The disposal tile should not be more than 18 to 24 inches below the
surface, and where the ground-water level rises to the bottom of the
trench special underdrains, described on page 16, are necessary.
Special provisions must also be made where tight soils are encountered.
These methods are described in the section entitled "Disposal methods
in tight or wet soils."
The table in figure 9, together with the information given in table 1,
below, may be used for estimating the number of tiles needed in any
particular soil type. If there is any doubt about this requirement, a
percolation test should be made in the disposal field, as follows:
Dig a hole 1-foot square and to the depth at which the tile is to be
laid. This depth in most instances will be about 24 inches and should
not exceed 36 inches. Fill the hole with water to a depth of 6 inches
and observe the time required for the water to seep away; divide by 6
to get the average time for the water to fall 1 inch. The test should
be repeated at three or four different points in the disposal field
and the average time noted for all tests used. The data in table 1 can
then be used to determine the number of tiles needed. Where 1 hour is
required for the water to fall 1 inch the soil is totally unsuitable,
and another site should be selected. Soil conditions at the time of
the test may vary from year-round average conditions, and this factor
must be taken into account. If the soil appears exceptionally dry,
greater depths of water may be used or the test repeated in the same
hole. In no case should tests be made in filled or frozen ground. Where
fissured rock formations are encountered, advice should be sought from
sanitation specialists.
Table 1.--_Determining tile-disposal field requirements from
percolation tests_[6]
------------+----------------------++------------+---------------------
Minutes | Effective absorption|| Minutes | Effective absorption
required | area required, per || required | area required, per
for water to| person, in bottom ||for water to| person, in bottom
fall 1 inch | of disposal trenches ||fall 1 inch | of disposal trenches
------------+----------------------++------------+---------------------
| _Square feet_ || | _Square feet_
| || |
2 or less | 26 || 10 | 52
| 30 || 15 | 63
| 36 || 30 | 90
| 40 || 60[7] | 120
------------+----------------------++------------+---------------------
[6] A minimum of 150 square feet should be provided, equal to 100 feet
of 18-inch trench.
[7] If more than 60 minutes, use special design with seepage pits or
sand-filter trenches.
Figure 9 suggests methods of arranging the tiles in disposal fields
under varying conditions and the length of tiles needed.
[Illustration: Figure 9.--Arrangements for tile-disposal fields, method
of laying tile, and length of tiles needed.]
SIZE AND MINIMUM SPACING REQUIREMENTS
FOR DISPOSAL TRENCHES
+---------+----------+----------------------+-------------+
| TRENCH | TRENCH | EFFECTIVE ABSORPTION | TILE LINES |
| WIDTH-W | DEPTH-D | AREA IN SQUARE FEET | SPACING-S |
|IN INCHES| IN INCHES| PER LINEAL FOOT | IN FEET |
+---------+----------+----------------------+-------------+
| 18 | 18 to 30 | 1.5 | 6.0 |
| 24 | 18 to 30 | 2.0 | 6.0 |
| 30 | 18 to 36 | 2.5 | 7.5 |
| 36 | 24 to 36 | 3.0 | 9.0 |
+---------+----------+----------------------+-------------+
Wider spacing of the lines desirable where available area permits