TM 5-814-3/AFM 88-11, Volume III
b. Site considerations. Table 6-1 summarizes soil and site factors that restrict mound systems. In using
table 6-1, percolation tests are usually run at 20-24 inches from the natural surface. As shown for slowly
permeable soil, if the percolation rate is less than 60 minutes per inch, the soil is permeable so that the slope
of the site may be cautiously increased to keep effluents in the upper soil horizons. If the percolation rate is
greater than 120 minutes per inch, then the soil is so impermeable as to disallow use of a standard mound
system. Soil characteristics, water table depth and amount of large fragments dramatically influence mound
design. In figure 6-7, a mound system using two trenches is illustrated; while in figure 6-8, the bed absorption
system is shown. For further information on design criteria and installation, see EPA Manual No. 625/1-80-
012. See also the sample design problem in appendix C.
6-12
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