Some Elements of Comparison between two ...

ICSE6 Paris - August 27-31, 2012

- Bennabi et al

Some Elements of Comparison between two Laboratory Devices for Soil Erosion Testing Abdelkrim BENNABI1, Tarek KAROUI1, Ahmed BENAMAR2, Hua-Qing WANG2 1

IRC-ESTP 28 Avenue Président Wilson 94234 Cachan - e mail:[email protected] 2

LOMC UMR 6294 CNRS-Université du Havre 53 rue Prony 76058 Le Havre- e mail: [email protected]

Abstract: The aim of this paper is to compare erosion tests results obtained with two experimental devices, Erosion Function Apparatus (EFA) and Hole Erosion Test (HET) and to highlight the influence of clay type and proportion on the initiation and the evolution of the erosion phenomena with these two devices. The tests were performed on mixtures of sand and clay (Kaolinite or Illite), in order to assess their sensitivity to erosion. EFA tests are performed in order to measure erosion rates for soils subjected to tangential water flow. HET results coming from a laboratory were compared to EFA results. The main parameters used in this comparison are the critical shear stress which initiates the erosion process in both devices and the critical velocity. Usual criteria are used to make a classification for the mixtures tested. Keywords: EFA, HET, Internal Erosion, Shear Stress, Erosion rate, Soil Mixtures.

I INTRODUCTION Erosion process is a major problem that affects all hydraulic constructions (Dams, Dykes, Levees…) and many important studies were conducted in order to understand the erosion process initiation and progression. In literature it is reported that erosion process is related to different parameters: grain size distribution, degree of compaction, composition of the eroded soil, variation of soil moisture, permeability, heterogeneity of soil, nature of fine particles in sandy soils… One of the most important parameters used to characterize the initiation of erosion process is the critical shear stress which is the value leading to the initiation of erosion. Many experimental studies were based on external or surface erosion devices like the JET (Jet Erosion Test Henensal & al, 1987 and USDA, REMR, 1985 and ARS, Hanson, 1992), the HET (Hole Erosion Test, Wan et Fell, 2002) and EFA (Erosion Function Apparatus Briaud et al., 1991 to 1993). Some studies were devoted to internal erosion using devices like the Triaxial Erosion Test, TET (Sanchez & al, 1983; Bendahmane, 2005) and the erosion columns (Benamar et al, 2010). Our purpose is to compare test results obtained on sandy-clay mixtures with both devices, the EFA and the HET.

II DESCRIPTION OF THE TESTS II.1 The Erosion Function Apparatus (EFA) The EFA device (Figures 1 and 2) was developed in the beginning of the nineties in order to measure the erodibility of soils and to predict bridge piers scour (Briaud and al., 1999, Briaud and al., 2001). It is used with site or reconstituted soils and provides a relation between the erosion rate and shear stress (erosion plot) which is called erosion function. In our study, the mixtures tested are poured and compacted in a Shelby tube (76.2 mm diameter, ASTM-D1587).The tube is then placed through a circular opening in the bottom of a rectangular cross section conduit (101.6 mm x 50.8 mm), which is 1.22 m long. A leak proof connection is obtained thanks to a snug fit and an O-ring. One millimeter of

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soil is extruded into the conduit (Figure 3). An initial velocity is chosen for the water flow into the conduit and is increased until soil erosion is initiated (range of mean flow velocities: 0.1 m/s to 6 m/s).

Fig.1: Erosion Function Apparatus (extracted from EFA user manual- Humboldt)

Fig.2: EFA owned and used by ESTP

Fig.3: Soil protrusion in the flow pipe (extracted from Briaud)

The typical EFA test procedure is recalled here: 1. Place the sample in the EFA, fill the conduit with water, and wait one hour. 2. Set a low velocity with the flow control valve (usually between 0.15 and 0.3 m/s). 3. Push the soil in the sampling tube so it is flush with the bottom of the conduit 4. Advance the piston as fast as the soil is being eroded by the water flow (visual inspection through the Plexiglas window). 5. Record the time t it takes for h mm of soil to be eroded. 6. When a few millimeters of soil have been eroded or after 1 hour of flow whichever comes first, increase the velocity. 7. Repeat step 4,5 and 6 in order to obtain a few values. The velocity when the soil starts to erode is called critical velocity and corresponds to the critical shear stress of the soil. Above this value, the velocity is increased incrementally and at each increment the sample erodes for a certain period of time. The recorded amount of erosion and test duration are used to calculate the erosion rate. This is obtained by dividing the length of eroded soil by the time required to do so. The shear stress is obtained with the Moody chart for pipe flows (Moody 1944).

  f    v2 8 where:  : Shear stress on the wall of the pipe f : Friction factor obtained from the Moody chart  : Mass density of the water (1000 kg/m3) v : mean flow velocity in the pipe

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The friction factor f is a function of the pipe Reynold’s number Re and the pipe roughness ε/D. The Reynold’s number is v D/ν where D is the pipe diameter and ν is the kinematic viscosity of water (10−6m2/s at 20°C). Since the pipe in the EFA has a rectangular cross section, D is taken as the hydraulic diameter D =4A/P where A is the cross sectional flow area, P is the wetted perimeter, and the factor 4 is used to ensure that the hydraulic diameter is equal to the diameter for a circular pipe. For a rectangular cross section pipe: D = 2ab/(a+b), where a and b are the dimensions of the sides of the rectangle. The relative roughness ε/D is the ratio of the average height of the roughness elements on the pipe surface over the pipe diameter D. The average height of the roughness elements ε is taken equal to 0.5D50 where D50 is the median grain size for the soil. The factor 0.5 is used because it is assumed that the top half of the particle protrudes into the flow while the bottom half is buried into the soil mass. We used the Colebrook relation to have an accurate value of the friction factor.  1 2,51  2 log  D  f  3,7 Re f 

   

Briaud proposed also a classification of erosion categories of soils and rocks based on 15 years of erosion testing experience (Briaud 2008).

II.2 The Hole Erosion Test (HET) In this test, a flow is established in a pre-drilled hole in the soil sample, in a manner similar to the pinhole test and to a piping erosion situation (Figure 4), with follow-up instrumented. This test leads to the characterization of the soil resistance against the surface erosion which is produced in the hole. Measuring and estimating a number of parameters (flow rate, hydraulic gradient and turbidity) and change in diameter of the hole, lead to determine the shear stress and erosion rate. The HET has been used by various researchers to carry out tests on mixtures of sand-clay (kaolinite and illite). These tests showed the strong dependence of erosion rate with the type and percentage of clay (Pham 2008). Wan and Fell (2004) conducted a study on the erosion resistance of different materials constituting the core of dams. An erosion rate Index was introduced for the classification of the resistance to erosion. Since then, other research teams have developed new devices equipped with more sophisticated instrumentation allowing more precise measurements of the test parameters. The HET of CEMAGREF (Benahmed - 2011) is one of them. It allows erosion testing both on specimens of reconstituted or undisturbed soil. In this device, the test apparatus consists of a cylindrical plexiglass cell, divided into three detachable parts. The soil sample is placed in the central part to be tested and the upstream portion is connected to a water supply. The downstream portion is connected to a turbidimeter placed close to the downstream part of the sample in order to characterize the effluent, and a mass flow meter for the flow control. Two pressure sensors are mounted upstream and downstream of the device for measuring the inlet pressure and outlet pressure, and thus the pressure gradient applied. The manufacture of reconstituted samples is very similar in both types of tests, EFA and HET. In the two cases, one begins by determining the values of dry density and water content of the soil sample to be tested, based on the values of dry density and water content of the Standard Optimum Proctor. As it will be explained a little further for the EFA, once dry density and water content set, the sample is introduced into the test cell, in several successive layers, compacted using a rammer. As the EFA, the HET is used to estimate the critical shear stress needed to initiate erosion. It allows also the determination of the rate of soil removal per unit of applied excess stress by estimating a detachment rate coefficient.

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Fig.4: Hole Erosion Test device (extracted from Bezzazi and al, 2010)

III EFA EXPERIMENTS Materials tested: The tests were conducted on sampled soil mixtures of sand and 2 types of clay. The mixtures were constituted in the Schelby tube (see procedure below). Two kinds of sand were used in this study: Fontainebleau sand and Hostun sand. Also two kinds of clay were used: Kaolinite and Illite. The main characteristics of these materials are recalled below. 100

% Passing

80

D50 Font = 0.154mm

60

D50 Hostun = 0.205mm

40

fontainebleau Hostun

20 0 0,01

0,1

Sieve (mm)

1

10

Fig.5 : Grain size distribution curve - Fontainebleau and Hostun sands Mixtures preparation procedure: The sand was mixed with the clay and the amount of water previously determined in a mechanical mixer at least for 3 minute in order to insure a homogeneous mixing. Then the Shelby tube was filled with 30 to 40 cm of soil height corresponding to 3 to 4 kg of material. To fill the tube, a small amount of soil between 200g to 400g was introduced and then compacted until reaching a certain height corresponding to the desired compaction value. This procedure is repeated until at least 30cm of soil height which is used for the testing. Once the filling is complete the sample is placed in the EFA. The soil is pushed until it reaches the top of the Shelby tube. Then, the soil is trimmed in order to obtain a homogeneous surface and the Shelby tube is placed inside the flume. At this moment the water flow can be established. During the experiment (as indicated before in the procedure) a maximum of soil is pushed within one hour of time, by step of 1 mm. Then the flow is stopped and the Shelby tube is pushed down. The soil is pushed up a little bit so it can be trimmed again before the next value of velocity. Then it is placed back in the flume. When the velocity increases leading to an increase of the erosion rate, the next value of velocity is chosen when we reach 50mm height of eroded soil.

IV RESULTS The results of EFA testing consist of the erosion rate dz/dt versus shear stress, called the erosion function. Figure 6 shows the erosion function obtained for Hostun sand – Kaolinite mixtures (5% and 25% Kaolinite). These two mixtures and the Hostun –Illite mixture added in Fig.7 were tested around

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80% Standard Proctor with a dry density of approximately 1,6 t/m3. The first comment that can be done is that there is a clear difference between the erosion behavior of these two mixtures. The erosion resistance is higher with 25% kaolinite. The values found for the critical shear stress are 0,1 Pa and 0,4 Pa for respectively 5% and 25% kaolinite. The second comment concerns the use of an approximation to relate the data of rate erosion and shear stress. These approximations can be used to estimate the critical shear stress. We used two approximations, a polynomial (Fig.6) and a logarithmic one (not shown) that give close curves and very similar values of the critical shear stress. However, there has been no systematic study on the best way to approximate the experimental curves obtained.

Fig.6: Erosion Function for Hostun sand – Kaolinite mixtures (5% and 25% Kaolinite) Figure 7 shows Hostun Sand - Clay (Kaolinite and Illite) mixtures with erosion classifications. The three mixtures are classified between “Very High erodability” and “High erodability”.

Fig.7: Classification of erodibility according to the shear stress Hostun-Kaolinite mixture (5% and 25% K) and Hostun-Illite mixture (5% I) For the second series of tests the mixtures were Fontainebleau sand and Kaolinite with 5%, 10%, 15% and 25% Kaolinite. The water content values were respectively 7%, 14,5%, 15,5% and 11% in approximately 80% Standard Proctor compacted samples. The results are represented in Fig.8. The critical shear stresses are between 0,1 Pa and 0,6 Pa. We can notice that the increase of Kaolinite amount increases the resistance against erosion. Figure 8 indicates that tested mixtures can be classified between Very High erodability and High erodability. The shear stress depends on the coefficient of friction in the pipe which itself depends on the roughness. In general, the value of that parameter is taken equal to 0.5 D50, but can be set by the operator. The tests were performed on mixtures of fine grained soils so that uncertainty about the value of D50 is not significant for that of the shear stress. For the 5% Kaolinite mixture a calculation was done with D50 (D50 = 0,154 mm for the four kaolinite mixtures), D30 and D70. The points obtained are almost alike (no more than 5% of difference for higher values of shear stress and less than 1% for lower ones). A larger variation (calculation with D20 or D80) is not more significant. In this study, we have not worked with coarse grained soils, in this case and particularly with a graded grain size distribution, the determination of the roughness of the pipe would have a greater importance.

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Erosion Rate (mm/h)

10000 High Erodability

Very High Erodability

1000 100

5%K

Medium Erodability

10

10%K

Low Erodability

1 0,1 0,01

0,1

1

10

15%K 25%K

100

Shear Stress (Pa)

Fig.8: EFA results for Fontainebleau sand – Kaolinite mixtures – Classification

Erosion Rate (mm/h)

Figure 9 show the tests performed on Fontainebleau sand-Illite clay mixtures. The percentages of Illite are: 5%, 15%, 20%, 25% for corresponding values of water content: 15,5%, 11,2% a, 15% and 13,8%. The first velocity value, at the beginning of the test, is 0,21 m/s. This value correspond to a shear stress of 0,2 Pa. But for this value no erosion is observed. The next velocity is 0,60 m/s and corresponds to a shear stress of 1,3 Pa. For this new velocity, erosion is observed and the measurement gives an erosion rate of 0,50 mm/h. Then, critical shear stress must have a value between 0,2 Pa and 1,3 Pa. It is not very easy, with the EFA device, to set low values of velocity; this is why we have encountered some difficulties in determining the critical shear stress with a good accuracy. Similarly to the result obtained with the Fontainebleau sand – Kaolinite mixtures, the increase of Illite amount increases the erosion resistance. We can also notice that the mixtures with Illite seem to be more resistant against erosion. For 20% and 25% Illite, the mixtures are classified as “Moderately Erodible” while 25% Kaolinite mixture is in “Very Erodible” domain. 10000 1000

High Erodability Medium Erodability

Very High Erodability

100

5% I 15% I

10 Low Erodability

1

25% I 20% I

0,1 0,01

0,1

1

10

100

Shear Stress (Pa)

Fig.9: EFA results for Fontainebleau sand – Kaolinite mixtures – Classification

V COMPARISON WITH HET TEST RESULTS AND DISCUSSION The table 1 gives HET test results performed in Geotechnical laboratory of CEMAGREF, Aix-enProvence (Benahmed, 2011), in the framework of an inter-comparison research program - ERINOH French National Project on Internal Erosion. The tests were performed on NE34 sand – kaolinite mixtures (NE34 is the sand called Fontainebleau in EFA testing). In this table, Ce, kd and Ie are the parameters of detachment-driven erosion equation which can be expressed in terms of either mass rate of erosion or volumetric erosion (Wahl and al. 2010): = ( − ) = ( − ) where: is the rate of mass removal per unit of surface area (kg/s/m2), -  and c are the applied shear stress and threshold shear stress for soil detachment,

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Test 5%K 20%K 25%K 30%K 5%I 10%I

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Table 1: HET tests for NE34 sand-Kaolinite Mixtures kd (cm3/N.s) Ce (s/m) C (Pa) d (T/m3) 3,24E-03 1,75 1,9E+00 1,2 1,26E-03 1,8 7,0E-01 3,7 2,5E-03 1,78 1,4E-01 5,2 1,77 1,75 -

Ie 2,5 2,9 3,6 -

- Ce is the coefficient of soil erosion, - kd detachment rate coefficient (Ce = kd.d); d is the dry density. - Ie is the erosion Index ( = − ) A first remark concerns the difficulty of achieving tests for mixtures containing a low percentage of clay. These mixtures often collapse during the test. The critical shear stresses found are higher than the ones found on EFA. This puts them in the category of “Very Rapid” erosion for 20% and 25% kaolinite mixtures and “Moderately Rapid” erosion for the 30% Kaolinite mixture.

Erosion Index Ie

0 1

Extremely

2 3 4 5

Very Rapid

20%K

Moderatly Rapid Moderatly Slow

25%K 30%K

Very Slow

6

Extremely Slow

7 1

10 Critical shear stress (Pa)

100

Fig.10: HET classification of Erodability (Wan and Fell – 2004) The EFA testing performed on sand-clay mixtures shows that the increase of amount of clay increases the resistance against erosion. This result is similar to the one obtained with other types of experiments and particularly HET test (Fig.9). Also, the type of clay affects the erosion rate of the mixtures. The Illite mixtures show more resistance against erosion. This is verified with tests performed with the EFA and confirmed with test results obtained with the HET (Pham, 2008). As the HET, EFA appears therefore discriminating since the two textures are well discernible between them. A rather clear difference between the EFA and the HET is that with the HET, it is difficult to test mixtures with low percentages of clay, because of their tendency to collapse. This is not the case with EFA with which coarser soils can be tested.

VI CONCLUSION We wanted to make a comparison of the two types of erosion test, EFA and HET, but ultimately, we do not have a sufficient number of results that would have allowed a more objective comparison. However, it is possible to make some comments explaining the differences between the two types of tests. The comparison is based on the comparison of key parameters and accuracy of their assessment. For both types of test, it seems that one of the most important parameter is the shear stress that causes the detachment of particles. This study, which is ongoing and will be deepened, focused on reconstituted fine-grained soil samples. We plan to test coarser soils, with larger percentages of clay as well as natural soils. The comparison HET/EFA is suggested by the similarities of the phenomena occurring during each test and by the similarity of their interpretation, particularly regarding the determination of the shear stress. Indeed, both types of tests involve surface erosion. The surface under erosion test for HET is that of the wall of the hole which is cylindrical and variable during the test. While for the EFA, the surface under test erosion is a circular area, smaller and constant, which is placed at the base of the pipe.

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For both tests, the phenomena are studied for increasing shear stresses, and erosion (eroded mass for the HET and erosion rate for EFA) is analyzed in terms of the shear stress acting on the surface under test. Practically, the evaluation of this shear stress is made using the same formula which involves parameters whose measurement or determination is made with more or less uncertainty. This is particularly true of the roughness of the hole for the HET or that of the sample surface subjected to the erosion for EFA. In terms of the erosion rate obtained with the EFA, it is accepted (Briaud and al. 2003) that below 10 mm/hr, the error on the erosion rate is about 0.5 mm/hr, while for high values (above 100 mm/hr) this error is estimated at 2 mm/hr. The relative error is less than 10%. More generally, it is recognized that the two main parameters determined in the EFA are measured with a relative error of about 10%. With the HET, it may be recalled that a major source of uncertainties is related to the difficulty of knowing the diameter of the hole at any time during the test. The use of equations reflecting the phenomena require the determination of the nature of the flow regime (laminar or turbulent), and to correctly estimate the friction coefficient of the water on the walls of the hole.

VII REFERENCES Bezzazi, M., Khamlichi, A., Vera, M.P., Cintas Rubio, M.D. and Olegario, C. L.: A Simplified Analytical Modeling of the Hole Erosion Test. American J. of Engineering and Applied Sciences 3 (4): 765-768, 2010. Briaud, J.-L., Ting, F. C. K., Chen, H. C., Gudavalli, R., Perugu, S., Wei, G., 1999 (a), “SRICOS: Prediction of Scour Rate in Cohesive Soils at Bridge Piers,” Journal of Geotechnical Engineering, Vol. 125, No. 4, April 1999, pp. 237-246, American Society of Civil Engineers, Reston, Virginia, USA. Briaud, J.-L., Ting, F. C. K., Chen, H. C., Gudavalli, R., Kwak, K., Philogene, B., Han, S. W., Perugu, S., Wei, G.,Nurtjahyo, P., Cao,Y., Li, Y., 1999 (b), “SRICOS: Prediction of Scour Rate at Bridge Piers,” Report 2937-1. Texas Department of Transportation, Texas A&M University, Civil Engineering, College Station, Texas 77843-3136, USA. Briaud, J.L., Chen, H.C., Li, Y., Nurtjahio, P.O., and Wang, J. (2004). “Pier and Contraction Scour in Cohesive Soils”, NCHRP Report 516, Transportation Research Boa, Washington, D.C.. Briaud, J.-L., Chen, H.-C., Li, Y. Nurtjahyo, P., Wang, J. (2003) - “Complex Pier Scour and Contraction Scour in Cohesive Soils” – NCHRP Report 24-15 - USA Benahmed, N. : « Essais d’Erosion de Trou avec le dispositif HET du CEMAGREF », Research Report ERINOH, 2011. Hanson, G.J. and Simon, A., 2001. Erodibility of cohesive streambeds in the loess of the Midwestern USA. Hydrological Processes, Vol. 15, pp. 23-38. Pham, T.L., « Erosion et Dispersion des Sols par un Fluide », Thesis 2008 - LCPC France. Wahl, T.L., “A Comparison of the Hole Erosion Test and Jet Erosion Test”. Joint Federal Interagency Conference on Sedimentation and Hydrologic Modeling. June 27 – July 1, 2010 – Las Vegas, NV. Wan, CF., Fell, R. 2004a. Investigation of rate of erosion of soils in embankment dams. J. Geotech. and Geoenvir. Engrg. N°30 (4), pp. 373-380. Wan, CF., Fell, R. 2004b. Laboratory tests on the rate of piping erosion of soils embankments. Geotech. Testing J. N°27 (3). Acknowledgements: Many tests were performed by FREMOND, A., SIMONNET, J. and STERIN, C.A., 2nd Year ESTP students, in the framework of a Research Project – ESTP 2012.

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