Case Study on Hydraulic Reclaimed Sludge Consolidation Using Electrokinetic Geosynthetics

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1 Case Study on Hydraulic Reclaimed Sludge Consolidation Using Electrokinetic Geosynthetics Y.F. Zhuang School of Civil Engineering, Wuhan University, Wuhan, , P.R.China ( Y. Huang, F. Liu, W. Zou & Z. Li School of Civil Engineering, Wuhan University, Wuhan, China ABSTRACT: A hydraulic reclaimed sludge of 19m long, 15m wide and 5.8m deep was treated using electro-osmosis technique. A novel Electrokinetic Geosynthetics (EKG) and automated electric power source was developed for the electro-osmotic consolidation. It took only 36 days to reduce water content from 62% to 36% and it would take 3 years for preloading consolidation to achieve the same effect. After the treatment, bearing capacity of the fluid-like sludge was increased to 70kPa. Besides EKG materials, design of electro-osmosis scheme is the key issue for the success of electro-osmotic consolidation. Issues of energy consumption and cost and challenges in large scale applications are discussed after presenting experience with the EKG. Keywords: Electro-osmosis; EKG; consolidation; reclamation 1 INTRODUCTION Increasing environmental concerns as well as land demands has led to a growth of hydraulic-filled areas. These areas are usually containment areas for dredged materials, sewage sludge or tailings. Hydraulicfilled areas are produced also from reclamation projects, which create lands from lake or sea. Land reclamation has played a significant role in many countries, such as Netherlands, Japan, Singapore, Korea and China (Hoeksema 2007; Yang 2003; Zhuang et al. 2012a; Zhuang et al. 2012b). Currently the most popular technique for consolidating hydraulic-filled areas is vacuum preloading. However, for hydraulic reclaimed sludge, vacuum preloading is too slow. Also for area of deep hydraulic reclaimed sludge (usually deeper than 4~5 m), the effect of vacuum preloading is limited. For this case, electro-osmotic consolidation can be an alternative option. Electro-osmosis is a phenomenon that moisture in the soil can be transferred from anode to cathode under DC field. This phenomenon was discovered very early by Reuss (1809). Electro-osmotic consolidation is much quicker than preloading method. Flow rate produced under electric field can be 100 to times greater than that under hydraulic gradient (Jones et al. 2008). People see significant potential for electro-osmosis and there are continued studies and trials since the discovery of electro-osmosis. However, electro-osmosis is still not a popular method in the market of soft ground treatment. This is because of high cost, difficulty of management in large scale application and complexity of soil which makes design of electro-osmotic consolidation difficult. Case study on electro-osmotic consolidation of hydraulic reclaimed sludge in this paper will present a novel electrokinetic geosynthetics (EKG), automated electric power source, experience of in-situ application of electro-osmotic consolidation as well as challenges in large scale application for further research. 2 EKG MATERIAL Concept of EKG was presented by Jones et al. in 1996 (Nettleton et al. 1998). The concept of EKG defined a new type of geosynthetics that can act as both electrode and drainage channel in the electroosmotic consolidation. Using this concept, Dr. Zhuang started to develop the EKG product since 2000

2 (Zhuang et al. 2004) and the first generation is shown in Figure 1 (Zhuang et al. 2006). The idea was to use electric conductive plastic as major material so that to solve the electrode corrosion problem for electro-osmotic consolidation. This EKG sample was made from electric conductive plastic rods stitching on non-woven geotextile and its resistivity was m. The problem of this EKG sample lies in two aspects: one is difficulty for mass production and the second is that the electric conductivity is not good enough. Analysis showed that the electric conductivity of EKG should be higher than m (Zhuang 2005). Further investigation and trials showed that the required conductivity was quite difficult to achieve in the polymer industry. The difficulty was that when electric conductivity increased, the physical properties such as strength and flexibility decreased. In the following years, with the help from geosynthetics industry from Jiangsu Province of China, a novel EKG was developed and it can be mass-produced in a factory. The EKG material developed is shown in Figure 2. The EKG looks like classical PVD (Prefabricated Vertical Drain), with the exception that it is made from conductive polymer with a conductivity of m -1. The EKG is 100 mm wide and 0.8 mm thick. Every groove on the both sides of EKG is 3 mm wide bound by thin webs 0.8 mm thick. As shown in Figure 2 there are two bumps of 6 mm wide and 2.5 mm thick on both sides of the EKG. Two copper wires of 1 mm are embedded inside the bumps. The embedded wires are for better distribution of electric current into the soil and for convenience of wiring (Zhuang et al. 2012c). Figure 1. EKG sample in 2004 Figure 2. EKG product patented in AUTOMATED ELECTRIC POWER SOURCE In contrast to the classical theory for electro-osmotic consolidation proposed by Esrig in 1968, which states that flow rate of electro-osmosis is a coupled result of voltage gradient and hydraulic gradient. Modern electro-osmosis theory suggests that electro-osmosis flow rate depends mostly on the mobility of ions in the soil. According to Esrig s theory, intermittent electro-osmosis can help for the dissipation of negative pore water pressure in the soil, so that it can improve the effect of electro-osmotic consolidation. However, many experiments have shown that this is not the case. Intermittence increases the electric current, but it will decrease in a short time. From the point of view of ion mobility in the soil, this is a process of rebalance of electric field in the soil and it has nothing to do with the variation of hydraulic gradient. If there are ions (such as Fe cations, like Fe 2+ or Fe 3+, which are easily trapped in the soil) trapped and accumulated somewhere in the soil, the increased current by intermittence helps very little for the effect of electro-osmotic consolidation(zhuang et al. 2012a). Another technique that may improve the effect of electro-osmotic consolidation is polar reversion. Again, it is not because of dissipation of negative pore water pressure but because of better ph condition created by neutralization due to polar reversion, which maintain higher mobility of ions in the soil and for a longer time as well. These facts indicate that electric field has to be adjusted as needed in a more flexible manner both in direction and intensity during electro-osmotic consolidation. For this purpose, a novel automated electric power source was developed (see Figure 3 and Figure 4). The power source is able to work under constant-current mode and constant-voltage mode. It can be manually or electronically controlled. The open programming interface allows people to customize their program so that to adjust the electric field as they want during the electro-osmosis process, including switching between constant-current and constantvoltage mode, polar reversion quickly (once per second for maxima) and alteration of electric field intensity.

3 Figure 3. Automated DC power source Figure 4. Control panel of program for power source 4 FIELD APPLICATION 4.1 Site Profile There was a 19m 15m hydraulic reclaimed area to be treated by electro-osmotic consolidation. The area was filled with 5.8 m thick of dredged pool sludge. The properties of sludge before electro-osmotic consolidation are shown in Table 1. Table 1. Properties of sludge before electro-osmotic consolidation Specific gravity Water content / % Dry density Permeability / g/cm 3 / cm/s Liquid limit / % Plastic limit / % Treatment EKG electrodes were square arrayed at a space of 1m. The treatment included two stages separated by 16 days of intermittence. Stage 1: Electro-osmotic consolidation began under a constant-current mode of 290 A and lasted for hours (~10 days). Then it was switched to a constant-voltage mode of 50 V and lasted for hours (~ 1 day). Stage 2: After the intermittence, electro-osmotic consolidation continued under a constant voltage mode of 80 V. It lasted for hours (~ 9 days). The treatment was controlled by computer software. Electrode polar was reversed according to the trend of electric current variation. Generally, stage 1 had longer time of current in reverse direction, while stage 2 had longer time of current in forward direction. Purpose of this scheme for electro-osmosis was to maintain as much as possible mobile cations in the soil. 4.3 Results After electro-osmotic dewatering treatment, soil was sampled from the field for measurement of properties in the lab. Test result showed that the water content of soil decreased from average 62% to 36% and the minimal water content after electro-osmotic dewatering according to borehole was 24%. Distribution of water content after electro-osmotic dewatering along middle cross-section from the West to the East and from the North to the South is shown in Figure 5 and Figure 6, respectively. Water content was relatively high to the East and low to the South. This is due to the boundary conditions. There was a pond at the East, while there was a road at the South. Unconsolidated-undrained shear strength of the soil increased from 0 to 25kPa; the soft ground was improved from a fluid-like status to a bearing capacity of 70kPa. The average energy consumption for this treatment was 5.6kwh/m 3

4 Figure 5. Distribution of water content after electroosmotic dewatering along middle cross-section from the West to the East (within borehole range) Figure 6. Distribution of water content after electro-osmotic dewatering along middle cross-section from the North to the South (within borehole range) 5 DISCUSSION 5.1 Consolidation Effect As a comparison, the preloading method was analyzed to produce the same consolidation effect as EKG. The soil had a coefficient of consolidation C V =0.0029cm 2 /s and a compression index C C = In order to achieve the same effect of consolidation (to reduce water content from 62% to 36%), there should be 132 kpa of preloading (around 6~7 m high soil surcharge); and it would take 1139 days, which is more than 3 years, to achieve 90% of consolidation. However, electro-osmotic consolidation took only 36 days, including 16 days of intermittence. For vacuum consolidation, theoretic limit of bearing capacity is 1 atm (~100kPa) and practical limit is around 80kPa considering the vacuum loss. Therefore, practically vacuum consolidation can achieve bearing capacity of 50~60kPa in 3~6 months for hydraulic reclaimed sludge. Comparisons above shows that electro-osmotic consolidation is much quicker and can achieve better consolidation effect. 5.2 Cost This case study shows that if the electro-osmosis scheme is properly design, the energy consumption is fairly acceptable. It is not higher than the energy consumption for vacuum consolidation. The high cost is due to the price of EKG product. The EKG product adopted in the field application is 10 times more expensive than traditional PVD. 5.3 Challenges in large scale application Consolidation for reclamation is often for large scale with area of tens of thousands square meters. This poses challenges in the following aspects Electric Power and Cable When the area increases, the electric power increases dramatically. Although the total energy consumption of electro-osmotic consolidation is similar to that of vacuum consolidation, the difference is that electric current for electro-osmosis at the beginning is much higher. Therefore, design of electro-osmosis scheme is the key issue. If the electric power source and cables are designed according to the maximal electric current that can be achieved, the power source and the cables would be unreasonably large; and it is not practical both in size and cost. High electric current at the beginning is uneconomical and it is also unsafe for the electric circuit. The maximal current a single EKG electrode can bear is 6~7A; higher current will result in burning of EKG. A properly designed electro-osmosis scheme includes current intensity, electro-osmosis and intermittence duration, polar reversion frequency, distribution and organization of power sources. For large scale application, the electro-osmosis scheme shall be controlled mainly by computer program.

5 5.3.2 Bearing Capacity and Cost In land reclamation project, there are usually two stages of treatment. The first stage is vacuum consolidation to improve fluid-like sludge to a bearing capacity above 50kPa so that it can support construction equipment on the ground. The second stage is further improvement of the soft ground to the designated bearing capacity according to the construction requirement. If electro-osmotic consolidation improves bearing capacity to 70kPa, the secondary treatment is still necessary for most of construction requirements. Although electro-osmotic consolidation is quicker and the effect is better than that of vacuum consolidation, the cost of treatment is almost 10 times higher than that of vacuum consolidation. Electro-osmotic consolidation can compete with vacuum consolidation only if the price of EKG decreases or effect of consolidation can be further improve so that the secondary treatment can be omitted Design of Electro-osmotic Consolidation Design of electro-osmotic consolidation is complicated. It is not like preloading consolidation that can be design under the frame of Terzaghi s consolidation theory. Electro-osmosis scheme is the key issue of design, but it varies case to case due to the complexity of soil. Theory of electro-osmosis is evolving. Compared with Esrig s theory, new theories such as electric charge accumulation theory (Zhuang & Wang 2005) and energy level gradient theory (Zhuang et al. 2011) are able to predict variation of electric current and to estimate energy consumption and distribution in the soil. New theory is in progress but the design relies still significantly on experience. 6 CONCLUSIONS Case study leads to following conclusions: (1) Electro-osmotic consolidation using the novel EKG improved the hydraulic reclaimed sludge to a bearing capacity of 70kPa. It took only 36 days to reduce water content from 62% to 36% and it would take 3 years for preloading consolidation to achieve the same effect for this specific case. (2) Electro-osmosis scheme is the key issue for the design of electro-osmotic consolidation. The scheme includes current intensity, electro-osmosis and intermittence duration, polar reversion frequency, distribution and organization of power sources. Therefore, for large scale application of electro-osmotic consolidation automated DC power source is necessary and the power source should be able to be controlled by customized program. (3) Average energy consumption for this electro-osmotic consolidation was 5.6kwh/m 3. It is not higher than the energy consumption for vacuum consolidation. However, the price of EKG is 10 times higher than that of PVD. EKG can compete with PVD in consolidation projects only if the price of EKG decreases or effect of consolidation further improved so that the secondary treatment of the ground is not necessary after electro-osmotic consolidation. ACKNOWLEDGEMENTS This work was supported by research grants from the National Natural Science Foundation of China (NSFC Grant No ) and the Ministry of Education of China (Ph.D. Programs Foundation of Ministry of Education of China Grant No ). REFERENCES Esrig, M.I Pore Pressure, Consolidation and Electrokinetics. Journal of the SMFD, ASCE, 94(SM4), p Hoeksema, R.J Three Stages in the History of Land Reclamation in the Netherlands. Irrigation and Drainage, Issue 56, p Jones, C.J.F.P., Lamont-Black, J., Glendenning, S., Bergado, D., Eng, T., Fourie, A., Liming, H., Pugh, C., Romantshuk, M., Simpanen, S., & Zhuang Y.F Recent Research and Applications in the Use of Electro-Kinetic Geosynthetics. Fourth European Geosynthetics Conference (CD-ROM), International Geosynthetics Society UK Chapter, United Kingdom. Nettleton, I.M., Jones, C.J.F.P., Clark, B.G. & Hamir, R Electro Kinetic Geosynthetics and Their Applications. 6th International Conference on Geotextiles Geomembranes and Related Products, Industrial Fabrics Association International, Georgia USA, p

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