USE OF ELECTRICAL TECHNIQUES FOR ACCELERATING

Transcription

1 USE OF ELECTRICAL TECHNIQUES FOR ACCELERATING THE CONSOLIDATION OF CLAYEY SOILS Nogueira Santos, João Alves IST, Technical University of Lisbon, Portugal ABSTRACT The aim of this study is to investigate the use of electrical methods to accelerate the consolidation of clayey soils. The electrical characteristics of clays cause the attraction of ions and polar molecules to their surface. This phenomenon explains the low permeability in clayey materials, which make the consolidation processes too slow and the materials very compressible. Several studies began with Reuss in 1809, continued by Casagrande and nowadays by Jones and Glendinning showed the efficiency of a soft clayey treatment process called electroosmosis. This process uses the application of an electrical field to a soil mass to increase the velocity of water molecules percolation and consequent hydrodynamic consolidation of the soil. The present work is based on the results of some oedometric tests where conventional conditions were adopted and where an electrical field was applied in a controlled manner to a mass of soil. Both one-dimensional and radial drainage conditions are studied, each considering the cases with and without electrical treatment. The aim of the tests is to show how much time electroosmosis can save when compared with the traditional methods. Esrig theory, developed in 1968, is used in the interpretation of the results and significant differences are expected when the times found for consolidation using electrical field application are compared with those needed to consolidate without using electrical current. It is also found that an increase in the voltage applied to the soil will result in accelerating the draining process, and this will also be accelerated if the applied current has varying direction. Keywords: kaolin, electrokinesis, electroosmosis and acceleration of the consolidation 1. INTRODUCTION When an electrical flow is applied to a saturated soil it results into ions displacement inside the soil mass. The positive ions (cations) are moved into the negative electrode (cathode) and the negative ions (anions) into the positive electrode (anode). The pore water is carried with the ions flow in a viscous manner. If the two opposite kind of ions are in the same proportion on pore fluid and both have the same size, no water movement will occur. However, once the clays have a negatively charged surface, their tendency is attract positive ions and immobilize them on the double layer to neutralize the electrical forces. Thus, a movement of cations will occur, which will carry the pore water in the same direction. This phenomenon is called electroosmosis, and is used to drain saturated soils and therefore to improve their mechanical properties. Electroosmosis can be studied has if it was a case of pore pressure increment, which cannot cause undrained failure in the soil. Esrig (1968) studied the different types of pore pressures that can result from this treatment as function of the drainage conditions and reached to three kinds of drainage conditions: through cathode and anode (both open), though cathode (anode closed) and the opposite, cathode closed and only through anode. The only condition viable in practice 1

2 corresponds to the second case. More authors studied deeply this subject and nowadays the applications where this technique is adopted are increasing. 2. CONSOLIDATION BY ELECTROOSMOSIS The first studies were focused on unidimensional consolidation, where the studies from Esrig (1968) are included, for the case where water drainage is done only through the cathode. The distribution of the pore water pressure in this case, in the beginning of the electroosmosis, is shown in Figure 1, where is the electroosmotic conductivity coefficient, measured when water percolates only due to electroosmotic effects, is the coefficient of hydraulic permeability and is the voltage. On that is shown the proportionality between the voltage in the soil and the pore water pressure developed. As can be seen, the pressure created on the anode is negative. The pressure at any point is proportional to the voltage at that point. EQ. 1 Eq. 2 comes from introducing Eq. 1 in Darcy s equation and both in the equation that governs the one-dimensional consolidation (Terzaghi et al., 1996). In this equation, is the compressibility of the soil and is the coefficient of consolidation. EQ. 2 The pressure developed during the electroosmosis is given by Eq. 3. ( ) ( ) ( ) [ ( ) ] [ ( ) ] EQ. 3 where is the maximum voltage applied to the soil and is the time factor (Eq. 4) depending on the distance between the electrodes and the time, given by EQ. 4 A solution for the Eq. 3 can be obtained on Mitchell et al. (2005). The average degree of consolidation is given by ( ) ( ) [ ( ) ] EQ. 5 A solution for this equation is shown in Figure 2. FIGURE 1 DISTRIBUTION OF THE VOLTAGE AND THE PORE PRESSURE (FROM MITCHELL ET AL., 2005) A counterflow is generated by this mechanism leading the water in the opposite way of the electroosmotic flow. The balance of those two flows will determine if there is or not water movement. However, considering the movement of water out of soil, and consequent reduction of the void ratio, electroosmosis, the balance of the two referred flows is equal to given by Eq. 1. FIGURE 2 - AVERAGE DEGREE OF CONSOLIDATION VERSUS DIMENSIONLESS TIME FOR ONE-DIMENSIONAL CONSOLIDATION BY ELECTROOSMOSIS (FROM MITCHELL ET AL., 2005) 2

3 Considering now the case where radial drainage is applied, the variation of voltage with the radial distance to the electrode during the electroosmosis is shown in Figure 3. FIGURE 3 - ASSUMED VARIATION OF VOLTAGE WITH DISTANCE DURING ELECTROOSMOSIS FOR RADIAL FLOW: RW IS THE DRAIN RADIUS AND RE IS THE DRAINAGE CYLINDER RADIUS (FROM MITCHELL ET AL., 2005) Equation 6 governs the consolidation by radial flow ( ) EQ. 6 The solution for this equation was obtained by finite differences for the ratio 20 and is presented in Figure 4. equal to 5, 10 and FIGURE 4 - AVERAGE DEGREE OF CONSOLIDATION AS A FUNCTION OF DIMENSIONLESS TIME FOR RADIAL CONSOLIDATION BY ELECTROOSMOSIS (FROM MITCHELL ET AL., 2005) 3. EXPERIMENTAL STUDIES 3.1. MATERIAL AND EQUIPMENT The material used in the tests is a commercial white kaolin (w L=75%, IP=40%). Reconstituted specimens were prepared with water content equal to 1,5 w L, normally consolidated with a maximum stress of 12 kpa. The electrical resistivity of the saturated soil for different water contents (and therefore void ratios) was also measured in order to confirm that this parameter does not changes significantly during the performance of the oedometer tests. Further details can be found in Nogueira Santos (2012). Few tests were performed first to ensure that the oedometer equipment was isolated from the electrical system. This motivated the modification of the trimming ring on the oedometer cell. The new ring is shown in Figure 5. and is made in nylon, which is an electrical insulator material. The results showed that the use of a nylon ring instead of one made of steel did not introduces a significant difference. The curves in this figure are the average degree of consolidation of the soil surrounding the drain as function of the time factor, where is the coefficient of radial consolidation, is the time and is the diameter of the drainage cylinder. 3

4 FIGURE 5 MODIFIED TRIMMING RING Two different types of tests were performed, where several different cases were tested. The specimens of the first type were tested in a normal oedometer cell adapted to apply an electrical field to the soil. Tests were performed with and without the application of electrical DC voltage and two different voltages were tested: 6.35V and 9V. A Wykeham Farrance oedometric cell was used, modified to include four silver electrodes (square plates) in the top and bottom porous stones, as shown in Figure 6. FIGURE 7 APPARATUS FOR THE RADIAL FLOW TRIALS FIGURE 8 ELECTRODES AND DRAINS FOR THE RADIAL FLOW TRIALS FIGURE 6 SILVER ELECTRODES ON THE POROUS STONE Two electrical power supplies were used. A commercial 9V battery cell was adopted to apply the electrical flow to the soil as well as a modified mobile phone battery charger, as shown in Figure 9. This source has a voltage of 6,39V and an intensity of 0,71A. The specimens studied in the second type were tested in a new consolidation cell developed to include vertical drains to force radial flow instead of vertical flow. This cell is made of acrylic and is shown in Figure 7. The top load plate of the cell was drilled to allow the inclusion of the drains and the settlement of the soil without interference. A geosynthetic material was placed between the specimen and the load plate to enable drainage from the top. The drains introduced allowed drainage only or drainage with the application of an electrical field. Medical needles were used to apply the electric current to the soil. The needle s cases were used to create the drain, by punching them and filling them with sand. On the Figure 8 is shown the electrodes and the drains used. FIGURE 9 - MODIFIED MOBILE PHONE BATTERY CHARGER 3.2. TESTS PERFORMED Several oedometric tests were performed in order to determine the influence of the electric current in soil consolidation speed. Besides the reference test (Tests EC1 and EC2) where there was no electrical current application, the cases with 4

5 electrical current allowed to study the influence of increasing voltage (EO1 with DC voltage 9V and EO3 with DC voltage 6.35V) and the influence of applying a reversible current flow (EO2 with DC voltage 6.35V). The stress path adopted was the same in all cases, consisting in increasing the vertical stress each 24h, as follows: 12kPa-25kPa-50kPa-100kPa-200kPa-400kPa- 800kPa-1600kPa-400kPa-12kPa. A fourth test (EO4) was performed where each increment of vertical stress was applied for very short periods of time (4h), aiming to reproduce the same vertical settlement observed when there was no current applied. Most of the tests were performed using the battery charger and the nylon ring. A complementary study was performed in an experimental apparatus developed for this purpose. The equipment (acrylic cell with 12cm diameter and 5cm high), described by Nogueira Santos (2012), allows radial consolidation using vertical drains. Two radial flow tests were performed. The first (EOR) was performed with the application of electrical field. The second (ER) was performed without applying the electric treatment to the soil. However the equipment broke during this test and therefore it wasn t completed. FIGURE 10 EFFECTIVE STRESS VS VOID RATIO CURVE FOR EC2 AND EO3 The main differences between the tests are in the values of the coefficient of consolidation shown in Figure 11. As shown in this figure, this coefficient does not depend on the applied tension and is higher for the EC3 tests where the electroosmotic pressure was applied. This proves that the current accelerates the consolidation process EXPERIMENTAL RESULTS The influence of electroosmosis in consolidation was studied by comparing the results of tests EC2 with those of EO3 (6.35V). Figure 10 shows the plot effective stress vs void ratio, where it can be seen that the electrical treatment applied for 24h slightly increases the magnitude of the settlements, however the compressibility characteristics of the soil are not much affected (without treatment: C c=0,541 and C s=0,146; with V=6.35V: C c=0,670 and C s=0,160). FIGURE 11 COEFFICIENT OF CONSOLIDATION AS FUNCTION OF THE APPLIED STRESS FOR EC2 AND EO3 The comparison of the results found in tests EO3 and EO1 allows understanding the influence of the applied voltage. The void ratio as function of the applied stress is shown in Figure 12 (with V=9V: C c=1,135 and C s=0,133; with V=6.35V: C c=0,670 and C s=0,160). As can be seen, the use of a higher voltage for the same period of time (24h) increases the magnitude of the settlements. 5

6 FIGURE 12 - EFFECTIVE STRESS VS VOID RATIO CURVE FOR EO3 AND EO1 FIGURE 14 - EFFECTIVE STRESS VS VOID RATIO CURVE FOR EO3 AND EO2 The comparison of the coefficients of consolidation measured in both tests is shown in Figure 13. The values are very similar for both tests, which indicate that the increment of voltage had no significant effect in the time of consolidation. This may be due to the fact that the two voltages are very similar. Adding to this, the two electrical fields compared are high. The comparison of the coefficients of consolidation measured in both tests shown in Figure 15 allows concluding that the reversibility of the electrical current slightly increases the coefficient of consolidation. This can be explained by a more homogeneous distribution of electrical charges in the soil. FIGURE 13 COEFFICIENT OF CONSOLIDATION AS FUNCTION OF THE APPLIED STRESS FOR EO3 AND EO1 The influence of reversible current flow was studied comparing the results of tests EO3 and EO2. The compressibility curves are shown in Figure 14 (with V=6.35V: C c=0,670 and C s=0,160; with reversible V=6.35V: 0,632 and 0,116). As it can be seen, the application of a reversible current flow increases the magnitude of the settlements. FIGURE 15 COEFFICIENT OF CONSOLIDATION AS FUNCTION OF THE APPLIED STRESS FOR EO3 AND EO2 The time necessary to reach, in tests EO3, the values of the settlements measured in EC2 after 24h were about 4h for each loading increment. In other words, 4h was the time needed in each load step of EO3 to achieve the same settlement measured in EC2. A new test was performed, EO4, where each load step was 4 hours. The comparison between the results of the tests is presented in Figure 16. This figure shows that test EO4 has similar behavior to the other tests, as expected. 6

7 FIGURE 16 EFFECTIVE STRESS VS VOID RATIO CURVE FOR EC2 AND EO4 FIGURE 18 - COEFFICIENT OF CONSOLIDATION AS FUNCTION OF THE APPLIED STRESS FOR EOR AND ER Finally, a similar study was made for the radial flow tests EOR and ER. The compressibility curves shown in Figure are incomplete because of the equipment breakage during test ER. FIGURE 19 - COEFFICIENT OF HORIZONTAL CONSOLIDATION AS FUNCTION OF THE APPLIED STRESS FOR EOR AND ER FIGURE 17 - EFFECTIVE STRESS VS VOID RATIO CURVE FOR EOR AND ER There is no significant difference in the values of the coefficient of consolidation in the vertical direction shown in Figure 18. This result leads to the conclusion that the electroosmosis had no significant influence on the vertical consolidation process on those trials. However, the values of the coefficient of consolidation in the radial direction shown in Figure 19 (higher values found for test EOR) prove that the electroosmosis increases the speed of consolidation. Moreover, the main mechanism of water drainage is this test is through the radial drains, as expected. Electrodes corrosion and the formation of an oxide were detected during the electroosmotic one-dimensional tests, as well as the formation of gas bubbles. Figure 20 shows some photographs of the gas formation (a) and the electrode corrosion (b). (a) (b) FIGURE 20 (A) GAS FORMATION AND (B) ELECTRODE CORROSION The silver oxide produced in test EO3 is shown in Figure 21. 7

8 FIGURE 21 OXIDE ON THE TOP OF THE SPECIMEN (EO3) Only electrodes corrosion was observed in the electroosmotic radial flow tests ELECTRONIC MICROSCOPE OBSERVATIONS There were performed some electronic microscope observations, in order to analyze the material structure and identify any possible change caused by the electric field applied. The material after being subjected to the same vertical stress is analyzed for the following cases: in Figure 22 the material without treatment, in Figure 23 the white part of the specimen in test EO3 (6.35V) and in Figure 24 the dark part (oxide deposit) of the specimen in test EO3 (6.35V). No significant differences were observed. FIGURE 24 - TREATED KAOLIN DARK PART 4. MODELING CONSOLIDATION GAIN WHEN USING DRAINAGE AND ELECTROOSMOSIS A very simple model simulating the settlements caused by the consolidation of the foundation of an embankment was developed. The cases without drainage, with preloading, with vertical electroosmotic treatment, with vertical drains and with drains incorporating electrodes to apply electroosmosis were studied. The main purpose of the cases studied was to compute the time necessary for the complete settlements to occur, and therefore to evaluate the efficiency of the treatments. Figure 25 presents the model studied. It was assumed that the drainage of the saturated clay layer of the foundation (5m deep) was done only through the top. The embankment, with very large area, applied 48kPa vertical stress. A thin layer of a fill material was considered in the contact between the embankment and the foundation soil to ensure drainage. FIGURE 22 RECONSTITUTED MATERIAL FIGURE 25 MODEL CONSIDERED (WITHOUT ANY DRAINAGE) FIGURE 23 TREATED KAOLIN WHITE PART For the case when there were no drains, the values obtained on EC2 test were adopted and time necessary for consolidation is 6.1 years. This 8

9 period is reduced to 2.3 years in case of executing a pre-load embankment with twice the high of the embankment. If there is the possibility to execute horizontal electrodes parallel to the embankment foundation, for example using recent techniques described in Nogueira Santos (2012), an electrical field similar to the one adopted in tests EO could be applied. In this case, assuming that the power in the construction site is similar to the one adopted in the laboratory tests, the consolidation time reduces to 1.1 years. The delayed response of the first case has the ratio of. Finally, considering the presence of the vertical drains without any electrical current, the values from test ER were used assuming that the drains in the field were placed to ensure the same drainage conditions. In this case, the consolidation times in the vertical and radial directions are respectively And Considering that and [ ] [ ([ ] )] EQ. 7 EQ. 8 EQ. 9 ( )( ) EQ. 10 ( )( ) the period of is computed. It is a very important reduction in time. If the electrical current is now included in the drains the values form test EOR can be used. In this case the consolidation times in the vertical and radial directions are respectively and Which results in ( )( ) ( )( ) EQ. 11 EQ. 12 EQ. 13 of the period of time is. As expected, this time reduces when electrosomosis is included, in the ratio. The comparison between the cases using drains with and without voltage show that the inclusion of electrical current brings no evident earning in the time saved. Nevertheless, the use of electrical current significantly increase costs, and for this reason its adoption in practical cases must be evaluated considering total daily cost of delays in construction. 5. CONCLUSIONS The studies presented confirm that electroosmosis accelerates the consolidation of clayey soils because the consolidation coefficient c v increases at least one order of magnitude. This is a very significant improvement. The studies where the electrical field was applied for 24h indicated that the settlement obtained can be larger than if there was no electrical current. For this reason the duration in time of the treatment must be controlled Regarding the value of voltage applied to the soil, higher voltages can result in larger settlements if they are applied the same period of time. It can be deduced that the time during which voltage is applied can be reduced for higher voltages. The small difference between the voltages applied in the tests, 9V and 6.35 V, did not allowed more contrasting results. Considering the reversibility of current, the gains are both in the settlement as in time. These findings may be related to the efficiency of the electrodes, which were further explored and by the alternated drainage of cathode and anode. Regarding the results of the tests with radial consolidation, the influence of electroosmosis is reflected mainly in the parameters related to the radial (horizontal) percolation. Considering the overlap between radial (horizontal) and vertical directions, taken as a hypothesis for Esrig (1968), the inclusion of radial direction has the most important role. The visualization of the structure using electron scanning microscope was inconclusive considering structural changes caused by the application of electrical current. Combining these 9

10 observations with test values obtained, can be assumed that electroosmosis only acts as an accelerator in this void reduction process without changing the material. The formation of gas due to water hydrolysis, corrosion of electrodes and formation of oxides in the sample should be investigated because their presence may interfere with the main processes of consolidation. To conclude, electroosmosis is a method which has high potential to be used to accelerate the rate of settlements, and therefore the strength earn in soft clayey soils. In spite of the energy and power consumption costs this is a relatively simple technique. Other limitations appear to be the corrosion of the electrodes and the occurrence of parallel phenomena (such as hydrolysis, electrophoresis, ion migration, among others), which are unknowns of this process. It is believed that the development and use of this technique in the future is more dependent on technology than on applicability. ACKNOWLEDGMENTS The author acknowledges to Professor Rafaela Cardoso the comments on the work presented, as well as the help provided by the laboratory Technician José Reis in the preparation of the experimental tests. REFERENCES Esrig, M Pore pressures, consolidation, and electrokinetics. Journal of the Soil Mechanics and Foundation Division. 1968, Vol. 94. Mitchell, J. and Soga, K Fundamentals of Soil Behavior, Third Edition: John Wiley and Sons, Nogueira Santos, J. (2012). Study on the use of electrical techniques for accelerating the consolidation of clayey soils. MSc Thesis, Instituto Superior Técnico, Universidade Técnica de Lisboa (in Portuguese). Terzaghi, K., Peck, R.B. and Mesri, G Soil Mechanics in Engineering Practice, Third Edition. : John Wiley and Sons, Inc.,