International audienceWithin 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

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.

Suffusion 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 processes: detachment, transport and possible filtration of finer fraction. It can induce a change in particle size distribution, porosity and hydraulic conductivity of the material. With the objective to characterize the suffusion susceptibility, downward seepage flow tests were conducted. Four different cohesionless soils were tested under hydraulic gradient controlled conditions or under flow rate controlled conditions. This study shows the significant effect of hydraulic loading history on the value of critical hydraulic gradient.Moreover, method characterizing the erosion susceptibility based on rate of erosion does not lead to a unique characterization of suffusion process for different types of hydraulic loading. The new analysis is based on energy expended by the seepage flow and the cumulative eroded dry mass. The results demonstrate that this approach is more effective to characterize suffusion susceptibility for cohesionless soils.

The jet erosion test (JET) and the hole erosion test (HET) are two tests used to determine soil erodibility classification, and results are commonly interpreted by two distinct methods. A new method based on fluid energy dissipation and on measurement of the eroded mass for interpreting the two tests is proposed. Different fine-grained soils, covering a large range of erodibility, are tested. It is shown that, by using common methods, the erosion coefficient and average critical shear stress are different with the JET and with the HET. Moreover, the relative soils classifications yielded by the two erodimeters are not exactly the same. On the basis of the energy method, an erosion resistance index is determined for both apparatuses, and a classification of surface-erosion resistance is proposed. For both apparatuses, values of the erosion resistance index are roughly the same for each soil, and a single classification of soil erodibility is obtained.

International audienceCohesionless granular matter subjected to internal flow can incur an internal erosion by suffusion characterized by a migration of its finest constituting particles. A series of suffusion tests is performed on assemblies of gap graded glass beads using a large oedo-permeameter device. Two successive processes of erosion can be observed during the tests. First, a suffusion process is characterized by a progressive and diffuse migration of fine particles over a long time period. The second process, induced by the first one, is characterized by a strong migration over a short time period (blowout of fine particles) and produces rapidly large settlement of specimen. Time series of hydraulic conductivity, longitudinal profile of specimen density, eroded mass and axial deformation are analyzed. The initial content of fine particles and the history of hydraulic loading appear as key parameters in the suffusion development. To characterize the suffusion development, erosion rate is investigated according to the power expended by the seepage flow, and a new law of erosion by suffusion is proposed

International audienceUnder the effect of internal flows, a liner can undergo a washing out of particles, which modifies the particle size distribution and affects hydraulic, chemical and mechanical characteristics. This paper discusses the effects of internal flows on sand/kaolin mixture, in terms of rate of erosion and modification of the hydraulic conductivity. A parametric study is conducted with a specific device that consists of three modified triaxial cells. These cells allow isotropically consolidating and confining specimens, they prevent a parasitic flow and survey large deformations of specimen. The tests reveal that suffusion of clay is accompanied by a clogging in the specimen that induces a drop in hydraulic conductivity. For high gradients the erosion of clay is accompanied by the backward erosion of sand and finally the specimen collapses. The erosion rate then depends on the values of the different parameters considered (hydraulic gradient, clay content and filter pore opening size)

Internal erosion in soils is characterized by a first step of detachment of solid particles from the granular skeleton under the action of a water seepage; then the detached particles are transported with the water flow. For some erosion processes, as suffusion, transported particles may finally be redeposited within the interstitial space of the soil itself acting as a filter. This paper focuses on the analysis and the description of the two first steps of particle detachment and transport in the cases of erosion by suffusion and piping erosion. The analysis is mainly based on direct numerical simulations performed with a fully coupled discrete element-lattice Boltzmann method (DE-LB method). Inter-particle interactions occurring in the solid granular phase are described with the discrete element method, whereas dynamics of the water flow is solved with the lattice Boltzmann method. Simulation results show that internal erosion of the solid phase can be described either from the hydraulic shear stress or from the power expended by the water seepage.The latter description based on the flow power is finally compared with experimental results from laboratory tests.

International audienceAn optical sensor was designed to measure the fine solid particles concentration contained in a fluid flow. Thissensor is composed of four light-emitting diodes and four associated light-dependent resistors, allowing the measurement offluid transparency. Given the small device dimensions, it can be placed close to the particles exit from the specimen. Theoptical sensor is able to instantaneously measure a large range of clay or silt concentrations in the effluent, without a significantinfluence of flow rate. The presence of sand grains in fluid flow can be detected. The use of this sensor with a specifictriaxial device allows precise characterization of the initiation and development of the suffusion process on clayey sandspecimens. It is shown that suffusion of clay particles induces a decrease of hydraulic conductivity, which is due to a diffuseprocess of eroded particles filtration. Clay suffusion and filtration processes are influenced by grain-size distribution andalso 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