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.

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)

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

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Internal erosion processes in earth structures and their foundations may increase the failure risk of such structures. Suffusion, one of the main internal erosion processes, selectively erodes the fine particles which move through the voids formed by the coarser particles. In literature, several suffusion susceptibility investigations were already published with various tested specimen sizes.However, the influence of the specimen size on suffusion susceptibility is not well established. The objective of this study is to investigate this influence by comparing results of suffusion tests performed on six different soils, with two different sized devices. First, this study highlights the complexity of suffusion process which is a combination of three processes: detachment, transport and possible filtration of the finer fraction. The results also show a decrease of the critical hydraulic gradient with the size of the specimen. The proposed interpretative method is based on the energy expended by the seepage flow and the cumulative loss dry mass. This method permits to obtain the same suffusion susceptibility classification for both specimen sizes.

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.

An experimental program was set up to study the suffusion process by using a triaxial erodimeter developed in our laboratory. This device is equipped with a multichannel optical sensor in order to characterize precisely the initiation and development of suffusion process. With the objective to characterize the erodibility of clayey sand, tests were performed under either flow-rate-controlled or hydraulic-gradient-controlled conditions. The test analysis allows distinguishing three steps. First, suffusion of a small quantity of clay is detected on downstream. In flow-rate-controlled conditions, this suffusion is immediately followed by a high increase of the hydraulic gradient which continues during the second step. The second step is characterized by very low amount of particles in the effluent. During the third step, high suffusion of clay appears and induces erosion of clay and sand. The results reveal the necessity to take into account the evolutions of hydraulic gradient (or pressure gradient) and also the evolutions of velocity (or flow rate) to represent the hydraulic loading. The analysis based on energy expended by fluid flow and eroded mass permits to characterize the soil sensitivity to clay suffusion and clayey sand erosion for tests under the two types of hydraulic loading.