EKG, in the form of vertical wick drains, has been used by Electrokinetic to consolidate super soft clay. A large hydraulic test vessel was constructed and filled to a depth of 2.4m with several tonnes of kaolin slurry, and EKG was installed as shown below. The clay was left to settle for 100 hours, during this time 50 litres of water was expelled, equivalent to hydraulic consolidation of 20mm. Electroosmosis was then conducted for 500 hours and resulted in a reduction in volume of approximately 930 litres resulting in a moisture content reduction to 62%. The resultant increase in shear strength of the material was from less than 1kPa to about 15 - 35kPa (depending on depth). Water removal and settlement of the surface was most pronounced in the first 150 hours of electroosmotic treatment.

Dewatering of super-soft kaolin clay (Lamont-Black, 2001; Jones et al., 2002).

Water flows by electroosmosis from the anode to the cathode thus the area around the anode experiences the greatest reduction in moisture content and improvement in shear strength. In order to minimise moisture content anisotropy, the trial was completed with a phase of polarity reversal in order to draw water away from the electrodes which were acting as anodes in the first phase. Polarity reversal resulted in a much more even distribution of shear strength in the test soil.

Consolidation - normal and reverse polarity (1.5m depth)

• The bottom axis above represents distance along the length of the test pit
• The grey background of the graph represents a cross section of the soil
• The blue vertical lines represent cathodes and the red line represents the anodes at the start i.e. during normal polarity. These swapped over during reverse polarity
• Plots of undrained shear strength, on different dates, measured at 1.5m depth along the long axis of the test pit

It can be seen that as treatment proceeded the shear strength improved from 2kPa to 34kPa adjacent to the anodes after 24 days of normal polarity treatment. During this time the material adjacent to the cathodes improved to approximately 22kPa within the treatment zone. Outside the treatment zone, normal hydraulic consolidation had improved the strength of the soil to only 6kPa.

At the end of the normal polarity phase, a distinct difference was seen in the strengths between the anode and cathode areas. Polarity reversal was used to ‘smooth out’ these differences. This resulted in a flatter curve. Part of this process also improved the shear strength of the soil immediately ‘beyond’ the treatment ( i.e. in the non electroosmotic zone ) from approximately 6kPa to between 10 and 15kPa.

This test shows the potential for the use of EKG for ground improvement by consolidating materials such as alluvial clays and silts in low lying or flood plain areas.

 

applications > civil engineering > consolidation