Didier Marot

Energy-Based Method for Providing Soil Surface Erodibility Rankings

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.

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).

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.

Study of scale effect in an internal erosion mechanism: centrifuge model and energy analysis

Suffusion is an internal erosion mechanism, which means detachment and transport of fine particles within the soil skeleton due to hydraulic seepage flows. Different researchers have observed that the value of the critical hydraulic gradient required to initiate suffusion decreases with the length of the specimen tested. A specific centrifuge bench was designed to study the suffusion process and to study this scale effect under a controlled effective stress. Clayey sand specimens were subjected to centrifuge acceleration and to a downward flow under a constant hydraulic head. The study underlines the influence of specimen length on critical hydraulic gradient and also on the rate of erosion. A new energy analysis of tests is developed, linking the erosion rate to the power expended by fluid flow, and the eroded clay mass to the energy dissipation. This method permits the effect of specimen length to be avoided. La suffusion est un mécanisme d’érosion interne qui correspond au détachement et au transport de particules fines au travers du squelette granulaire sous l’action d’un écoulement. Différents chercheurs ont observé que la valeur du gradient hydraulique critique nécessaire à l’initiation de la suffusion décroît avec la longueur de l’échantillon testé. Un dispositif d’essais en centrifugeuse est spécialement développé afin de caractériser la suffusion et d’étudier cet effet d’échelle sous contrainte effective contrôlée. Des échantillons sablo-argileux sont centrifugés et sont soumis à un écoulement vertical descendant sous charge hydraulique constante. L’étude met en évidence l’influence de la longueur de l’échantillon sur le gradient hydraulique critique et sur le taux d’érosion. Une nouvelle approche énergétique est proposée et permet de relier le taux d’érosion avec la puissance dissipée par le fluide ainsi que la masse érodée avec l’énergie dissipée. Cette approche permet de s’affranchir de l’effet d’échelle.

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).

Investigation of interface erosion rate by Jet Erosion Test and statistical analysis

ABSTRACT Erosion is one of the main causes of instabilities within hydraulic earth structures. Two types of erosion can be distinguished: suffusion and interface erosion. This paper deals with the interface erosion phenomenon and the Jet Erosion Test is used in order to evaluate the erodibility of fine soils. A new energy analysis of the test is developed, relating the total eroded mass to the dissipated fluid energy. A new erosion resistance index is proposed. The erodibility is evaluated for twelve natural soil specimens which are compacted with the Proctor protocol. They represent a large range of erosion sensitivity. A wide dissipated hydraulic energy scale appears and a statistical analysis is carried out which gives a correlation of the erosion resistance index with three physical parameters.

A comparative analysis of interface erosion tests

Interface erosion is one of the main phenomena in dams, dikes and their foundations which may increase their failure risk. In laboratories, the jet erosion test (JET) and the hole erosion test (HET) are commonly used for the evaluation of the sensibility of interface erosion of fine soils. The results are interpreted by two distinct methods that are valid for one test only. A new energy analysis of the tests is developed, relating the total eroded mass to the dissipated fluid energy, and a new erosion resistance index is proposed. Seven naturally occurring fine-grained soils, covering a large range of erodibility, are compacted with the Proctor protocol, and they are tested with the two devices. It was shown that by using the commonly used methods, the values of the erosion coefficient are systematically higher with the JET than with the HET and the HET critical shear stress is about fifty times higher than the JET critical shear stress. Thus, the relative soil classifications yielded by the two erodimeters are not exactly the same. Based on energy analysis, values of erosion resistance index are roughly the same for each tested soil with the two apparatuses and a single classification of soil erodibility is obtained.

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...

A method to assess the suffusion susceptibility of low permeability core soils in compacted dams based on construction data

Abstract Suffusion, as one of the main internal erosion processes in earth structures and their foundations, may increase their failure risks. The paper aims at presenting a general method to assess the suffusion susceptibility of core soil samples belonging to zoned hydraulic embankment dams. On one hand, the suffusion susceptibility of the soil samples is evaluated by an erosion resistance index. Thanks to existing statistical analyses, the erosion resistance index is estimated from several soil parameters that can be easily measured in situ or in laboratory during the construction of a dam. On the other hand, the saturated hydraulic conductivity of the soil samples is evaluated based on the amount of fines content and on available construction data. Moreover, the power dissipated by the flow is inferred based on the saturated hydraulic conductivity and simplified fluid boundary conditions. The combined consideration of the erosion-resistant index and of the power dissipated by the flow permits to identify zones characterised with a relatively larger suffusion potential (lower erosion resistance index and larger power than their respective average). Throughout, the method is applied to a particular zoned dam with a till core, from Northern Quebec, as a proof of concept.