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

deep trenches with a greater depth of rock below the distribution pipe
to increase the sidewall area is more suitable.
Reserve Area: When planning and locating the absorption system, consid-
eration should be given to reserving a suitable area for construction of
a second system. The second system would be added if the first were to
fail or if the system required expansion due to increased wastewater
flows. Care must be used in constructing the second system so that the
original system is not damaged by the construction equipment.
The reserve area should be located to facilitate simultaneous or alter-
nating loading of both systems. If the reserve area is used because the
initial system has failed, the failing system should not be permanently
abandoned. With time, the initial system will be naturally rejuvenated
and can be used alternately with the reserve system. Reserve areas can
be provided very easily with trench systems by reserving sufficient area
between the initial trenches as shown in Figure 7-4.
Dimensions: The absorption system should be dimensioned to best fit the
lot while maintaining separation distances and avoiding excessive depths
of excavation. Commonly used dimensions are given in Table 7-3.
The depth of excavation is determined by the location of the most perme-
able soil horizon and flow restricting layers or the high water table
elevation. Unless a deep, more permeable horizon exists, the trench or
bed bottom elevation should be maintained at about 18 to 24 in. (46 to
61 cm) below the natural ground surface. To prevent freezing in cold
climates, 6 to 12 in. (15 to 30 cm) of cover should be backfilled over
the aggregate (11).
If the water table or a very slowly permeable layer is too near the
ground surface to construct the system at this depth, the system can be
raised. Very shallow trenches 6 to 12 in. (15 to 30 cm) deep can be in-
stalled and the area backfilled with additional soil (see Figure 7-5).
Adequate separation distance must be provided between the trench bottom
and the seasonally high groundwater level to prevent groundwater
contamination.
The length of the trench or bed system depends on the site characteris-
tics. The length of the distribution laterals is commonly restricted to
100 ft (30 m). This is based on the fears of root penetration, uneven
settling, or pipe breakage which could disrupt the flow down the pipe to
render the remaining downstream length useless. However, these fears
are unwarranted because the aggregate transmits the wastewater (4)(13)
(17). To assure adequate transmission and distribution of the
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