several tests to determine the best location and average
conditions should be made. From test results, the rate of
sewage application to the total bottom area of the tiled
trenches may be taken from the data below. Soil testing
over 30 minutes is not suitable.
Time for water to fall 1
Allowable rate of sewage
application in gallons per
square foot per day, bottom
of trench in tile field
TRENCH WIDTH.The minimum width of a
trench on the basis of the types of soil is as follows:
Sand and sandy loam, 1 foot
Loam and sand and clay mixture, 2 feet
Clay with some gravel, 3 feet
FROST LINE.Placing tile below the frost line
to prevent freezing is not necessary. Tile placed 18
inches below the ground surface operated successfully
in New England for many years. Subsurface tile should
never be laid below groundwater level.
PIPE SIZE.Design and construction should
provide for handling and storage of some solid
material, eliminating, as much as practical, the
opportunity for clogging near pipe joints. Pipe 4 to 6
inches in diameter is recommended. The larger pipe
gives greater storage capacity for solids and a larger
area at the joint for solids to escape into the surrounding
LAYING THE PIPE.To provide for free
discharge of solids from the line to the filter trench, lay
the pipe with 3/8-inch clear openings. The top of the
space is covered with tar paper or similar material to
prevent entry of gravel. Bell-and-spigot pipe is laid to
true line and grade easily. Good practice calls for
breaking away two thirds along the bottom of the bells
at the joint and rising small wood-block spacers. The
pipe is commonly laid at a slope of about 0.5 foot per
100 feet when taking the discharge directly from the
septic tank and 0.3 foot per 100 feet when a dosing tank
is used ahead of the field.
BEDS.The tile is laid on a bed of coarse-
screened gravel at 6 inches deep with 3 inches of
coarse gravel around and over the pipe. Coarse-
screened stone passing 2 1/2-inch mesh and
retained on a 3/4-inch mesh is recommended. This
gravel bed gives a relatively large percentage of
voids into which the solids pass and collect before
the effective leaching area becomes seriously
clogged. The soil that fills the trench must not fill
the voids in the coarse-screened gravel around the
pipe. A 3-inch layer of medium-screened gravel
over the coarse stone and 3 inches of either fine-
screened gravel or suitable bank-run gravel over
the medium stone is recommended.
LAYOUT.The layout of the tile in the field
should be designed carefully. Generally, the length of
laterals should NOT exceed 100 feet. When tile is laid
in sloping ground, distribute the flow so each lateral
gets a fair portion. Flow must be prevented from
discharging down the slope to the lowest point.
Individual lines should be laid nearly parallel to land
contours (fig. 2-14). Tile fields are laid out either in a
herringbone pattern or with the laterals at right angles to
the main distributor. The distance between laterals is
three times the width of the trench. Distribution boxes
to which the laterals are connected may be desirable.
Trenches, 24 inches wide or more, are economical.
When a trenching machine is practical on a large
installation, base the design on the width of the trench
excavated by the machine.
P R O T E C T I N G T H E F I E L D .Once
constructed, all traffic must be excluded by fencing or
posting the tile field to prevent crushing of the tile.
Planting shrubs or trees over the field is not good
practice since the roots tend to clog the tile lines; grass
over the lines aids in removing moisture and keeping
the soil open.
Upon arrival at an advanced base, temporary
facilities must be provided immediately for the
disposal of human waste. A number of designs of field-
type latrines are used for this purpose. A 16. by 32-foot
wood-frame tent may be used to shelter the field-type