Fateh Bendahmane

Internal Flow Effects on Isotropic Confined Sand-Clay Mixtures

Under 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).

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.

Characterization of internal erosion in sand kaolin soils: Experimental study

ABSTRACT The appearance of breaches in recent hydraulic earthstructures due to internal erosion confirms that improving our physical understanding of the phenomenon is urgent. Under the effect of internal flows, earthstructures can undergo a migration of their particles, which modifies the particle size distribution and affects both hydraulic and mechanical characteristics. This paper initially presents a parametric study conducted on sand/kaolin samples. The experimental device described here consists of three modified triaxial cells used to control sample confinement, prevent any parasitic flow and survey sample large deformations. The tests reveal that internal erosion (suffusion or piping) is initiated by the transport of clay particles. The erosion rate, then, depends on the values of the different parameters considered (hydraulic gradient, clay content and filter pore opening size).

Suffusion susceptibility characterization by triaxial erodimeter and statistical analysis

Suffusion process corresponds to the coupled processes of detachment-transport-filtration of the soil’s fine fraction within the voids between the coarse fraction. Because of the great length of earth structures and because of the heterogeneities of soils, it is very difficult to characterize the suffusion susceptibility of soils all along the earth structures. So, a statistical analysis can be performed in order to optimize the experimental campaign. By using a specific triaxial erodimeter, an experimental program was setup to study suffusion susceptibility of thirty two specimens. The suffusion susceptibility is determined by the erosion resistance index. Ten physical parameters are determined and a statistical analysis is performed in order to identify the main parameters for a correlation with erosion resistance index. The multivariate statistical analysis leads to an expression of the erosion resistance index as a function of eight physical parameters, and by distinguishing the gap-graded and widely-graded soils, another new correlation is obtained with five physical parameters.

New Apparatus for Assessing Soil Suffusion Susceptibility Under Two Flow Directions

Suffusion is the process of selective erosion of fine particles within the matrix of coarse soil particles under the effect of seepage flow. Suffusion can induce important modifications in the hydraulic and mechanical characteristics of the soil in different flow directions. Thus, to ensure the safety assessment of hydraulic earth structures, the experimental models need to match the reality of the body of dikes or dams with horizontal flow. In this paper, a new multi-directional flow apparatus is described for characterizing soil sensibility for the suffusion process and to study the effect of flow direction. A series of suffusion tests was performed using this new device with vertical flow or horizontal flow on both gap graded soils and widely graded soils. All tested specimens were subjected to a multi-stage hydraulic gradient. The comparison of the results of the new device with results obtained from another device in vertical flow was realized. The specimens have the same initial hydraulic conductivity and suffusion susceptibility classification with both devices. Furthermore, for specimens characterized by limited soil anisotropy, the suffusion susceptibilities of these soils are quite identical under vertical flow and horizontal flow. These results permit to validate the new device and the experimental method.

Investigation of Local Processes and Spatial Scale Effects on Suffusion Susceptibility

Many failures of earth structures are caused by the internal erosion occurring in these structures and their foundations. Suffusion, one of four internal erosion types, is a selective erosion of fine particles which move through the matrix formed by the coarser particles. In literature, most investigations on suffusion took it as a single erosion process. However, the suffusion is a complex process due to the combination of three processes: detachment, transport and possible filtration of finer fraction. The influence of the local processes on suffusion susceptibility, especially the filtration process, is not well established. The objectives of this study are investigating the filtration process by verifying results of filtration tests with the basic filtration equation and analyzing the influence of spatial scale effects on the filtration process by performing tests with two different-sized devices. The filtration tests results show the consistency with the basic filtration equation on suspended particle concentration. And suffusion tests indicate the significant effect of specimen size on filtration process. The interpretative method based on the energy expended by the seepage flow and the cumulative loss dry mass is more appropriate with filtration process than those based on the geometric shape of the particles.

Modelling the mechanical strength development of treated fine sediments: a statistical approach

ABSTRACT Sediments valorization (recycling) has revealed limitations due to different restrains and practical difficulties. When it comes to different recovery methods, the possibility of valuing diverse types of sediments still needs to be defined. Using a statistical approach, the present study aims to quantitatively estimate the mechanical resistance of stabilized sediments. A database that included 22 fine sediments is selected and assembled from the literature. These sediments were treated with distinct types and quantities of additives (fillers and/or binders). The present study includes two parts. On one hand, using multivariate linear regression tool of XLstat software, an analytical model that highlights the effects of various parameters influencing the mechanical resistance of treated sediments after 28 days is obtained. This model showed that organic matter content and plasticity index are the most significant factors of sediments characteristics, while cement is the best mechanical strength booster. On the other hand, the evolution of treated sediments mechanical resistance over time is modelled by an exponential relationship using a least square regression method. Both models showed acceptable accuracies compared to a panel of selected experimental values.