Large scale soft ground consolidation using electrokinetic geosynthetics
Introduction
With stricter regulation for sludge disposal and increasing of land demands, demands of hydraulically filled ground consolidation increase as well. Hydraulically filled areas are generated from disposal of dredged materials, sewage sludge, slurry for tunnelling and piling, tailings, etc.; Hydraulically filled areas come also from reclamation project. There was a report in 2015 declared that due to improvement of reclamation technique and increase of land demands, it became cheaper to build land on sea compared to acquiring existing waterfront land in the past decade and inflation is not the reason (Malherbe 2015). Land reclamation has played a significant role in many countries, such as Netherlands, Japan, Singapore, Korea and China (Hoeksema 2007; Yang 2003). In China, reclamation by hydraulic-fill has been going on around coastal cities like Xiamen, Tianjin, Wenzhou, Guangzhou, etc. For example, reclamation area for the Hong Kong-Zhuhai-Macau Bridge project reached up to 350ha (Liu et al., 2019; Li et al., 2020a; Li et al., 2020b; Yu and Song 2019; He et al., 2019).
It is very likely that hydraulically filled ground contains fine grains with very low hydraulic permeability. Ground with fine-grained deposits and high water content is very soft and it takes long time to consolidate under hydraulic gradient provided by preloading or vacuum, which are commonly applied in soft ground consolidation using PVD. Consolidation of soft ground from reclamation using PVD is a big market all over the world. It was reported that usage of PVD in China reached up to 30 billion RMB (~5 billion USD) per year (Zhuang 2015).
Soft soil improvement using PVD has been extensively studied for more than two decades and study continues with innovations in recent years. For example, booster PVD presented a technique with air injection to improve the effect of vacuum consolidation with PVD (Cai et al., 2018; Anda et al., 2020); thermo PVD presented a technique of producing heat using electricity and flexible wire heater to improve vacuum consolidation (Bergado et al., 2020); drain-to-drain vacuum consolidation presented a membraneless system for preloading and vacuum consolidation (López-Acosta et al., 2019). Besides these innovation in technique, there were also advances in mechanism research and theoretical research, e.g. research on mechanism of clogging phenomenon surrounding PVD (Xu et al., 2020; Wang et al., 2020), lateral deformation under vacuum consolidation (Wang et al., 2019), deformation after vacuum removal (Ni et al., 2019), prediction of long term performance of PVD (Kumarage and Gnanendran 2019).
Among these advances, electro-osmosis is an efficient way to accelerate the consolidation as well. It was reported by Mitchell that electro-osmotic permeability is insensitive to particle size while hydraulic permeability decreases rapidly with decreasing particle size. Therefore, electro-osmosis is effective in fine-grained soils, which are difficult to consolidate under hydraulic gradient (Mitchell and Soga 2005).
Electro-osmosis is a phenomenon that water in fine-grained soil can migrate from anode to cathode under DC electric field. This phenomenon was firstly reported by Ferdinand Friedrich Reuss in an unpublished lecture entitled “notice on a new hitherto unknown effect of galvanic electricity” to the Physico-Medical Society of Moscow in November 1807. The first publication on electro-osmosis was in 1809 by Reuss in French, whose tile translated in English is “notice on a new effect of galvanic electricity” (Reuss 1809). For fine-grained deposits, consolidation by electro-osmosis can be 100 to 10,000 times faster than that under hydraulic gradient (Jones et al., 2008). Dewatering and consolidation with electro-osmosis are very remarkable in bench-scale experiments; soil can be quickly consolidated to a very high extent with low water content and high shear strength if the scale of model test is in tens of centimetres along three dimensions. There were also some field applications of electro-osmosis since Casagrande in 1983. However, application of electro-osmosis in large scale consolidation of soft ground remains some difficulties to be tackled.
In 80–90s last century, field trials of electro-osmotic consolidation indicated that the difficulties were electrode corrosion and high energy consumption. In the past decade, there were a lot of progresses in this area and those two difficulties seemed to be solved. Development of Electro-Kinetic Geosynthetics (EKG) solved the problem of electrode corrosion. EKG presents a new category of geosynthetics. Concept of EKG was presented by Jones et al., in 1996 (Nettleton et al., 1998); and ability of mass production of EKG was achieved recently in China (Zhuang et al., 2012b). EKG is made from electrically conductive polymer, which is corrosion proof during electro-osmosis. General requirements for EKG products are that electric resistively should be under 10−3 Ω·m and durability of corrosion proof should be higher than 3 months. These requirements are now basic quality control of electrically conductive polymer materials for producing EKG (Zhuang 2016a). Electrically conductive polymer can be extruded into different shapes as desired for different application purposes. There are currently two types of EKG products available in China for soft ground consolidation, which are E-board and E-tube. E-board looks similar to normal PVD and only that E-board is electrically conductive. E-tube is perforated tube made from electrically conductive polymer.
First field trial in China with EKG showed that energy consumption to complete the electro-osmotic consolidation was 5.6 kW·h/m3 (Zhuang et al., 2014). This value is acceptable and it is not as high as reported in some early literatures, which were tens of kW·h/m3. The reasons of high energy consumption may lie in two aspects: one was passivation of metal electrodes because of corrosion, which led to poor conductivity of electrodes; the other was unreasonable strategy of DC field input, which led to inefficient energy utilization during electro-osmotic consolidation (Zhuang 2015). Innovation of EKG materials solved the first problem, namely there is no corrosion and electrode passivation during electro-osmosis. The second problem is related to the design of DC power source, which is going to be presented in this paper in detail. In the field test presented in this paper we can see that energy consumption was less than 1 kW h/m3.
Section snippets
DC power source
Electro-osmosis requires direct current and for safety reason only low voltage can be applied into field. Field trials showed that DC voltage input into ground should not exceed 80 V for safety reason (Zhuang et al., 2014; Zhuang et al., 2018). Therefore, DC power source is needed to convert alternating current with high voltage to direct current with low voltage. Design of smart DC power source is currently greatest challenge for large scale application of electro-osmotic consolidation.
Site profile and soil properties of the field test
The experiment site was located in a hydraulically filled area of 294.52 ha in Taizhou city in China. The hydraulically filled sludge was 5~8 m deep and there was inhomogeneous soft soil underneath with maximal depth of 29 m. The soft soil was composed of sludge, clay and silty clay.
The area for the field test was 100 m long and 10 m wide. The area was divided into 5 sections labeled as K1~K5. Different treatment of the sections is listed in Table 1. The surrounding area of the field test site
Field test design
EKG used in the field test is shown in Fig. 2. Installation of EKG was similar to installation of PVD in vacuum consolidation. EKG electrodes were designed to be installed with 1 m spacing. Experiment design focused on estimation of electric current, consolidation time and ground settlement.
Field test
As designed, electro-osmotic consolidation was carried out alternately in turns for 3 sections: K2 and K3 was merged as section 1; K4 and K5 were section 2 and section 3, respectively. Rolling frequency was an issue stilled to be studied. Theoretically, it could be determined by looking at how long would the water migration last after current input stopped. However, it was difficult to observe water migration in the field test. The other indicator could be used was discharging current
Electric current and power
Output of power from the DC power source during electro-osmosis is shown in Fig. 7. Although there was interval on a regular basis for every of the three sections during electro-osmosis, the DC power source was running on a continual output status under the control of roll polling program. It is shown in Fig. 7 that the maximal power used in the field test was about 6 kW. However, the DC power source had to be designed with power capacity of 80 V/400 A, which was 32 kW. This was because the
Energy consumption
The electro-osmotic consolidation lasted for 54 days and the total energy consumption was 3397.69 kW·h. The volume of soil to be treated by electro-osmosis was 800 m2 × 7.75 m, so the energy to volume ratio was 0.55 kW·h/m3. As comparison, energy consumption used to be 5–10 kW·h/m3, so there was great improvement in energy saving. This was due to the roll polling program of the DC power source. The roll polling program improved energy efficiency by following ways: 1) made use of interval time
Conclusions
Field test in this paper shows progresses on electro-osmotic consolidation including design of DC power source, design method for field application of electro-osmosis and energy saving by using roll polling program. Major conclusions are:
- 1)
There are two strategies to build DC power source, one is building a large DC power source with high power; the other is building cluster of DC power sources with small power. The latter one is adopted in the field test.
- 2)
Roll polling program is an essential
Acknowledgments
This work was supported by research grants from the National Natural Science Foundation of China (NSFC Grant No. 41472039 and 51109168). This research was also supported by Huadong Engineering Corporation Ltd. and Hangzhou Shenyuan Environmental Sci-Tech Co. Ltd.
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