If the soil is heavy clay or has tight formation, yet shows some
porosity from percolation tests, the efficiency of the field may be
increased by placing below the tile lines 12 to 15 inches of additional
filter material (washed gravel, crushed stone, slag, clean cinders, or
clean bank-run gravel 3/4 to 2-1/2 inches in size). When the surface
soil is tight and is underlain by porous soil, sufficient drainage
is sometimes obtained for the smaller installations by omitting the
tile field and providing a dry well at the end of the effluent sewer,
provided the water table will not be contaminated. Larger systems under
such soil conditions should have a tile field, and absorption can be
increased by boring 6- or 8-inch holes down to the porous stratum and
filling them with gravel or sand; the holes should be 4 to 6 feet
apart. Another and perhaps the best practice is to excavate the tile
trenches 4 to 6 feet and install a lower tile line, as shown in figure
10. This latter method is especially desirable if the upper tight
stratum is especially thick, or if there is no porous lower stratum, or
if in irrigated regions and where the disposal field is limited in area.
Where the underdrain tile is not used, the absorption capacity of the
field can be increased by providing a rock-filled trench across the
lower end of the tiles for the full width of the field. The depth
should be not less than 5 feet and the width not less than 3 feet.
On account of the beneficial action of bacteria in the upper soil
layers it is highly desirable to confine the effluent near the surface
rather than to use underdrains. Purification becomes slower and less
effective, the deeper the drains.
In situations where the soil contains considerable moisture or is even
saturated, the field may be improved by partially encircling it with a
tile line laid to serve as a drain. Such a line should be on the high
side and have surface outlets for removing the water from the soil. It
should not be laid so close to a disposal tile line that it will drain
the sewage effluent from the disposal field onto the surface of the
ground.
[Illustration: Slope of disposal tile 2 to 4 inches per 100 feet. Slope
of underdrain tile not less than above.
Plug upper end of underdrain tile lines, lower end to discharge into
rock-filled seepage pit or into other approved outlet.