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

FIGURE 7-12
PROPER ORIENTATION OF A MOUND SYSTEM ON A COMPLEX SLOPE
of Flow
c. Sizing the Filled Area
Mound Shaped to Co
the Contour and to S
The dimensions of the mound are dependent on the size and shape of the
absorption bed, the permeability of the natural soil, the slope of the
site, and the depth of fill below the bed (see Figure 7-13). Depths and
dimensions are presented in Table 7-9.
The downslope setback (I) in Figure 7-13 is dependent on the permeabili-
ty of the natural soil. The basal area of the mound must be sufficient-
ly large to absorb the wastewater before it reaches the perimeter of the
mound or surface seepage will result. On level sites, the entire basal
area (L x W) is used to determine I. However, on sloping sites, only
the area below and downslope from the absorption bed is considered C(B)
x (A + III. The infiltrative rates used for the natural soil to size
the downslope setback are given in Table 7-10. These rates assume that
a clogging mat forms at the fill/natural soil interface, which may not
be true. Since the percolating wastewater can and does move laterally
from this area, these values are conservative. However, for soils with
percolation rates faster than 60 min/in. (24 min/cm), the side slope
criteria determine the basal area instead of the infiltration rate of
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