TM 5-814-3/AFM 88-11, Volume III
h. 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 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.
i. 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. While, for level sites, the basal area
equals the mound area; for sloping sites, only the basal area downslope from the bed or trenches may be
considered effective. It includes the area enclosed by Bx (A+C+I) for a trench system (fig 6-7), or Bx (A+
I) for a bed system (fig 6-8). The percolation rate for the natural soil will determine how much area is
required. For percolation rates applicable for mound systems, the design loading rates are:
6-15
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