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Evaluation of shear strength characteristics of coarse-grained soils by performing laboratory tests resorts often to a scalping or scalping/replacement procedure due to the presence of oversized particles in comparison with the device size. This work aimed to study consequences of these two procedures on the resulting shear strength characteristics. Experimental tests were performed on soils having gap-graded particle size distributions by using triaxial apparatuses with small and medium diameters [PHI] = 50 and 100 mm. Original soils were mixtures of fine particles (sand and glass beads) and coarse particles (two types of gravel). The original soils were scalped and (or) scalped/replaced by two different procedures. Different factors that influenced the mechanical properties of the scalped and scalped/replaced soils in comparison with those of the original soils were analyzed: (i) the compactness parameter controlled for scalped and scalped/replaced soils, (ii) the fine content of the original soils, and (iii) the procedure used for the scalping/replacement. Key words: coarse-grained soils, scalping, scalping/replacement, mechanical behavior, triaxial device L'evaluation des caracteristiques de resistance au cisaillement des sols a gros grains par des essais de laboratoire recourt souvent a une procedure d'ecretage ou de substitution en raison de la presence de particules surdimensionnees par rapport a la taille du dispositif. Ce travail vise a etudier les consequences de ces deux procedures sur les caracteristiques de resistance au cisaillement qui en resultent. Des essais experimentaux sont realises sur des sols dont la distribution granulometrique est lacunaire, a l'aide d'appareils triaxiaux de petits et moyens diametres [PHI] = 50 et 100 mm. Les sols d'origine sont des melanges de particules fines (sable et billes de verre) et de particules grossieres (deux types de gravier). Les sols d'origine sont ecretes et/ou substitues par deux procedures differentes. Differents facteurs qui influencent les proprietes mecaniques des sols ecretes et substitues par rapport a celles des sols d'origine sont analyses : (i) le parametre de compacite controle pour les sols ecretes et substitues, (ii) la teneur en fines des sols d'origine et (iii) la procedure utilisee pour la substitution. [Traduit par la Redaction] Mots-cles : sols grossiers, ecretage, substitution, comportement mecanique, dispositif triaxial, Introduction Coarse-grained soils are characterized by widely graded particle size distributions. For this particular class of soils, large grains (stone and gravel) coexist with fine particles (sand, silt, and clay). [...]

Internal erosion is one of the main causes of instabilities within hydraulic earth structures. Four internal erosion processes can be distinguished, and this study deals with the process of suffusion, which corresponds to the coupled processes of detachment-transport-filtration of the soil's fine fraction between the coarse fraction. Because of the great length of earth structures and the heterogeneities of soils, it is very difficult to characterize the suffusion susceptibility of the different soils. Nevertheless, a statistical analysis can be performed to optimize the experimental campaign. By using a dedicated erodimeter, an experimental program was set up to study suffusion susceptibility of 31 specimens of nonplastic and low-plasticity soils. The suffusion susceptibility is determined by the erosion resistance index, which relates the total loss of mass with the total energy expended by the seepage flow. Fourteen physical parameters are selected, and a multi-variate statistical analysis leads to a correlation between the erosion resistance index and all these parameters. A statistical analysis is performed to identify the main parameters and to focus on those that can easily be measured on existing structures. By distinguishing gap-graded and widely graded soils, two correlations are proposed to estimate the erosion resistance index. Key words: laboratory testing, suffusion, physical parameter, statistical analysis, energy. L'erosion interne est l'une des principales causes des instability dans les ouvrages hydrauliques en terre. Quatre processus d'erosion interne peuvent etre distingues et cette etude porte sur le processus de suffusion qui correspond aux processus couples de detachement-transport-filtration de la fraction fine du sol entre les grains de la fraction grossiere. En raison du grand lineaire des ouvrages en terre et des heterogeneites des sols, il est tres difficile de caracteriser la susceptibilite a la suffusion des differents sols. Neanmoins, une analyse statistique peut etre effectuee afin d'optimiser la campagne experimentale. A l'aide d'un erodimetre dedie, un programme experimental a ete realise pour etudier la susceptibilite a la suffusion de 31 echantillons de sols non plastiques ou de faible plasticite. La susceptibilite a la suffusion est determinee par l'indice de resistance a l'erosion, qui relie la perte de masse totale avec l'energie totale dissipee par l'ecoulement interstitiel. Quatorze parametres physiques sont selectionnes et une analyse statistique multivariee abouti a une correlation entre l'indice de resistance a l'erosion et l'ensemble de ces parametres. Une analyse statistique est effectuee afin d'identifier les principaux parametres tout en privilegiant les parametres aisement mesurables sur les ouvrages existants. En distinguant les sols de distributions granulometriques discontinue et continue, deux correlations sont proposees pour estimer l'indice de resistance a l'erosion. [Traduit par la Redaction] Mots-cles: essais en laboratoire, suffusion, parametre physique, analyse statistique, energie., Introduction Internal erosion is one of the main causes of instabilities within hydraulic earth structures such as dams, dikes, or levees (Foster et al. 2000). According to Fell and Fry [...]

In this paper, we present a micro-mechanical study of the effect of fine content on the behavior of gap graded granular samples by using numerical simulations performed with the Discrete Element Method. Different samples with fine content varied from 0% to 30% are simulated. The role of fine content in reinforcing the granular skeleton and in supporting the external deviatoric stress is then brought into the light.

We investigate the transport of moderately large passive particles through granular assemblies caused by seeping flows. This process can only be described by highly nonlinear continuum models, since the local permeability, the advection and dispersion mechanisms are strongly determined by the concentration of transported particles. Particles may sometimes get temporally trapped and thus proper kinetic mass transfer models are required. The mass transfer depends on the complexity of the porous medium, the kind of interaction forces and the concentration of transported particles. We study these two issues by means of numerical and laboratory experiments. In the laboratory we use an oedo-permeameter to force sand grains to move through a gravel bed under conditions of constant hydraulic pressure drop. To understand the process, numerical experiments were performed to approach particle transport at the grain scale with a fully coupled method. The DEM-PFV combines the discrete element method with a pore scale finite volume formulation to solve the interstitial fluid flow and particle transport problems. These experiments help us to set up a continuum transport model that can be used in a boundary value problem.

An optical sensor was designed to measure the fine solid particles concentration contained in a fluid flow. This sensor is composed of four light-emitting diodes and four associated light-dependent resistors, allowing the measurement of fluid transparency. Given the small device dimensions, it can be placed close to the particles exit from the specimen. The optical sensor is able to instantaneously measure a large range of clay or silt concentrations in the effluent, without a significant influence of flow rate. The presence of sand grains in fluid flow can be detected. The use of this sensor with a specific triaxial device allows precise characterization of the initiation and development of the suffusion process on clayey sand specimens. It is shown that suffusion of clay particles induces a decrease of hydraulic conductivity, which is due to a diffuse process of eroded particles filtration. Clay suffusion and filtration processes are influenced by grain-size distribution and also by grain shape of the coarse fraction. Under a high hydraulic gradient, clay suffusion can be accompanied by sand erosion. The critical value of mean pore velocity to initiate clay suffusion was determined for the types of soils. Key words: laboratory testing, seepage, transparency, solid particle concentration, suffusion, pore velocity. Un capteur optique a ete concu pour mesurer la concentration en particules fines d'un ecoulement de fluide. Ce capteur est compose de quatre diodes emettrices de lumiere, et de quatre photo resistances associees qui mesurent la transparence du fluide. Puisque l'appareil est de faible dimension, il peut etre place pres de l'endroit ou les particules sortent de l'echantillon. Le capteur optique peut mesurer instantanement une large gamme de concentrations d'argile ou de limon dans l'effluent sans influence significative du taux d'ecoulement. La presence de grains de sable dans l'ecoulement peut etre detectee. L'utilisation de ce capteur avec un appareil triaxial specifique permet de caracteriser precisement l'initiation et le developpement du processus de suffusion sur des echantillons de sable argileux. Il est demontre que la suffusion des particules argileuses induit une diminution de la conductivite hydraulique en raison d'un processus diffus de filtration des particules erodees. La suffusion de l'argile et le processus de filtration sont influences par la granulometrie et aussi par la forme de la fraction grossiere. Sous un gradient hydraulique eleve, la suffusion de l'argile peut etre accompagnee par l'erosion du sable. La valeur critique de la vitesse moyenne dans les pores pour initier la suffusion de l'argile a ete determinee pour les types de sol. Mots-cles : essais en laboratoire, ecoulement, transparence, concentration en particules solides, suffusion, vitesse dans les pores. [Traduit par la Redaction], Introduction Detection of the initiation of solid particle transport in compacted soils subjected to seepage and determination of solid particle concentration in fluid flow are important, notably in the field [...]

Within hydraulic earth structures (dikes, levees, or dams), internal seepage flows can generate the entrainment of the soil grains. Grain transportation affects both particle size distributions and porosity, and changes the mechanical and hydraulic characteristics of the earth's structure. The occurrence of failures in new earth structures due to internal erosion demonstrates the urgency of improving our knowledge of these phenomena of erosion. With this intention, a new experimental device has been developed that can apply hydraulic stresses to reconstituted consolidated cohesive soils without cracks in order to characterize the erosion evolution processes that might be present. A parametric study was conducted to examine the influence of three critical parameters on clay and sand erosion mechanisms. When the hydraulic gradient was low, it was concluded that the erosion of the structure's clay fraction was due to suffusion. When the hydraulic gradient increased, it was concluded that the sand fraction erosion initiation was due to backward erosion. The extent of the erosion was dependent on the clay content. The study underlines the complexity of confinement stress effects on both erosion phenomena. DOI: 10.1061/(ASCE)1090-0241(2008)134:1(57) CE Database subject headings: Erosion; Triaxial stress; Cohesive soils; Soil structure; Hydraulic structures.

Granular soils subjected to seepage flow may suffer suffusion, ie, a selective internal erosion. Extending the classical approach of poromechanics, we deduce a new form of the Clausius‐Duhem inequality accounting for dissipation due to suffusion, and we deduce restrictions on the constitutive laws of the soil. We suggest (a) a possible coupling between the seepage forces and the suffusion kinetics and (b) an extension of an existing elastoplastic model for the skeleton mechanical behaviour. Numerical integrations of the elastoplastic model are carried out under drained axisymmetric triaxial and oedometric conditions. As a result, we prove that the extended model is able to qualitatively reproduce the suffusion induced strains and the strength reduction experimentally observed. Predictions on the oedometric behaviour of suffusive soils are also provided. [ABSTRACT FROM AUTHOR]