MIL-HDBK-1005/16
6.4.8
Special Siting Considerations. Construction of mound
systems as well as conventional systems is not recommended in
flood plains, drainage ways, or depressions. Generally, sites
with large trees, numerous smaller trees, or large boulders are
unsuitable for the mound system because of the difficulty in
preparing the surface and the reduced infiltration area beneath
the mound. As with rock fragments, tree roots, stumps and
boulders occupy space, thus reducing the amount of soil for
proper purification. If no other site is available, then it is
recommended to cut the trees off at ground level, leaving the
stumps. A larger mound area may be necessary if too many stumps
are involved for sufficient soil to be made available to accept
the effluent. Separating distances should be considered between
the toe of the fill and the respective features, such as a
building, well, slope, or stream. When the mound or fill is
located upslope from a building or other features on soils with
slow percolation rates or slowly permeable subsoil layers, the
separating distances should be increased.
6.4.9
Basal Area Calculation. The natural soil-fill area
interface is the basal area. The effluent is accepted from the
overlying mound fill through this area into the subsoil beneath.
For level sites, the basal area equals the mound area. For
sloping sites, the basal area downslope from the bed or trenches
is used in basal loading rate calculations. It includes the area
enclosed by B*(A+C+I+J)for a trench system (Figure 15) or
B*(A+I+J) for a bed system (Figure 16). The percolation rate for
the natural soil will determine how much area is required. For
percolation rates applicable for mound systems, the design basal
loading rates are provided in Table 11.
Table 11
Percolation Rates and Corresponding Design Loading Rates
3 to 29 min/in.
Use
1.2 gpd/sq ft
(1.1 to 11 min/cm)
(0.049 cu m/d/sq m)
30 to 60 min/in.
Use
0.74 gpd/sq ft
(12 to 24 min/cm)
(0.03 cu m/d/sq m)
60 to 120 min/in.
Use
0.24 gpd/sq ft
(24 to 48 min/cm)
(0.0098 cu m/d/sq m)
103
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