Experimental investigation on electro-osmotic treatment combined with vacuum preloading for marine clay
Introduction
Marine clay is formed in coastal regions and exhibits characteristic properties, including low bearing capacity, large compressibility, and high water content. With the rapid economic and social development, the demand for building land increases and the coastal regions have been used for building construction. Thus, mechanical properties of marine clay should be improved for the land demands. Various methods and techniques have been developed by geotechnical engineers to consolidate soft soils and improve their mechanical properties: (i) dynamic drainage consolidation (Chu et al., 2006; Lefebvre and LeBoeuf, 1987), (ii) grouting (Bayesteh and Sabermahani, 2018), (iii) vacuum preloading (Chu et al., 2000; Indraratna et al., 2012b; Lei et al., 2019; Shang et al., 1998; Shibata et al., 2014), (iv) surcharge preloading (Liu, 2015), and (v) electro-osmotic consolidation (Jeyakanthan and Gnanendran, 2013; Joshua and Kara, 2020; Otsuki et al., 2007; Wu and Hu, 2014; Yuan and Hicks, 2016). According to a survey report of soil depth in coastal regions of eastern China (Dong, 2017), the depth of soft clay is thicker than 10 m in the backshore area. The traditional treatment methods would require a large amount of time for soil improvement, significantly increasing the cost and reducing the consolidation efficiency. Research studies (Wang and Vu, 2010; Wang et al., 2018) indicated that the VE and EO are the effective techniques to strengthen marine clay given soil characteristics and site conditions.
In the VP technique, pressure difference is utilized as a surcharge to drive pore water to vertical drains, resulting in the reduced pore water pressure and the increased effective stress to improve soil strength (Nguyen et al., 2021). The effectiveness of VP in improving mechanical properties of various soils has been investigated (Artidteang et al., 2011; Indraratna et al., 2012a). Chu et al. (2000) completed an experimental study that improved the soft clay for an oil storage station using VP. The results showed that the shear strength of the treated soil was 2–3 times higher than that of the untreated soil. Indraratna et al. (2012b) carried out numerical investigations of soft ground improvement and proposed a design procedure for soft ground enhancement using VP with vertical drains. Sun et al. (2018) performed pilot tests at a land reclamation site to improve the dredged marine clay using VP combined VPSL. The proposed method in the study efficiently improved the dredged clay with a uniform soil strength profile. Ding et al. (2019) conducted a case study on Yangtze River floodplain using the vacuum preloading combined with surcharge preloading. The results showed that the thick muddy-silty clay was effectively improved with the increased shear strength using the combined method. It was noted that several discrepancies exist in the VP method, such as the inadequate improvement for the deep soil, vacuum loss near the vertical drains, and difficulties in maintaining effective vacuum pressure (Chu et al., 2000; Indraratna et al., 2012b; Shang et al., 1998; Shibata et al., 2014). These problems limit the application and utilization of VP in the engineering projects.
The feasibility of EO for the soil improvement has also been proved (Lefebvre and Burnotte, 2002; Micic et al., 2001; Peng et al., 2015; Wu et al., 2016), in which the pore water flows from the anode towards the cathode when subjected to the electric current in the soil. As the water is removed from the consolidation system, the soil properties are improved with decreased water content and increased shear strength. Research has been completed that uses the EO to improve marine clay (Rittirong et al., 2008; Wu and Hu, 2014; Xue et al., 2015). Kaniraj and Yee (2011) performed laboratory tests on the organic soil that was strengthened using EO combined with chemical stabilization. The use of EVDs with the reduced pumping interval could improve the efficiency of the EO for organic soil. Flora et al. (2017) utilized EO to strengthen the fine-grained dredged sediment, with the increased soil yielding strength. Hu et al. (2019) experimentally investigated the effects of soil pH and ionic species on the EO for the kaolin. These studies also indicated that the application of EO was limited due to excessive energy consumption and severe electrode erosion, which need to be addressed for improving its efficiency (Zhang, 2018; Zhang et al., 2017).
A few methods were reported to explore the combined utilization to solve their disadvantages (Chang et al., 2010; Chien et al., 2015; Huang et al., 2021; Liu et al., 2020b) and further improve the consolidation efficiency for marine clay. Peng et al. (2015) developed a combined method of electro-osmotic grouting with vacuum drainage at the cathode and conducted experimental test to examine the efficiency in improving the soil properties. Zhang et al. (2017) carried out experimental tests and filed studies that examined EO combined with chemical grouting for marine clay. This study indicated that the electro-osmotic chemical treatment is a useful technique for clay improvement and the efficiency was influenced by the concentration of the salt solution. Liu et al. (2014) developed a new technique that combined EO, VP, and surcharge preloading for the ground improvement. Zhang and Hu (2019) proposed EO and VP using electrokinetic geosynthetics, to improve the kaolinite properties and demonstrated its ability to increase the water removal efficiency. As suggested in these studies, the combined method is a promising technique to strengthen marine clay and solve various geotechnical issues for the soft foundation (Chien et al., 2015; Liu et al., 2020a).
This study developed a combined method of vacuum preloading with electro-osmotic consolidation to improve marine clay. A prefabricated device was designed to efficiently combine the EO and VP and facilitate the combined method for the construction practices. Laboratory tests were conducted to evaluate behavior of the treated soil and investigate the effectiveness of the combined method for marine clay. The properties of the treated soil in the terms of water removal efficiency, shear strength, and bearing capacity were examined to provide an optimal combination for marine clay.
Section snippets
Soil properties
The marine clay with black-gray color which had low bearing capacities was collected at an average depth of 2 m from the Xinyang Port, Yancheng, Jiangsu Province, China, and used in the laboratory tests. Before laboratory tests, physical tests were conducted to measure mechanical properties of the soil samples. The material properties, as listed in Table 1, were the results of disturbed soil samples. The soil parameters indicated that the collected soil was the silty clay with high organic
Water discharge and discharge rate
The weight of the removed water was measured to calculate the discharge rate for three tests, and Fig. 4 shows the water discharge time histories in the three tests. The water discharge rate is the amount of the removed water for every hour and calculate using Eq. (1).where v is the water discharge rate; m1 and m2 are the weight of the removed water at time t1 and t2, respectively; and ρw is the water mass density. Fig. 5 illustrates the water discharge rate time histories for
Water content
The water content of the treated soil was measured at the depths of 5 cm, 15 cm, and 25 cm, as shown in Fig. 3. Fig. 9 illustrates water contents at the selected points for three tests. The water content of the untreated soil for three testes reached approximately up to 26.8%. After the tests, the water contents of the tested soils significantly reduced, with the lowest water content of 17.1% using the VP-I-ECM. In test 1, the water content of the treated soil slightly increased as the soil
Conclusions
This study conducted laboratory tests on vacuum preloading combined with electro-osmotic consolidation to improve mechanical properties of marine clay. The results were analyzed to investigate the effectiveness of the combined method in consolidating marine clay. In this study, three techniques were examined to identify an optimal consolidation method for marine clay improvement. The conclusions were obtained:
- 1.
The combined consolidation method fully utilized advantageous characteristics of VP
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This project was supported by Natural Science Foundation of Jiangsu Province (Grant No. BK20200996), China Postdoctoral Science Foundation (Grant No. 2020M681566), National Natural Science Foundation of China (Grant No. 51978317), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 20KJB560033), Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant No. KYCX19_1703), and Excellent Scientific and Technological Innovation Team of
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