Electroosmosis

(1) An electroosmotic dewatering system consists of anodes (positive electrodes, usually a pipe or rod) and cathodes (negative electrodes, usually wellpoints or small wells installed with a surrounding filter), across which a d-c voltage is applied. The depth of the electrodes should be at least 5 feet below the bottom of the slope to be stabilized. The spacing and arrange­ment of the electrodes may vary, depending on the dimensions of the slope to be stabilized and the voltage available at the site. Cathode spacings of 25 to 40 feet have been used, with the anodes installed midway be­tween the cathodes. Electrical gradients of 1.5- to 4- volts-per-foot distance between electrodes have been successful in electroosmotic stabilization. The electri­cal gradient should be less than about 15 volts per foot of distance between electrodes for long-term installa­tions to prevent loss in efficiency due to heating the ground. Applied voltages of 30 to 100 volts are usually satisfactory; a low voltage is usually sufficient if the groundwater has a high mineral content.

(2) The discharge of a cathode wellpoint may be estimated from the equation

Qe = keieaz (4-Ю)

where

ke= coefficient of electroosmotic permeability

(assume 0.98 xlO-4 feet per second per volt per foot)

ie= electrical gradient between electrodes, volts per foot

a = effective spacing of wellpoints, feet

z = depth of soil being stabilized, feet Current requirements commonly range between 15 and 30 amperes per well, and power requirements are generally high. However, regardless of the expense of installation and operation of an electroosmotic de­watering system, it may be the only effective means of dewatering and stabilizing certain silts, clayey silts, and clayey silty sands. Electroosmosis may not be applicable to saline soils because of high current re­quirements, nor to organic soils because of environ­mentally objectionable effluents, which may be un­sightly and have exceptionally higbpH values.