Overview

EKG Geochemical Soil Improvement

Environmental Sustainability

After Abdullah and Al-Abadi, 2010)

EKG geochemical soil improvement

EKG with chemical treatment

The concept for the EKG chemical treatment is to use EKG materials to introduce chemicals via electroosmotic flow and ion migration to take part in cation exchange and cementation reactions on the clay mineral surfaces in order to modify the surface chemistry, change the plasticity of the clay, reduce the shrink swell behaviour and increase drained shear strength.

The use of chemical conditioners in the form of lime to improve soils including fine grained soils is an established technique but requires the mechanical mixing of the conditioning material with the ground.

The critical difference between conventional lime stabilisation and the EKG approach is that by the use of the electrokinetic phenomena of ion migration and electroosmotic flow, the introduction of the conditioning chemical species is achieved without mechanical disturbance to the ground (except of course for the introduction of discrete and separated boreholes, which represent a very small fraction of the overall volume of soil being treated).

The literature shows that this idea has attracted some recent attention and has come to be known as electroosmotic chemical treatment ECT.

The effects of electroosmotic treatment on plasticity of soils are known in the literature. Morris Hillis & Caldwell (1985) used a simple laboratory set up with soft silty clay and demonstrated substantial reductions in water content and liquidity index, particularly in the zones of the anodes. Static and dynamic triaxial tests indicated major improvements in static strength and resistance to cyclic loading of the soil. This work was conducted with seismic dynamic loading in mind, but the dynamic nature of seismic loading has similarities to the passage of trains over rails and roads with heavy traffic, therefore is of direct interest. Similarly Wade (1975) noted a reduced sensitivity to dynamic loading and susceptibility to liquefaction of clayey silt as a result of electroosmotic treatment.

Thomas and Lentz (1990) used a synthetic soil of 30% fine sand, 30% silt, 30% kaolinite and 10% montmorillonite (smectite). The shrinkage limit, plastic limit and liquid limit of this material were initially 14%, 15% and 39% respectively. After electroosmotic treatment, the shrinkage limit had risen to 16 – 22%; the plastic limit had risen to 19 – 23% and the liquid limit 39 – 43%. Overall the plasticity index showed a small reduction, falling in the range 19 – 24%. The authors noted:

“ the increase in SL [shrinkage limit] is believed to be a major benefit of electroosmosis. Little or no mention of this effect has been made in the literature. The increase is caused by a change in soil structure from dispersed to flocculated. By increasing the SL one has effectively reduced the shrink-swell potential of a soil”

Using repeated particle size distribution tests Pugh (2002) recorded an increase in the particle size distribution of London Clay after electroosmotic treatment. Such an effect increases the effective shear strength. Casagrande (1983) similarly demonstrated changes in grain size of quartz as a result as a result of electroosmotic treatment. And Wade (1975) noted a significant shortening of the fine grained tail of the particle size distribution curve in silt after electroosmotic treatment.

The soil between the electrodes can be further improved by the use of chemical conditioners, whose application is ideally facilitated by the design of EKGs.

Recent work using potassium hydroxide has identified the possibility for reductions in the plasticity of some clays. The basal silicate layer of certain clay materials contain holes in the crystal lattice with a diameter of 1.32Å. Potassium has a ionic radius of 1.33Å. By introducing a chemical conditioner of KCl, the K+ ions can bind strongly and permanently into the lattice holes with the effect of allowing adjacent layers to bind more strongly to each other. The effect is known as ion fixation. Smectite clays have a high fixation power, illites have a higher still power, kaolinites tend to fix ions less strongly. Abdullah and Al-Abadi (2010) recorded some exceptional results on clays treated with KOH (Table 1). Implementing ECT using a KCl conditioner, they noted changes in plasticity index from 40% reducing to 8% after treatment with KCl and 8 – 32% after treatment with KCl. More importantly the swelling potential was dramatically reduced.

Summary

From the literature it is clear that changes in plasticity, strength and stiffness of soil can be brought about by cation exchange associated with electroosmosis and that these effects can be enhanced to effect soil modification using chemical additives in conjunction with electroosmosis. For application to the different clay-rich soils, the correct conditioner would need to be identified and improvements quantified by laboratory testing prior to a full scale treatment.

The application of EKG and chemical treatment

EKG with chemical treatment is applicable in situations where consolidation of disturbed or soft soils is unlikely to be either possible or would yield the required strength improvements. An example would include deep seated failures in landslides or cases where a reduction in plasticity to reduce shrink – swell characteristics would be beneficial.