AbstractSuffusion is a selective erosion of fine particles under the effect of seepage flow within the matrix of coarser particles. This complex phenomenon appears as a combination of three process...

Role of blockages in particle transport through homogeneous granular assemblies

This letter deals with the transport of particles through granular assemblies and, specifically, with the intermittent formation of blockages originated from collective and purely mechanical clogging of constrictions. We perform numerical experiments with a micro-hydromechanical model that is able to reproduce the complex interplay between the carrier fluid, the transported particles and the granular assembly. The probability distribution functions (PDFs) of the duration of blockages and displacements give the time scale on which the effect of blockages is erased and the advection-dispersion paradigm is valid. Our experiments show that these PDFs fit exponential laws, reinforcing the idea that the formation and destruction of blockages are homogeneous Poisson processes.

Microscale Analysis of the Effect of Suffusion on Soil Mechanical Properties

Suffusion is a particular internal erosion process that can lead to important disorders in water retaining structures such as embankment dams and levees. It causes modifications in the soil micro-structure and may modify the mechanical behaviour of the soil leading to deformations at the macroscopic scale. Therefore, the aim of this study is to investigate the consequences of internal erosion on the mechanical properties of the soil. We present such an investigation through numerical and experimental approaches. For the experimental approach, a newly developed suffusion test apparatus is used while for the numerical approach, a model is established based on the discrete element method (DEM) with a one-way fluid-solid coupling.

Phenomenological interpretation of internal erosion in granular soils from a discrete fluid-solid numerical model

Internal erosion in granular soils may involve different steps: the detachment of solid particles from the granular skeleton under the action of water seepage; the transport of the detached particles carried with the water flow in the pore space; and eventually, for some erosion processes, such as suffusion, the possible reattachment of some transported particles to the solid skeleton of the soil, acting as a filter. The first part of this paper is devoted to the description and interpretation of the first step about the particle detachment. The analysis is mainly based on direct numerical simulations performed with a fully coupled discrete element-lattice Boltzmann method. Dynamics of the solid granular phase is represented thanks to the discrete element method in which each solid particle is explicitly described, whereas dynamics of the interstitial water flow is solved with the lattice Boltzmann method. Interactions between the solid phase and the fluid phase are handled at the particle scale avoiding the introduction in the model of some phenomenological constituents to deal with fluid-solid interactions. Numerical modellings of hole erosion can be interpreted similarly to laboratory hole erosion tests where the erosion rate is linearly related to the hydraulic shear stress. Further investigations from the numerical results suggest that the erosion rate for hole erosion in granular soil, can also be interpreted as a function of the water flow power according to a power law. The latter interpretation is applied to experimental data from suffusion tests on a cohesionless soil and glass bead mixtures. Here again, if change of erosion rate due to filtration is discarded, erosion rate is correctly described by the water seepage power according to a power law. Finally, a simple phenomenological model is suggested to describe the whole suffusion process, based on the previous results, to describe the particle detachment, and completed to take also into account the transport and filtration phases. Predictions of this model are compared with experimental results from suffusion tests on glass bead mixtures.

Micromechanical Analysis of Second Order Work in Granular Media

This paper examines the evolution of instabilities in granular materials from a microscopic point of view, using the discrete element method. The relationship between the unstable loading directions that result in negative values for the macroscopic expression of the second order work and the terms in the microscopic expression of the second order work was investigated. Good agreement was found between the microscopic and macroscopic expressions of the second order work for the two three-dimensional specimens studied, in both the elastic and plastic tensorial zones. The vanishing of the microscopic and macroscopic second order work is shown to coincide in both specimens.

To Which Extend the Failure Mode Originates from Microstructure

The theoretical study of instabilities in random heterogeneous media has proved to be an interesting pursuit in the development of a proper continuum framework for defining instability in geomaterials. Its influence transcends the scales down to the micro level and extends beyond applications that are being herein contemplated. In this present work, we are specifically interested in establishing a linkage between macroscopic and microscopic instabilities and their respective mathematical expressions through a theoretical analysis which bridges the two scales.