Chemical Improvement of Clay Soils in situ Using Electrokinetic Processes Researcher: Christina Liaki Supervisor: Professor CDF Rogers, Dr DI Boardman Sponsors: The University of Birmingham and Foundation Piling Ltd Introduction and Problem Statement Research has come up with many stabilising mechanisms both to strengthen the soil and to remove its sensitivity to water. Chemical stabilisation of clay soils is traditionally carried out using mix-in-place processes.In the case of lime and cement stabilisation of clay, these processes have been widely researched as surface and deep soil treatment techniques (Figure 1). For certain practical situations, such as house foundations on expansive soil, mixing is not possible. Current research has shown that chemical treatment using a potential difference to transport chemical ions between bespoke electrodes can overcome such problems, whilst achieving the desired results. However, a major problem this technique has to overcome is metallic electrode corrosion, because this can cause ions detrimental to the reaction process to be introduced into the clay-water-stabiliser system. Figure 1: Deep clay-reagent mixing (After This study aims to explore the science, and thereby 'prove the Bachy Environmental advertisement video of ICI Explosives site remediation concept' and establish the practical possibilities, of recreating project, 1995) the classical chemical modification and stabilisation reactions in situ using such a technique, while overcoming any problems caused by electrode degradation. Aims and Objectives The ultimate target of this research study is to create volume stability and/or strength gain in clay soils in situ using electrokinetic processes. In order to achieve this, it has been decided to: Evaluate the behaviour of various clay-stabilising ion systems when subjected to electric current. Determine how much the process improves the properties of the clay in both a chemical and a geotechnical sense. Determine the time-dependent nature of this improvement. Examine the associated changes in water content. Explore the effects of using different electrode types. Attempt to create a block of stabilised clay mid-way between the electrodes. Electrokinetic Stabilisation Electrokinetic stabilisation is essentially a combination of the processes of electroosmosis and chemical grouting. The electrokinetic processes reportedly cause a decrease in the water content and an acceleration of consolidation of the clay, an increase in the plastic limit, an increase in the shear strength of the clay and formation of insoluble salts in the clay. Along with the main effects of the electrokinetic method, there are some others which significantly affect the stabilisation procedure. These include the electrolysis of the water molecules and the reduction and oxidation reactions occurring at the electrodes. The electrolysis results in the formation of an acid front around the anode and a base front around the cathode. The oxidation of the anode also causes release of Fe3+ ions from steel electrodes, which, due to the applied electric current, travel towards the cathode. These ions react with the clay minerals and cause weakening of the soil. Case Study A research programme was performed to assess the reasons why settlement, indicated by cracks, had occurred underneath a garage structure on a site in Bath and the possibilities of performing a field trial. A trial pit was excavated, wherefrom samples were extracted. Testing was conducted to explore how the soil-water system would behave when mixed with Fe3+ ions released into the system due to the anode corrosion occurring. The results clearly showed that the Fe3+ ions weaken the soil. The main experimental programme involved the application of electrokinetic injection to soil samples extracted from the site. A representative model of the site (Figure 2) was created and it was attempted to change its water regime. Two hollow, perforated mild steel tubes were used as electrodes. Initially, it was decided to conduct the electrokinetic tests by providing the anode with water (Water Electric Current Test). A second type of experiment was then carried out in which the two electrodes were fed with appropriate chemical solutions that could potentially cause stabilisation of the soil sample (Chemical Electric Current Test). A constant direct current of a value of 5 Volts was used for all experiments, based on previous work done by Rogers et al (2002). Figure 2: Experimental apparatus used for the case study It has been demonstrated from this stage of the research that the application of electric current between two electrodes causes movement of the pore water in the soil from the anode to the cathode, as expected, and that this causes a related change in the shear strength of the clay. It has also been shown that the application of electric current with the addition of appropriate stabilisers at the electrodes has resulted in significant improvements in the strength characteristics of the clay, and potentially resistance to the effects of volume change. One method of determining the relative performance of the clay in terms of strengthening is to plot a graph of cone penetration, which is a direct indicator of shear strength, against water content for both untreated and treated clay samples. Figure 3 was obtained by combining the penetration and water content data. It illustrates that any points that lie significantly below the trend line for untreated clay represent clays that have been beneficially altered, whereas any points that lie significantly above it have been weakened and/or their performance has been otherwise compromised. It is interesting to note that where water alone was used, and thus the only chemical influence derived from anode degradation, a distinct weakening of the soil occurred. Figure 3: Relationship between penetration (i.e. shear strength) and water content (After Rogers et al, 2003) Future Work In the future, it will be assessed what happens when appropriately selected stabilisers react with different types of clay soils, both when mixed and when transported using electric currents, and the amount of stabilisers needed for these reactions to occur. Further investigation will be conducted to isolate the effects of such parameters as the iron release by replacing the steel electrodes with electrodes that are less susceptible to the redox reactions. Investigation will also take place to assess the optimum voltage needed for this process to be successful. The testing will be conducted with different curing periods in order to establish the time needed for the ion migration and the stabilisation reactions to occur. References Rogers, C.D.F., Barker, J.E., Boardman, D.I. and Peterson, J. 2002. Electrokinetic Stabilisation of a Silty Clay Soil. Proceedings of 4th International Conference on Ground Improvement Techniques. Kuala Lumpur, Malaysia, Vol. 2, pp. 621-628. Rogers, C.D.F., Liaki, C., and Boardman, D.I. 2003. Advances in the Engineering of Lime Stabilised Clay Soils. Keynote Paper, International Conference on Problematic Soils, Nottingham, UK.