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

The depth of the porous media may vary. A.minimum of 6 in. (15 cm)
below the distribution pipe invert and 2 in. (5 cm) above the crown of
the pipe is suggested. Greater depths may be used to increase the
sidewall area and to increase the hydraulic head on the infiltrative
surface.
Gravel or crushed rock is usually used as the porous media, though other
durable porous materials may be suitable. The suggested gravel or rock
size is 3/4 to 2-l/2 in. (1.8 to 6.4 cm) in diameter. Smaller sizes are
preferred because masking of the infiltrative surface by the rock is
reduced (13). The rock should be durable and resistant to slaking and
dissolution. A hardness of 3 or greater on the Mob's Scale of Hardness
is suggested. Rock that can scratch a copper penny without leaving any
residual rock meets this criterion. Crushed limestone is unsuitable
unless dolomitic. The media should be washed to remove all fines that
could clog the infiltrative surface.
To maintain the porous nature of the media, the media must be covered
with a material to prevent backfilled soil from entering the media and
filling the voids. Treated building paper was once used but has been
abandoned in favor of untreated building paper, synthetic drainage fab-
ric, marsh hay or straw. These materials do not create a vapor barrier
and permit some moisture to pass through to the soil above where it can
be removed through evapotranspiration. All these materials, except for
the drainage fabric, will eventually decay. If they decay before the
soil has stabilized, the value of the materials is lost. To ensure the
barrier is not lost prematurely, heavy duty building paper of 40 to 60
lb (18 to 27 kg) weight or a 4 to 6 in. (10 to 15 cm) layer of marsh hay
or straw should be used. In dry sandy soils, a 4 in. (10 cm) layer of
hay or straw covered with untreated building paper is suggested to pre-
vent the backfill from filtering down into the rock.
f. Inspection Pipes
Inspection pipes located in the subsurface soil absorption system pro-
vide limited access to observe the depth of ponding, a measure of the
performance of the system, and a means of locating the subsurface field.
If used, the inspection pipes should extend from the bottom infiltrative
surface of the system up to or above final grade. The bottom should be
open and the top capped. The portion of the pipe within the gravel
should be perforated to permit a free flow of water (see Figure 7-6).
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