Electrolysis: Desaturation as Mitigation Technique Against Liquefaction

It is thus important to consider liquefaction potential of dams or embankments to prevent this kind of phenomena. Various techniques have been developed during the last centuries, such as: densification; draining; soil reinforcement and desaturation. One of the most promising techniques against liquefaction is desaturation. As well known when the degree of saturation (Sr) increases, the resistance to liquefaction increases. Desaturation seems to be a useful remediation against liquefaction, especially in cyclic liquefaction as shown by several research works (Chaney, 1978; Yoshimi et al., 1989; Okamura and Soga, 2006; Mele et al., 2018). In fact, a small decrease in the degree of saturation of a fully saturated sand can result in a significant increase in shear strength against liquefaction. Recently, new interpretations of liquefaction phenomena in unsaturated conditions have been provided by Mele et al., 2019 using an energetic approach, which is able to simulate the resistance to liquefaction of unsaturated sandy soils as reported by Mele and Flora (2019). Nevertheless, few researches have been performed to study how desaturate the soils and to apply in situ this effective mitigation technique. One of the most interesting techniques could be an induced desaturation by means of electrokinetic phenomena. Desaturation can be reached by introducing small amounts of gas through electrolysis. By lowering the degree of saturation from full saturation to about 90%, volumetric strain and pore pressure generation can be reduced by several times.

Desaturation Of Soil Deposits Through Electrolysis

When an electric field is applied to the soil for some time through electrodes, electrokinetic phenomena are generated (electroosmosis, electromigration and electrophoresis). In particular, pore water is transported by electroosmosis through the porous medium and, at the same time, electrolysis of pore water occurs near the electrodes. This leads to the formation of OH- ions at the cathode and H+ ions at the anode, which are transported by the flow and by the electric field, and there is therefore a change of pH, that is not homogeneous in the porous medium. It means that electrolysis may be used to entrap gas molecules in saturated specimen. Electrokinetic treatments are increasingly being used in geotechnical and geoenvironmental engineering for site remediation and dewatering of clays (Casagrande 1949; Esrig 1968; Flora et al. 2016; 2017; Gargano et al. 2018; 2019; 2020). Recently, Thevanayagam and Jia (2003) have explored the use of electrokinetics to grout soils for liquefaction mitigation applications.

While the gas quantities produced by electrolysis are not high enough to produce any safety hazard (especially H2 gas), they are significant enough to change the degree of saturation. Furthermore, the electrolysis process can generate, at least under laboratory conditions, a controlled amount of gases without disturbing the specimen (Bayat et al., 2009). Although the effectiveness of electrolysis to desaturate sandy soils has been investigated in laboratory (Yegian et al., 2007) new tests can be useful to verify the effectiveness and the applicability in a bigger scale, realizing, for example, field trials in liquefiable areas.

Applicazione

There have been limited studies investigating the practical considerations for field implementations and the factors affecting the process. In particular, some technical problems could hamper its application at the industrial scale. These problems include the electrode corrosion and high power consumption (Raats et al., 2002). Process parameters such as the timing of the electric field application, total energy input and energy input distribution in time will affect the dewatering results and overall the energy efficiency.

Different operating conditions are possible, such as the use of inert electrodes like graphite and “pressed carbon-coated” electrodes, in order to reduce the corrosion; or the use of intermittent current for the reduction of power consumption and electrode corrosion. Compared to conventional remediation technologies, EK this desaturation technique has several advantages, such as being less expensive (cost-effective), being applicable both in-situ and ex-situ, rapid installation and easy to operate (simplicity), having silent operation, having the advantage of not disturbing the site activities, and relatively short treatment duration. It is also worth noting that electrolysis, apart from desaturation, could be extremely important also to remove polluting substances.

Conclusions

Liquefaction is one of the most critical issues for tailing dams in all parts of the world as demonstrated by several case histories. Sometimes, traditional mitigation techniques (such as densification or soil reinforcement) seem not to be effectiveness or easy to apply. Owing to that, new technologies have been investigated and desaturation is considered one of the most promising techniques. To desaturate soil deposits electrolysis may be used. Even though it is studied in small scale, demonstrating its effectiveness, new studies have to be performed to investigate its application in field.