TM 5-814-9
Materials. Local sands and gravels should he
surface of the filters as evenly as possible (fig
(6)
6-15). The number of distribution troughs
used whenever possible. The material may
needed will depend on the area of the filters
have to be washed and screened to obtain
and the volume of water to be filtered. Plastic
clean and properly sized aggregate. The top
pipes with drainage holes may have to he
36 inches (0.9 meter) of the filter should
installed in the troughs to extend the
contain sand with less than 1 percent of the
distribution system and apply the water
fines passing the number 200 sieve. The D10
should be between 0.2 and 0.4 millimeters
uniformly over the surface. The holes in the
with a uniformity coefficient (D60/D10)1 of less
pipes should be the same size as those in the
grid system. The filter distribution system
than 3.5. The next layer contains 4 inches
used should he capable of being disassembled
(10.2 centimeters) of 0.5-inch (1.3-
when maintenance of the filters is required
centimeter) aggregate. The next layer is 3-
and the supply pipe to the distributors should
inches (7.6- centimeter) of 1-inch (2.5-
he buried.
centimeter) aggregate. The bottom layer
consists of 11 inches (27.9 centimeters) of
Underdrain system. Perforated collection
(5)
aggregate ranging in size from 1.5- to 2.0-
pipes for the filters are installed near the
inch (3.8-to 5.1-centimeter). The depth of the
bottom of the filter embedded in gravel just
bottom layer should be measured from the
above an impervious layer. Water will per-
bottom of the lowest collection pipe. The
colate downward through the sand and collect
bottom and walls of the filter must be
in the pipes (fig 6-16). These underdrains
relatively impervious. Liners of clay soil or
should be placed on a minimal grade (less
geotextile membranes should be provided
than 0.25 percent) to provide gravity flow.
similar to those described for the earthen ba-
The designer must consider the surface area
sins (para 6-26). Figure 6-17 shows an inter-
of the filters and design the collection system
mittent sand filter in cross section.
to also handle stormwater falling on the filter
6-18
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