Influence of soil parameters on erosion Van-Nghia Nguyen, Laboratory Mechanics of Soils - Structure and Materals (MSSMat), Ecole Central Paris (ECP), Grande Voie des Vignes, 92295 Châtenay Malabry, E-mail: nghiahre@gmail.com Jean-Robert Courivaud, Engineer, Geotechnics and Underground Structures Service, EDF, Savoie Technolac, 73373 Le Bourget du Lac Cédex, France Patrick Pinettes, Geophyconsult, 12 Allée du Lac de Garde, BP 231, 73074 Le Bourget du Lac cedex, France Hanène Souli, Doctor, Laboratory of Tribology and System Dynamics, ENISE 58 Rue Jean Parot, 42023 Saint Etienne Cedex, France Jean-Marie Fleureau, Professor, Laboratory Mechanics of Soils - Structure and Materals (MSSMat), Ecole Central Paris (ECP), Grande Voie des Vignes, 92295 Châtenay Malabry, E-mail: jean-marie.fleureau@ecp.fr Abtract: The influence of some soil parameters on the erosion of a silty soil, characterized by the erosion coefficient kD, the critical shear stress τc, and the equilibrium scour depth Pe, is studied A series of submerged Jet Erosion Tests was carried out to examine the influence of dry density ρd, compaction water content w, and degree of saturation Sr The erosion parameters were derived using method of Hanson The results show that the erosion coefficient and the equilibrium scour depth decrease when the dry density, the compaction water content or the degree of saturation of the soil increase, whereas the critical shear stress increases with these parameters We also observed an inverse relationship between the two parameters kD and τc Keywords: Jet Erosion Test (JET), erosion parameter, erosion coefficient, critical shear stress Introduction: The erosion parameters depend on the properties of the soil, such as the type of soil (Lim, 2006; Wahl et al., 2009; Hanson and Hunt, 2007), the percentage of clay and its mineralogy (Bradford and Blanchar, 1999; Benahmed et Bonelli, 2012; Wan and Fell, 2002, 2004; Lim, 2006; Ansati et al., 2007; Pham, 2008) , as well as the dry density (Hénensal and Duchatel, 1990; Hanson and Robinson, 1993; Hanson et al., 2002; Lick and McNeil, 2001; Al-Madhhachi et al., 2013, 2014; Regazzoni, 2009) moisture content (Hanson and Robinson, 1993, Robinson et al., (2002), Lim (2006), Ansati et al (2007), Bendahmane and Bonelli, 2012), compaction energy (Hénensal and Duchatel (1990), Lim (2006), Hanson and Hunt, 2007), etc To estimate erosion parameters, several devices were used to quantify soil erosion like the Hydraulic Flume Test (Shaikh et al., 1988), the Erosion Function Apparatus (Briaud, 2001), the Rotating Cylinder Test (Chapuis and Gatien, 1986; Lim and Khalili, 2009), the Jet Erosion Test (Hanson and Cook, 2004; Mazurek and Rajaratnam, 2001; Al-Madhhachi et al., 2013), and the Mobile Jet Erodimeter (Hénensal and Duchatel, 1990), etc… We were interested in the Jet Erosion Test which was developed by Hanson and Cook (2004), but with this device, we could only measure the depth erosion in the center of the sample but not depth erosion profiles This could lead to bad values of erosion parameters Concerning the factor which impacted erosion resistance, we studied compaction water content, dry density and compaction degree of saturation However, in previous research, to study the influence of water content and dry density, the samples were compacted, with both dry density and water content varying following the compaction curve Therefore, it was very difficult to assess the role of each parameter on soil erosion In the tests presented in this paper, one variable was maintained constant while the other was changed The samples were compacted by means of a hydraulic press Then, the Jet Erosion Tests were carried out using an improved device developed at Ecole Centrale Paris which measures not only the scour depth in the center of the sample but also the scour profile The erosion parameters (kD and τc) were detemined by Hanson’s method based on the Blaisdell’s solution approach Soil characterization and experimental apparatus 2.1 Soil characterization The soil was a silty soil which was taken in a dike in the south of France Soil testing was performed to determine the soil properties: Atterberg limits (XP CEN ISO/TS 17892-12), particle size distribution (XP CEN ISO/TS 17892-4), methylene blue value (NF P94-068), Standard Proctor compaction test (NF P94-093), oedometer test (XP CEN ISO/TS 178925) The liquid limit (wL) ranges from 30 to 35%, the plastic limit (wP), from 14 to 16%, the plasticity index (IP), from 13 to 16%, and the methylene blue value, VBS = 1.8, which is consistent with the relatively high soil plasticity The optimum water content (wOPN) is 17.2%, and the corresponding maximum unit weight γdOPN is between 16.6 and 17.2 kN/m3 2.2 Experimental apparatus The experimental device (Figure to Figure 3) was built at Ecole Centrale Paris (ECP) based on the original apparatus of Hanson and Cook (2004) The apparatus consists of the following parts: acquisition data unit (A), injection cell (B) with angle sensor (E), displacement sensor (F), pressure sensor (G), hydraulic pump (C), reservoir (D), point gauge (H), deflector (I) and Jet tube (J) The schematic view of the experimental setup is showed in Figure below For this JET device, the injection cell is composed of a perspex tube, 50 mm in diameter and 800 mm in height, ended at the base by the injection nozzle with an air purge valve at the upper part (Figure 3) The injection nozzle was made of a conical hole in a plate of mm thickness, Its diameter is 6.35 mm and the angle of the cone is 45o The arrival of the water is at half height, upwards, so as to remove more easily the dissolved or entrapped air The tube is supported by two clamps attached to a movable axis placed a few inches from a fixed axis The injection cell (B) can rotate around an axis, which allows us to measure the depth of erosion not only in the center of the sample but also in other points on a circle (Figure 6) At its base, in the injection nozzle, the cell is equipped with a rotary valve (or deflector (I), not shown in the photo) that can stop manually the jet very quickly The Proctor mold containing the sample, 117 mm in height and 101.6 mm in diameter is placed in a perspex downstream reservoir, 500 mm in diameter and 600 mm in height (Figure 4) The depth of immersion is determined by the discharge of water in the upper part of this reservoir To vary the depth of immersion, the mold is placed on a parallel displacement lift It is therefore the first parameter Once the height of the mold is fixed, it sets the distance between the injection nozzle and the sample at the selected value The injection cell is supplied with water from a constant level reservoir, which can be arranged at different levels on a metal shelf (Figure 1) For constant level, the reservoir D is alimented by a pump C which withdraws water from the immersion reservoir The improvements, compared to the original device developed by Hanson, consist in the possibility of changing the values of real hydraulic head, h1, depth of immersion in water of the specimen, h2, and distance between the nozzle of jet and the specimen, h3 (Figure 5) more accurate measurements, and the acquisition of the erosion profiles as shown in Figure Moreover, it is not necessary to close supply valve which was operated manually while we measured the depth of erosion and the erosion profiles D G Air purge Valve B E A C F Figure 1: Supply system of jet in closed-circuit Figure 2: Pressure and angle sensors Downstream reservoir Soil speciment J Figure 4: Sample submerged D H B Supply valve I Sample Figure 5: Schematic representation Figure 3: Photo of the injection cell of h1, h2, h3 Fixed axis Rotating axis Nozzle of jet Displacement sensor Measurement points Proctor mold Figure 6: Displacement principle of the injection and measurement devices Figure 7: Example of erosion profiles corresponding to different measurement times Test procedures The Proctor mold with the sample is placed in a reservoir under the jet (Figure 4), the test parameters and hydraulic parameters were fixed in this work As soon as the test starts, the variations of three parameters: angle, displacement and pressure will be recorded by a center of acquisition (Figure 8) As soon as the test starts, the acquisition system was ready and the jet was closed by the deflector The air purge valve was closed, the supply valve was opened slowly to fill with water in the tube, after the air purge valve was opened to remove the air in the tube When the tube is filled with water, air purge valve was closed, supply valve was opened entirely The initial distance between the nozzle of jet and the specimen, maximal and minimal angle were noted to calculate the value of scour depth profile and its position Then a spreadsheet in Microsoft Excel was established to determine hydraulic head, evolution of depth erosion at specimen center and depth erosion profiles 50 45 Angle (°) 40 35 30 25 20 200 400 600 800 Time (s) 1000 1200 1400 Figure 8: Example of recorded raw data Results and discussion 4.1 Influence of dry density and gravimetric compaction water content Case of constant gravimetric compaction water content The tests were carried out at different constant water contents to study the influence of dry density It found that the trend of equilibrium erosion depth (Pe) is similar with trend of erosion coefficient (kD) so we only show relationship of kD and critical shear stress (τc) with soil parameters In from Figure to Figure 10, we note that increasing the dry density leads to a decrease in the erosion coefficient (kD), an increase in the critical shear stress (τc), which means that the erosion resistance of the soil increases with the dry density When density increases, the void ratio decreases which leads to the increase in suction (Fleureau et al., 2011; Taibi, 1994; Taibi et al., 2011) and soil strength These results confirm those of previous researchers (Hénensal and Duchatel, 1990; Lim, 2006; Mostafa et al., 2008; Robinson and Hanson, 2001; Robinson et al., 2002; Benahmed and Bonelli, 2012) It is found that at low dry density (ρd = 1.55 g/cm3), the values of kD are widely dispersed while the value of τc is not widely dispersed, because the penetration occurs quite quickly in the above soil layer which influences the erosion velocity This shows that, at low dry density, moisture content plays an important role in the erosion mechanisms while, at high dry density, the influence of water content is negligible because the penetration occurs very slowly 30 20 20 τ c (Pa) kD (cm3/N/s) 30 10 10 0 1.5 1.55 1.6 1.65 1.7 1.75 1.5 1.8 1.55 1.6 1.65 1.7 1.75 1.8 ρ d (g/cm ) ρ d (g/cm ) Figure 9: Relationship between erosion coefficient, kD, and dry density, ρd, at different constant gravimetric water content Figure 10: Relationship between critical shear stress τc and dry density ρd, at different constant gravimetric water content Case of constant dry density The influence of gravimetric compaction water content on τc, kD is complex (Figure 11, Figure 12) When the dry density is high (ρd ≥ 1.6 g/cm3), the erosion coefficient (kD) decrease while the critical shear stress (τc) increases with the water content Soil resistance to erosion increases with water content because the change of water content leads to a change in the fabric of the soil and the soil possess a structure more homogeneous when water content increases because the grains were more hydrated (Lambe, 1958; Shresta and Arulanandan, 1988; Cui, 1993) In this case, it is the fabric of the soil which plays the major part in the erosion phenomena At very high dry density (ρd = 1.75 g/cm3), it is found that the erosion resistance hardly varies with the compaction water content because the variation of suction caused by variation of water content is very small compared to the initial suction of the soil Moreover, for very dense samples, the change of water content does not change the soil structure In this case, water content has little effect on the erosion parameters But when the dry density is low (ρd < 1.6 g/cm3), the critical shear stress (τc) decreases while the erosion coefficient (kD) increase with the gravimetric compaction water content It means that the soil resistance to erosion decreases when the gravimetric compaction water content increases because there is an increase of water content in soil with time of test and depth of sample caused by a penetration of water in the soil which decreases its suction and changes its properties (Fleureau et al., 1993, 2002, 2011) This increase decreases the suction of soil occurs more quickly on low density (Figure 14) than on high density (Figure 13) In this case, the initial suction and the penetration of water play the most important role 35 35 30 30 ρ d =1.55 ρ d =1.7 25 ρ d =1.75 20 τ c (Pa) kD (cm /N/s) 25 15 20 ρ d=1.65 15 ρ d=1.6 10 10 ρ d =1.6 ρ d=1.7 5 ρ d =1.65 ρ d=1.75 ρ d=1.55 12 14 16 18 w (%) 20 22 12 14 16 w (%) 18 20 22 Figure 11: Relationship between erosion coefficient (kD) and water content (w) Figure 12: Relationship between critical shear stress (τc) and water content (w) winitiale = 16.6%, ρ d = 1.7g/cm winitiale = 16.6%, ρ d = 1.7g/cm 210 35 timmersion=10min Suction (kPa) water content (%) 180 30 timmersion=60min 25 timmersion=30min 20 150 120 timmersion=10min 90 timmersion=60min 60 timmersion=30min 15 30 10 0 10 12 Depth along of sample (cm) 10 12 Depth along of sample (cm) (a) (b) Figure 13: Evolution of water content and suction in function of depth of soil at different immersion time with hight density soil winitiale = 17%, ρ d = 1.55g/cm winitiale = 17%, ρ d = 1.55g/cm 35 50 timmersion=60min 40 timmersion=300min Suction (kPa) Water content (%) timmersion=10min 30 25 20 15 30 20 timmersion=10min timmersion=60min 10 timmersion=300min 10 12 Depth along of sample (cm) Depth along of sample (cm) 12 Figure 14: Evolution of water content and suction in function of depth of soil at different immersion time with low density soil 4.2 Influence of the compaction degree of saturation For a series of tests with similar parameters except the compaction degree of saturation, we note that, globally, the critical shear stress (Figure 16) increases with compaction degree of saturation while the erosion coefficient (Figure 15) decrease It means that the resistance to erosion increases with the compaction degree of saturation because increasing the compaction degree of saturation leads to change of soil fabric from aggregate structure to homogeneous structure and reduces the void ratio, which increases the resistance to erosion of soil (Lambe, 1958; Cui, 1993) These results are in agreement with the conclusion of former works (Wan and Fell, 2002, 2004; Lim, 2006; Lim and Khalili (2009); Regazzoni, 2009) However, we remark on from Figure 15 to Figure 16 that, at low degrees of saturation, the values of kD of loose samples are scattered because of water penetration during the tests 30 30 -05 -4.66 τc = 2x10 Sr 20 3.073 R = 0.39 τ c (Pa) R = 0.49 kD (cm /N/s) kD = 2.02x10 Sr 20 10 10 0 45 55 65 75 85 95 45 55 65 75 85 95 Sr (% ) Sr (% ) Figure 16: Relationship between critical shear stress (τc) and degree of saturation (Sr) Figure 15: Relationship between erosion coefficient (kD) and degree of saturation (Sr) 4.3 Relationship between kD and τc 40 30 kD (cm3/N/s) Based on the results of these tests, we have established a relationship between the erosion parameters There is an inverse relationship between the erosion coefficient and the critical shear stress (Figure 17), fitting these relationships with an exponential function yields good correlation coefficients R2, of 0.894 It shows that a loose soil which has a large erosion coefficient has a small critical shear stress and, on the other hand, a dense soil which has a small erosion coefficient has a large critical shear stress 20 kD = 83.23τc -1.31 R = 0.89 10 0 10 15 20 25 τc (Pa) Figure 17: Relationship between erosion coefficient and critical shear stress Conclusion This work presents the result of laboratory tests using an improved Jet Erosion Test device with an impinged jet These tests highlight the influence of the dry density on the erosion coefficient, the critical shear stress and the equilibrium scour depth On the other hand, the influence of the water content on the erosion parameters appears more complex The erosion coefficient and the equilibrium scour depth decrease with an increase in dry density whereas the critical shear stress increases with dry density The erosion coefficient and the equilibrium scour depth decrease when the water content increase while the critical shear stress increases with water content when the soil is dense (ρd ≥ 1.6 g/cm3) The effect is opposite when the soil is loose (ρd < 1.6 g/cm3) The erosion coefficient and the equilibrium scour depth decrease when the degree of saturation increase while the critical 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