# Experimental study of internal erosion on cohesive soils

Fateh Bendahmane, Didier Marot, Alain Alexis, Frédéric Rosquoët

► To cite this version:

Fateh Bendahmane, Didier Marot, Alain Alexis, Frédéric Rosquoët. Experimental study of internal erosion on cohesive soils. Poromechanics III Biot Centennial, 1905-2005, 2005, Oklahoma, United States. pp.607-612. hal-05044579

HAL Id: hal-05044579

https://hal.science/hal-05044579v1

Submitted on 23 Apr 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Experimental study of internal erosion on cohesive soils

Fateh Bendahmane, Didier Marot, Alain Alexis & Frédéric Rosquoët

GeM, Institut de Recherche en Génie Civil et Mécanique, I.U.T. de Saint-Nazaire,
Saint-Nazaire Cedex, France

ABSTRACT: Under the internal flow, hydraulic earthstructure (dike, levee, or dam) can incur a migration of particles. This migration induced a modification of particles size distribution and a variation of hydraulical and mechanic characteristics. This modification can generate instabilities: sliding, overtopping or even failures. The occurrence of breach in new earthstructure shows the urgency to improve the design rules of this point. We propose a parametric study on sand/kaolinite samples. The experimental device consists of three modified triaxial cells which allow controlling the sample confinement, preventing any parasitic flow and surveying great deformations of samples. The first tests carried out show the good repeatability and the validity of the experimental devices. Different aspects of the initiation of internal erosion are clarified: if migration of fine particles is important, the skeleton of sample is unstable and causes the breaking down.

# 1 INTRODUCTION

The water presence in the earthstructures are likely to undergo irreversible damages due to three different mechanisms (sliding, overtopping and internal erosion). Internal erosion appears to be one of the main cause of in-situ instabilities. Among the 11,192 already surveyed dams by Foster et al. (2000), 136 show dysfunctions, which are divided up as 5.5% for sliding, 46% for internal erosion and 48% for overtopping. Internal erosion is due to the transport and migration under the action of flow, of some particles constituting the structure. Improvements in the understanding of internal erosion mechanisms are hindered by their complexity and detection difficulties. The earthstructure material hydraulic and mechanical characteristics are then altered. The material permeability, for instance, undergoes sharp changes, which cause losses of tightness or interstitial overpressure. The migration of particles is prejudicial to the structural stability and induce structure failure.

Internal erosion mechanisms are complex and involve many parameters, coupled or not. Laboratory test sample to identify specific mechanisms are no less easier and account for the differences in interpretation within a same test campaign (Monnet, 1998; Skempton & Brogan, 1994). The two main phenomena responsible for migration of particles are piping and suffusion. In order to reduce internal erosion risks, hydraulic earthstructures often include a filter component usually laid below the downstream face.

To design the earthstructure, the internal erosion criteria do not consider the hydraulics/mechanics coupling and the simultaneous influence and interactions of the main parameters (porosity, grain size, confining, gradient, filter). The great number of parameters affecting the different phenomena shows the importance of taking experimental precautions.

A parametric study started in GeM (Saint-Nazaire France) to provide a new data on the effect on internal erosion is realized with a new experimental device.

Because of their implementation, use and money-saving capabilities, most authors have conducted tests using rigid cell permeameters (Lafleur, 1999; Tomlinson & Vaid, 2000; Reddi et al. 2000). Drawbacks come from the confinement characteristics, which are uniaxial, and from the existence of parasitic flows between the sample and the cell. In order to reduce such flows, a rubberlike diaphragm has been introduced between the rigid cell and the sample compacted within the cell (Kenney & Lau, 1985). It follows from this that the cell should preferably consist of a supple wall to prevent the appearance of parasitic flows, on the one hand, and control the sample confinement, which affects soil structure, on the other hand. Some authors (Tomlinson & Vaid, 2000; Hameiri & Fannin, 2002) have tested samples consisting of glass spheres because of satisfying test repeatability. They, however, cannot account for cohesive soils and grain angularities. Yet, their employment capabilities make it possible to anticipate satisfactory results as to their use for phenomenological tests. The
use of naturally heterogeneous and specific samples confront us with the problem of the generalization of the results and test repeatability. Consequently, using reconstituted samples (Reddi & Bonala, 1997) appears to be a satisfactory way to represent natural cohesive soils while satisfying sample preparation repeatability (homogenization by mixing before consolidation). So as to prevent liquefaction and identify the actions of both suffusion and piping more efficiently, samples must be subjected to downflows. Grain entrainment is increased when both the flowrate and the external stress grow (Papamichos et al. 2001). Moreover, Tomlinson and Vaid (2000) have underlined that the influence of the pressure gradient increases velocity. The size of the filter pore opening and its thickness are determining in phenomenon variability (Lafleur, 1999). Therefore, testing different openings and thicknesses is essential. Considering the length of the tests, automation is advisable.

## 2 EXPERIMENTAL DEVICE

### 2.1 Description

The device consists of modified triaxial cells coupled to two pressure/volume controllers (GDS), placed in a temperature-controlled chamber (20°C). Three triaxial cells, which can be used simultaneously for the best possible management of the test duration. The saturation, consolidation, the hydraulic and the mechanical tests are carried out inside the same cell without deconfining to avoid disturbances on the samples (figure 1).

Triaxial cells have been modified to permit the flow in the samples while limiting head losses and to discharge fine particles washed away by the process without clogging the drainage system.

The injection head consists of a 22 mm glass sphere layer to diffuse the fluid on the sample uniformly. At the bottom of the sample, the funnel-shaped draining system is specially designed to avoid clogging or the

<figure><img src="image_2.png" /><figcaption>Figure 1. Photograph of the experimental device.</figcaption></figure>

formation of a layer of particles within the system. This draining system makes it possible to use different filters, which can be either rigid or geotextile with openings ranging between 0.001 mm to 8 mm and a maximum thickness of 17 mm (figure 2).

The hydraulic system consists of different elements generating pressures and depressions, which can operate on the three triaxial cells simultaneously. The GDS is used to inject up to 200 cm³ of statically or dynamically pressurized fluid with a resolution of 1 kPa and 1 mm³. Three pressure regulators for the triaxial cells are connected to air/water interface cylinders. These pressure regulators are used to generate and maintain constant pressures. The maximum pressure is 600 kPa with a maximum flow-rate of 74 mm³/s.

In order to detect internal erosion initiation point and carried out real-time measurements of the eroded grain quantity, an optical sensor has been developed. Placed on the transparent pipe connecting the triaxial cell to the effluent tank, it measures the transparency of the fluid coming through the pipe. Thanks to a preliminary calibration, it possible to find a relationship between transparency and the effluent solid contents (figure 3).

The data acquisition is performed automatically by a computer.

### 2.2 Test procedure

The cohesive soil samples are reconstituted using a washed Loire sand (table 1) and two different type of clay: kaolinite (table 2) and bentonite (table 3).

<figure><img src="image_3.png" /><figcaption>Figure 2. The injection head and the draining system.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3. Triaxial cell and the effluent tank.</figcaption></figure>

<table><caption>Table 1. Loire sand characteristics.</caption><thead><tr><th>Grain size (mm)</th><th>Frequency (% finer)</th></tr></thead><tbody><tr><td>1</td><td>100</td></tr><tr><td>0.5</td><td>60</td></tr><tr><td>0.4</td><td>45</td></tr><tr><td>0.2</td><td>25</td></tr><tr><td>0.1</td><td>4</td></tr><tr><td>0.08</td><td>0</td></tr><tr><td>Uniformity coefficient</td><td>3.125</td></tr><tr><td>Volumic weight of grains (kN/m³)</td><td>26</td></tr></tbody></table>

<table><caption>Table 2. Kaolinite characteristics.</caption><thead><tr><th>Grain size (mm)</th><th>Frequency (% finer)</th></tr></thead><tbody><tr><td>0.060</td><td>100</td></tr><tr><td>0.010</td><td>94</td></tr><tr><td>0.005</td><td>80</td></tr><tr><td>0.002</td><td>47</td></tr><tr><td>0.001</td><td>18</td></tr><tr><td>Liquid limits. wl (%)</td><td>55</td></tr><tr><td>Plastic limit. wp (%)</td><td>22</td></tr><tr><td>Volumic weight of grains (kN/m³)</td><td>27</td></tr></tbody></table>

<table><caption>Table 3. Bentonite characteristics.</caption><thead><tr><th>Grain size (mm)</th><th>Frequency (% finer)</th></tr></thead><tbody><tr><td>0.010</td><td>98</td></tr><tr><td>0.005</td><td>89</td></tr><tr><td>0.001</td><td>76</td></tr><tr><td>0.0002</td><td>60</td></tr><tr><td>Liquid limits. wl (%)</td><td>440</td></tr><tr><td>Plastic limit. wp (%)</td><td>60</td></tr><tr><td>Volumic weight of grains (kN/m³)</td><td>27.6</td></tr></tbody></table>

## 3 TESTS AND ANALYSIS

The preparation of cohesive sample is divided into three steps:

- First, production and installation: the repeatability of the production is guaranteed by the following procedure. The sand is first mixed during 3 minutes at a water content of 8%. While mixing continues, clay is progressively added and mixing is then carried on during 10 minutes. This method has been validated through confirmation of the size distribution homogeneity achieved after mixing.

The installation of the sample inside the cell requires preliminary forming using a 50 mm diameter and 50 mm high mould under the action of two pistons until required dry density is reached (Bendahmane, 2001).

- Second, saturation: this phase begins when a light confinement pressure is applied to prevent any parasitic flow between the sample and the membrane. Saturation with distilled water is then slowly completed (24 hours) to prevent small particles from migrating.

- Third, consolidation: the cohesive sample is consolidated at 200 kPa during 24 hours. Pressure, therefore, increases in stages for a length of time depending on clay percentage and properties of sample.

### 3.1 Comparison and validation tests

The validation of tests is carried out by reproducing an experiment described by Kenney et al. (1992). The test chosen consists in testing samples made of sand and bentonite with a sand/bentonite weight ratio of 8%. The device used by Kenney is a rigid cell permeameter with an axial consolidation of 70 kPa, a hydraulic gradient of 15 m/m and distilled water as fluid. The sample dimensions are 50 mm² for the section and 25 to 30 mm for the height. The hydraulic permeability measured ranges between $2.1 \cdot 10^{-10}$ to $2.2 \cdot 10^{-10}$ m/s. The samples used for the validation have been reconstituted in
<table><caption>Table 4. Performed tests.</caption><thead><tr><th>Sample</th><th>kaolinite (%)</th><th>Filter (mm)</th><th>i (m/m)</th><th>n<sub>0</sub> (.)</th></tr></thead><tbody><tr><td>23</td><td>10</td><td>0.08</td><td>4</td><td>0.33</td></tr><tr><td>31A</td><td>10</td><td>0.08</td><td>27</td><td>0.33</td></tr><tr><td>29B</td><td>10</td><td>4</td><td>2</td><td>0.33</td></tr><tr><td>27</td><td>10</td><td>4</td><td>4</td><td>0.33</td></tr><tr><td>47</td><td>10</td><td>4</td><td>8</td><td>0.33</td></tr><tr><td>36A</td><td>10</td><td>4</td><td>27</td><td>0.33</td></tr><tr><td>20</td><td>30</td><td>0.08</td><td>32</td><td>0.30</td></tr><tr><td>19</td><td>30</td><td>0.08</td><td>110</td><td>0.30</td></tr><tr><td>14</td><td>30</td><td>4</td><td>44</td><td>0.30</td></tr><tr><td>17</td><td>30</td><td>4</td><td>48</td><td>0.30</td></tr><tr><td>16</td><td>30</td><td>4</td><td>92</td><td>0.30</td></tr><tr><td>21</td><td>30</td><td>4</td><td>168</td><td>0.30</td></tr></tbody></table>

the same sand/bentonite proportions and with an identical sand size distribution. The permeability measured on three different samples with the new device is near to $2.37 \cdot 10^{-10}$ m/s. Finally a good agreement between the results obtained by test realized in triaxial cell modified and the value found in the literature, permit a validation of the experimental procedure.

## 3.2 Results and discussion

The tests are performed on the sand kaolinite samples with 10% and 30% of kaolinite. The moisture content of 8% was used, which corresponds to a compacted dry unit weight of $17 \text{ kN/m}^3$. The confinement pressure are fixed at 200 kPa, that represent for the soil sample a 12 m depth. The water injection is performed with a constant hydraulic gradient. We use two filter opening sizes: 4 mm to study the migration of all particle (sand and kaolinite), and 0.08 mm to analyze the behaviour of clay particles only. The parametric study is summarized in table 4.

### 3.2.1 Test for 10% of kaolinite

For 0.08 mm filter opening size and 4 m/m applied hydraulic gradient (sample 23) no internal erosion appears. Whereas, with a 27 m/m hydraulic gradient (sample 31A), migration of clay particles appears. The evolution of instantaneous solid concentration of the effluent (figure 4) is transformed to obtain a more significative parameter: the erosion rate $q_s(t)$ (eroded mass per second or solid flow rate).

$$q_s(t) = C(t) q_{wi}(t) \quad (1)$$

with:

C(t): Concentration;

q<sub>wi</sub>(t): Fluid flow rate.

For a 4 mm filter opening size, a hydraulic gradient of $i = 2 \text{ m/m}$ do not modify the sample (no particles

<figure><img src="image_5.png" /><figcaption>Figure 4. Evolution of instantaneous concentration of effluent.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 5. Erosion rate with 0.08 mm filter and 10% kaolinite (samples 31 A, 23).</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Figure 6a. Erosion rate with 4 mm filter and 10% kaolinite (low gradient, samples 27, 29B).</figcaption></figure>

migration observed for the 29B sample) whereas, an $i = 4 \text{ m/m}$ induces a little migration of clay particles (figure 6a, sample 27). An $i = 8 \text{ m/m}$ generates a strong clay migration (sample 47).

An important migration of particles under a high hydraulic gradient ($i = 27 \text{ m/m}$) induces a drastic erosion rate (figure 6b), the optical sensor reach thus the maximum of signal (corresponding to 6 mg/s). This important erosion is accompanied by a structural modification on the sample base (migration of sand and clay particles) and finally generates collapses (figure 7).
<figure><img src="image_8.png" /><figcaption>Figure 6b. Erosion rate with 4 mm filter and 10% kaolinite (high gradient, samples 36 A, 47).</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Figure 7. Photography of structural modification (sample 36 A).</figcaption></figure>

### 3.2.2 Test for 30% of kaolinite

The same influence of the hydraulic gradient and the filter on internal erosion is observed on the samples with 30% of kaolinite. No erosion with a 0.08 mm filter opening size is observed with a hydraulic gradient up to 110 (sample 19, 20). Moreover, with a 4 mm filter opening size, very little rate (figure 8a) of erosion is observed up to 92 m/m hydraulic gradient, whereas a very high hydraulic gradient (168 m/m) generate an important clay erosion (figure 8b).

All the requests of performed test underline very different magnitude of erosion rate some test show no erosion ($q_s < 1\mu g/s$), some other tests show high

<figure><img src="image_10.png" /><figcaption>Figure 8a. Erosion rate with 4 mm and 30% kaolinite.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Figure 8b. Erosion rate with 4 mm and 30% kaolinite (very high gradient).</figcaption></figure>

erosion ($q_s > 1$ mg/s). The rests of test show moderate erosion the clay fraction appears to strongly influence the soil erodibility. The performed test tend to show that for 10% kaolinite samples the erosion rate is about to times higher than for 30% kaolinite samples. Erosion rate increases with hydraulic gradient from moderate erosion (above a certain gradient value strongly depending on granulometry and filter) to high erosion. With 4 mm filter opening size this critical gradient is about 3 m/m for 10% of kaolinite and 45 m/m for 30% of kaolinite. With 0.08 mm filter this critical gradient seems to reach 2 or 3 times the precedent values. Consequently the filter opening size is not the important parameter to qualify erosion rate, in complement with the hydraulic gradient.

## 4 CONCLUSIONS

Mechanisms responsible for internal erosion are many evolutionary and depend on different parameters. In the face of such complexity, internal erosion initiation and development criteria found in the literature are closely dependent on the structure studied and, therefore diverse.

The experimental device can be used to saturate and consolidate reconstituted sand/clay samples. The triaxial cells used for this parametric study have been modified to let the flow comes through up to the sample.
The first test realized permit on the hand, the comparison between the test carried out with the modified triaxial cells and results obtain by Kenney et al. (1992) on sand/bentonite. This comparison demonstrates the good repeatability of the tests and validates the experimental procedure. On the other hand, we have studied the mechanism which induce the internal erosion for a sand/kaolinite sample.

All tests are performed for sand/clay sample with 10 and 30% and kaolinite for a hydraulic gradient between 2 and 168 m/m.

- We have shown for sand/kaolinite samples that it is possible to reduce the migration of particles if we used a geotextiles filter. Moreover, if we increased the clay percentage in sample we established that sample have a better erosion resistance.

- The internal erosion begins by a migration of fine particles to the downstream side of sample. If the quantity of eroded fine particles is important, the sample structure is modified and then the transport of sand particles occurs. Finally the skeleton of sample is unstable and causes the breaking down. Sand erosion can't be characterized by the present configuration of a device. Further developments are needed by the mean of a mass balance include in the chain of measurements.

To extend this study, we have to carried out some complementary tests with different sand/clay ratio and filter opening sizes. We intend to use the capacity of our device to take into account the effect of confining pressure on the internal erosion kinetics.

## REFERENCES

Bendahmane, F. 2002. Influence de l'écoulement sur la résistance des sols: étude expérimentale préliminaire. Master report, Université de Nantes.

Foster, M. Fell, R. & Spannagle, M. 2000. The statistics of embankment dam failures and accidents. Canadian Geotechnical Journal, Vol. 37:1000-1024.

Fry, J.J., Degoutte, G. & Goubet, A. 1997. L'érosion interne: typologie, détection et reparation. Barrages & Réservoirs, no 6:0-126.

Hameiri, A. & Fannin, R.J. 2002. A cyclic gradient ratio test device. Geotechnical Testing Journal; 25(3):266-276.

Kenney, T.C. & Lau, D. 1985. Internal stability of granular filters. Canadian Geotechnical Journal; 22:215-225.

Kenney, T.C., Van veen, W.A., Swallow, M.A. & Sun-gaila, M.A. 1992. Hydraulic conductivity of compacted-sand mixtures. Canadian Geotechnical Journal, Vol. 29: 364-374.

Lafleur, J. 1999. Selection of géotextiles to filter broadly graded cohesionless soils. Geotextiles and Geomembranes; 17:299-312.

Monnet, A. 1998. Boulance, érosion interne, renard. Les instabilités sous écoulement. Revue Française de Géotechnique; 82:3-10.

Papamichos, E., Vardoulakis, I., Tronvoll, J. & Skjærstein, A. 2001. Volumetric sand production model and experiment. International Journal for Numerical and Analytical Methods in Geomechanics; 25:789-808.

Reddi, L.N. & Bonala, M.V.S. 1997. Critical shear stress and its relationship with cohesion for sand-Kaolinite mixtures. Canadian Geotechnical Journal; 34:26-33.

Reddi, L.N., Lee, I. & Bonala, M.V.S. 2000. Comparison of internal and surface erosion using flow pump test on a sand-kaolinite mixture. Geotechnical Testing Journal; 23(1):116-122.

Skempton, A.W. & Brogan, J.M. 1994. Experiments on piping in sandy gravels. Géotechnique; 44(3):440-460.

Tomlinson, S.S. & Vaid, Y.P. 2000. Seepage forces and confining pressure effects on piping erosion. Canadian Geotechnical Journal; 37:1-13.