Page 210 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Water
Soil Water 205

Figure 10.7
Electrophoresis
moves negatively
charged clay
particles toward a
positive electrode.

Figure 10.8
Setup for drainage
by electroosmosis.

As negatively charged clay particles move, positively charged ions move in the
opposite direction and take their water hulls with them. This is called
electroosmosis, and was put to practical use for draining soils in 1939 by Leo
Casagrande in Germany. Because electroosmosis moves water that is adsorbed in
the double layer, it can extract water that cannot be drained by gravity.

Electroosmosis can be accomplished in the field using a d.c. welding generator to
supply the voltage. Electrodes are spaced to give a maximum voltage gradient of
about 1 volt per 25 mm (1 volt per inch) to avoid loss of energy by heating; thus a
40 volt generator translates into an electrode spacing of 40 in. or 1 meter.
Perforated pipes are used as cathodes so cations and their associated water hulls
reaching a cathode are plated out, releasing their water. A schematic diagram is
shown in Fig. 10.8. Water flows up through the pipes to a manifold and is
conducted to an external drain. The anode must supply cations to the system, so
iron water pipe usually is used and must be replaced periodically.
Because electroosmosis mobilizes cations and water in the double layer it is more
effective than gravity drainage in fine-grained soils. However, because of its high
cost, electrical drainage is not used where well points or vacuum well points will
do the job.

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