On-Site Wastewater Treatment and Disposal Systems - Forced ...

high water table elevation. To prevent silt from entering the pipe
while the disturbed area is stabilizing, the tops of the joints or per-
forations should be covered with waterproof building paper or the pipe
jacketed with mesh. Natural soil material is used for the remainder of
the backfill (27).
The outlet must be protected from small animals. The outlet may be
covered with a porous material such as rock or gravel. Various com-
mercial outlet protection devices are also available (26).
b. Vertical Drains
Vertical drains are dug to the desired depth and width, and are back-
filled with a coarse, porous media such as coarse sand, l/4- to l/2-in
(0.6 to 1.3 cm) gravel, or similar material, to a level above the high
perched water table elevation. Natural soil materials are used for the
remainder of the backfill.
7.2.6.5 Maintenance
A well-designed and constructed drainage system requires little mainte-
nance. The outlets should be inspected routinely to see that free
drainage is maintained. If a silt trap is used, it should be inspected
annually to determine the need for cleaning.
7.2.7 Electra-Osmosis
7.2.7.1 Description
Electra-osmosis is a technique used to drain and stabilize slowly perme-
able soils during excavation. A direct current is passed through the
soil, which draws the free water in the soil pores to the cathode
(28). The water collects at the cathode and is pumped out. Steel well
points serve as cathodes, and steel rods driven between wells are used
as anodes. Common practice is to install the electrodes approximately
15 ft (4.6 m) apart, and apply a 30- to 180-volt potential. Current
flow is 20 to 30 amps (28).
Recently, a similar technique has been applied to onsite wastewater dis-
posal in soils with percolation rates slower than 60 min/in. (24
min/cm). A galvanic cell is constructed out of natural materials, which
requires no external power source. This cell is capable of generating a
268