Effect of test parameters on the results of the «Jet Erosion Test»

Transcription

1 ICSE Paris - August 7-, ICSE- Effect of test parameters on the results of the «Jet Erosion Test» Linh Quyen DAO, Hanène SOULI, Jean-Robert COURIVAUD, Jean-Jacques FRY, Patrick PINETTES, Jean-Marie FLEUREAU Laboratoire de mécanique des Sols, Structures et Matériaux, CNRS UMR 579, Ecole Centrale Paris, Grande voie des vignes, 995 Châtenay-Malabry, France daolinhquyen@gmail.com; jean-marie.fleureau@ecp.fr Laboratoire de Tribologie et Dynamique des Systèmes, CNRS UMR 55, Ecole Nationale d Ingénieurs de Saint Etienne, 5 rue Jean Parot, Saint Etienne Cedex - hanene.souli@enise.fr Electricité de France, Centre d Ingéniérie Hydraulique, 77 Le Bourget du Lac cedex jean-robert.courivaud@edf.fr; jean-jacques.fry@edf.fr GeophyConsult Allée du Lac de Garde, BP, 77 Le Bourget du Lac cedex pinettes@geophyconsult.com In order to gain a better insight into the role of some main parameters of the Jet Erosion Test with a submerged impinging jet, and their effect on the erosion parameters k D and τ c, a sensitivity study was funded by Electricité de France. The study was carried out on two soils: a natural clayey silt and a mixture of clay and sand. The results clearly highlight the importance of the test conditions such as the strength of the jet, the distance between the injection nozzle and the sample, the wetting time before the jet, etc. and show that these parameters must be carefully controlled to get reliable and reproducible values. Wetting tests carried out under the same conditions as the jet tests confirmed the link between wetting and erosion. Finally, the study leads to a better understanding of the influence of the various parameters on the results of the jet erosion test and may result in the future in some standardization of the conditions in which these tests are performed. Key words Erosion, Impinged jet, jet conditions, wetting I INTRODUCTION External and internal erosion of earthworks such as dams, dikes, etc. represent a major risk for these works [Courivaud et al., 9] that it is necessary to understand, quantify and prevent. Modelling of erosion phenomena is mainly based on the following equation [Pham, ]: ε = k ( τ τ ) () D e c where ε represents the mass (or volume) of soil eroded by water by unit of time and surface, k D the erosion coefficient, τ e the effective shear stress at the soil-water interface and τ c a threshold stress. Many questions remain unresolved concerning the use of this equation: - are k D and τ c reliable parameters for characterizing the «erodibility» of the soil independently from the type of test and from the test parameters (hydraulic head, flow )? - how to determine in a reliable way the effective shear stress? Presently, this determination is made on the basis of experimental results which were obtained under conditions different enough from those of the jet test (plane surface perpendicular to the jet) ; - how to relate the parameters k D and τ c to the usual geotechnical properties of a soil, i.e. how to predict from geotechnical data if a soil will be erodible or not? 57

2 ICSE Paris - August 7-, Following the pioneering work of Greg Hanson and USDA-ARS Hydraulic Laboratory, the Jet Erosion Test is increasingly used to characterize the erodibility of soils, due to its capacity to be performed both insitu and in the laboratory. In order to gain a better insight into the role of the main parameters, and their effect on the erosion parameters k D and τ c, in the case of an impinged jet, a sensitivity study was defined and funded by Electricité de France. The study was carried out on two soils: a natural clayey silt and a mixture of clay and sand. In both cases, several parameters were studied, either related to the test conditions (e.g. the strength of the jet, the conditions of wetting of the sample, the duration of the test) or to the specimen state (compaction density and water content, etc.). The jet tests were associated with independent wetting tests under the same conditions. II EXPERIMENTAL DEVICE The experimental device is derived from that of [Hanson and Cook, ]. One of the main objectives of the study was to allow several measurements to be carried out during the tests, and in particular to be able to measure the scour depth in different points of the specimen surface and not just in the center. To achieve this, the scour depth is measured by means of a displacement transducer (LDVT) which rotates around a fixed vertical axis, with a radius of about cm. An angular sensor is used to locate the position of the transducer, one of which corresponds to the point where the jet test is carried out. The injection cell is made of a Perspex tube, 5 cm in diameter and cm in height, with a nozzle at the lower end and an air drain at the upper end (Fig. ). Water arrives at mid-height of the column, from bottom to top, in order to eliminate more easily the air dragged by water or dissolved. The nozzle is made of a conical hole pierced in a plate of 5 mm thickness. Its smallest diameter is.5 mm and the angle of the cone is 5. This disposition permits to have a perfectly cylindrical jet as soon as it gets out of the orifice. Under the nozzle, the cell is equipped with a rotary deflector that allows us to interrupt the jet very quickly. The feed system is also equipped with floodgates, one manual at the entry of the column, the other electromagnetic. The Proctor (or CBR) mold containing the tested sample is placed in a reservoir made of perspex. The height of immersion is fixed by the tipping of water above the upper part of this reservoir. To vary the depth of immersion, the mold is placed on a variable height support. The cell is alimented in water by a constant level reservoir which can be placed at different heights. All the elements are linked together by large section hoses to minimize head losses. In addition to the transducers mentioned before, the column is equipped with a third one, a pressure transducer placed at the top of the column to measure the real head applied to the nozzle, and also to know precisely the periods of jet, as the beginning and stopping of the injection correspond to sharp changes in pressure. Finally, a peristaltic pump is used to take particles extracted from the specimen in order to measure their grain size distribution. All the data are recorded with an Agilent HP97A data logger at a frequency of Hz. An example of record is given in Fig., which shows the changes in pressure, scour depth and position of the transducer versus time. Moreover, a visual count-down on the screen of the computer help the operator open and close the rotary deflector Fig. : Photo of the device according to a given time schedule, so as to have fixed times for the jet and measurement operations. The small plateaus observed in the angle vs. time curve correspond to the scour depth measurements, visible in the depth vs. time plot, carried out at different points of the specimen. From these data, it is possible to derive the curve representing the increase in the scour depth under the jet (i.e. at the center of the specimen) (Fig. ), but also the profiles of scour depth along the (curved) measurement line at different times, as shown for instance in Fig. 5. The last curve is compared with the photo of the specimen taken after the test. 5

3 ICSE Paris - August 7-, Scour depth (cm) Scour depth at the center of the specimen Angle of the transducer ( ) Time (s) Time (s) Pressure (cm CE) Time (s) Fig. : Example of recorded data during the jet erosion test Cumulated jet time (s) Fig. : Scour depth versus cumulated jet time Scour depth (cm) Angle ( ) 9 9 5s 7s 9s 7s 7s s 57s 9s s s 7s Fig. 5: Profiles of erosion along the measurement line at different times 59

4 ICSE Paris - August 7-, III MATERIALS Two materials were used to perform the analysis : - a natural clayey silt, - a mixture of two laboratory soils containing 5 in weight of pure kaolin (P from Dousselin, Rhône) and 5 of Hostun sand (Hostun RF from Sibelco, Drôme). The properties of the two soils are summarized in Table and Fig.. D max μm D μm D μm < μm Natural clayey silt Mixture of 5 clay and 5 sand w L w P I P w OPN γ dopn kn/m, 7, 7, 5, 5,,9,7, Table : Properties of the two tested soils Natural clayey silt Sand-clay mixture Passing ().... Diameter D (mm) Fig. : Grain size distribution of the two tested soils The samples were compacted to different dry densities and water contents around the Standard Proctor Optimum, in layers, inside the Proctor mold. Homogeneity was verified. Then, the samples were turned, so as to expose the lower face to the jet. IV INFLUENCE OF TEST CONDITIONS To analyze the effect of the parameters, the results of several tests performed under various conditions will be presented either as the evolution of the scour depth versus jet time, as in Fig., or versus the studied parameter value. h represents the hydraulic head applied to the jet whereas h is the distance between the nozzle and the upper face of the specimen. h is the static head, i.e. the water height above the upper face of the specimen. In all the cases, comparison will be made between tests under similar conditions, except for the parameter being studied. Finally, the values of the erosion parameters k D and τ c were derived from the USDA-ARS method [Hanson and Cook, ] for each test. IV. Reproducibility of the results To test the reproducibility of the Jet Erosion Test, tests were carried out under the same conditions on similar samples of sand-clay mixture (with the same initial densities and water contents), as well as tests on the clayey silt. The results of the first ones are shown in Fig.. The curves are slightly different but the final values of scour depth vary from.5 to.77 cm, the parameter k D remains around cm /N/s and the main change is that of τ c which ranges between.5 and 5. Pa. Globally, the accuracy on k D and τ c is about and, respectively, if tests are carried out under the same experimental conditions.

5 ICSE Paris - August 7-, Time of jet (s) kd (cm /N/s) τc (Pa) Fig. : Results of tests performed under the same conditions in case of sand-clay mixture IV. Influence of the water head h applied to the jet In real case studies, the hydraulic head applied to the jet must be of the same order of magnitude that the head in the real work. However, in order to test the validity of the analysis and its robustness, it was chosen to test the influence of this parameter. Normally, whatever the head value (within reasonable limits), the theory should yield similar values of k D and τ c. Tests were carried out at values of head (h =, 9, cm), for different distances between the nozzle and the specimen (h = 5 cm and h = cm on one hand, h = cm and h = 5 cm on the other hand). As a first step, the flow of water through the nozzle was measured, so as to derive the real value of the hydraulic head, h _real, from equation () [Hanson and Cook, ]: v h = () _real g where V is the velocity of water at the jet nozzle. The results are shown in Table. The head loss increases with the applied water head but may stabilize for high velocities. The impact of the head loss on the parameters k D and τ c can be seen in Fig. 7. Considering the theoretical water head instead of the real one results in the value of k D being divided by.7 and that of τ c being multiplied by.. Theoretical water head h _th (m) Water velocity at nozzle (m/s) Real water head h _real (m) Head loss (m) Table : Comparison between theoretical and real applied water head 5 5 kd (real water head) 5 y =.7x R =.9 τc (real water head) 5 y =.x R = k D (theoretical water head) 5 5 τ c (theoretical water head ) Fig. 7: Effect of head loss on the values of k D and τ c

6 ICSE Paris - August 7-, h = 5 cm Distance between nozzle and sample: h = cm Head applied to the jet: h (cm) kd (cm /N/s) Labels: h th / h h = h + 5 cm /5cm 9/cm /5cm /cm 9/5cm /cm τc (Pa) Fig. : Influence of the hydraulic head h applied to the jet for different distances h between nozzle and specimen (sand-clay m.) Fig. shows the effect of the water head h (here, h _th ): the scour depth vary from. to.9 cm, whereas the parameters k D and τ c range from. to.5 cm /N/s and from. to.5 Pa, respectively. These values cannot be considered as constant. In all the cases, the rating of the soil erosion does not change (very erodible), but a problem arises when the eroded volume of a real dam versus time is to be predicted. IV. Influence of the distance h between the nozzle and the specimen Six tests were carried out with distances h between the nozzle and the upper face of the specimen for each of the head values h. The results, shown in Fig. 9, highlight the influence of this parameter, especially for the largest head, as the scour depth double when the distance decreases from to 5 cm. The impact on the parameters k D and τ c is also very noticeable: k D and τ c can be multiplied or divided by. In the same way as for the effect of the hydraulic head, the model should take into account the role of the force of the jet and give similar values of k D and τ c in all the tests, which is clearly not the case. h th = cm h th =-9 cm Distance between nozzle and sample: h (cm) kd (cm /N/s) Labels: h th / h h = h + 5 cm /5cm 9/cm /5cm 9/5cm /cm /cm τ c (Pa) Fig. 9: Influence of the distance h between the nozzle and the specimen for different values of the head h applied to the jet (sand-clay m.) IV. Influence of wetting time The time during which wetting is maintained before the beginning of the test may be important because a longer time will allow the saturation front to penetrate deeper in the specimen, resulting in a decrease in suction and in the capillary binding forces between the grains. Therefore, wetting could facilitate the separation of grains and thus, erosion. The results of tests on the clayey silt are shown in Fig.. A significant increase in the wetting time results in an increase in the scour depth (by a factor greater than ) and in the parameter k D (by a factor greater than ) and a decrease in τ c (by a factor around 5), which reflect the large loss of mechanical strength of the soil.

7 ICSE Paris - August 7-, Initial wetting time (min) kd (cm /N/s) long τc (Pa) short Fig. : Influence of the wetting time prior to jet test in the case of the clayey silt However, the same test carried out on the sand-clay mixture did not show the same phenomenon: in that case, there was practically no change when the wetting time was increased. These results can be explained by the infiltration curves of the soils, which consist in measuring the volume of water infiltrated in the specimen as a function of time (Fig. ). The kinetics of wetting is much quicker in the case of the clayey silt, and the saturated permeability of the sand-clay mixture is times lower than that of the silt. To predict the infiltration time, it is important to know the permeability of the unsaturated soil, which is the product of the saturated permeability by the relative permeability. The latter was derived from the wetting curves of the soil, derived from classical tests in which known suction is applied to a specimen and the characteristics of the soil (e.g. volumetric water content) are measured once the suction equilibrium is reached [Fleureau et al., 99]. Then the wetting curve was modeled using [van Genuchten, 9] model. Finally, the parameters of the model were injected in [Mualem, 97] model to obtain the relative permeability versus suction curve: n n n n r α θ θr K ( Θ) = Θ w Θ, where Θ = () θ θ s r θ is the volumetric water content and θ r, θ s, α and n are parameters derived from [van Genuchten, 9] model. Therefore, a much larger wetting time is necessary in the case of the mixture to observe a significant decrease in the strength of the specimen and lead to larger erosion. Injected volume (mm ) E+5 E+5 E+5 ρ d =.5 g/cm k sat =.. - m/s t inf = 7 min ρ d =. g/cm k sat =.. -9 m/s t inf = min IV.5 Interpretation E+ Time (min) Fig. : Comparison between wetting of sand-clay mixture and clayey silt Some new developments are required to improve the theory and explain the large effect of the test conditions. From a theoretical point of view, if k D increases with the hydraulic head h, k D is no longer a state parameter and the simple erosion law () is no more valid. From a practical point of view, it is recommended to perform the tests in the nearest conditions to those of the real work, and to control very carefully those conditions to get reliable values.

8 ICSE Paris - August 7-, Volumetric water content θ () 5 Clayey silt Sand-clay mixture. Suction u c (kpa) Relative permeability k r (u c )... Clayey silt Sand-clay mixture... Suction u c (kpa) Fig. : (left) Wetting curves of the two materials, starting from standard Proctor optimum; (right) relative permeability curves derived from the wetting curves using the van Genuchten-Mualem model V CONCLUSIONS The jet erosion test developed and used at Ecole Centrale Paris improves the control of test conditions. The most important finding is that the erosion parameters k D and τ c depend on the conditions of the test, such as the hydraulic head, the distance between the nozzle and the specimen, the time of wetting prior to the test, etc. This means that: Erosion coefficient does not appear to be a state parameter and the simple erosion law () is no more valid and may be an approximate of the true erosion law. the condition tests must be selected and controlled very precisely in order to be as close as possible from the site conditions. The accuracy of the measurements is jeopardize both by the variability of state of soils and by the difference of limit conditions between laboratory and site. VI NOMENCLATURE h : hydraulic head applied to the jet h : static head applied to the specimen h : distance between the nozzle and the upper face of the specimen VII REFERENCES AND CITATIONS Courivaud J.R., Fry J.J., Bonelli S., Benahmed N., Regazzoni P.L., Marot D. (9). - Measuring the erodibility of soil materials constituting earth embankments: a key input for dams and levees safety assessment. Hydro 9 conference, Lyon, - october. Fleureau J.-M., Kheirbek-Saoud S., Soemitro R. & Taibi S. (99). Behaviour of clayey soils on dryingwetting paths. Can. Geotechnical J. (): 7-9. Hanson G.J., Cook K.R. (). Apparatus, test procedures and analytical methods to measure soil erodibility in-situ. Applied Engineering in Agriculture, (): 55 Hanson G.J., Hunt S.L. (7). - Lessons Learned using Laboratory JET Method to Measure Soil Erodibility of Compacted Soils. Applied Engineering in Agriculture. (): 5-. Mualem Y. (97). A new model for predicting the hydraulic conductivity of unsaturated porous media. Water Resour. Res. : -5. Pham T.L. (). Erosion et dispersion des sols argileux par un fluide, PhD thesis, Ecole Nationale des Ponts et Chaussées, Paris, France. Van Genuchten M. (9). A closed form for predicting the hydraulic conductivity of unsaturated soils. Soil Sc. Am. Soc. : 9-9.