TM 5-814-3/AFM 88-11, Volume III
C-20. Mound systems. (Refer to para 6-5.)
a. Design requirements and criteria. Munitions storage guard house. Maximum capacity: 9 persons.
Area is underlain by coarse sand and irregular rocks. Bedrock is over 50* below site. Soil excavations reveal
no hard-pan, but water table is only 2* below ground.
Slope = 5 percent;
Percolation rate = 55 min/in at 24 in;
Manifold length = 125 ft;
Septic tank anticipated volume = 2,000 gal.
(1) Choose a location: Using soil maps, boreholes and soil pits, choose a location free of trees with
no clay lenses or large boulders in the soil profile. Choose mound site before building the housing.
(2) Calculate waste load: Consider fill occupancy. Whoever is on guard lives there, so anticipate 9
berthing areas.
150 gal/day/room 9 rooms = 1,350 gal/day.
(3) Select fill material: When quality fill is not readily available, the mound system is excessively
expensive. Therefore, utilize the soil maps to locate areas of medium sand. If the sand is not >25 percent
coarse sand and <5 percent fine sand, then it should bbe "up--graded" by screening out fines and materials
greater than 2mm in size, using a trommel. Final stockpiled material should have an infiltration rate of 1.0
to 1.5 gal/ft2/day.
(4) Size the absorption area: Use filtration rate of 1.20 gal/ft2/day (table 6--3). Absorption area
required = 1,350 gal/day 1.20 gal/ft2/day = 1,124 ft2. Use a conservative value of 1,200 ft2. Since the area
is permeable and of constant slope, a bed system could be used (trenches might be used if the topography
required following a contour or was very narrow). Two long narrow beds, A = 10" wide; each bed will be
60" long (B).
(5) Mound height: At upslope side, minimum depth = 1 ft (see fig 6-7). At downslope side, depth
= E = D + slope (A). Side depth = E = 1 + 0.05 (25) = 2.25. Bed depth (F) = 1 ft minimum with at least 8
in of aggregate (not limestone) beneath the distribution piping. Bed total thickness = 1 ft. Cap should be a
minimum of 6 in of subsoil and 6 in of reclaimed topsoil. To provide drainage, use 2.0" at centerline (see fig
C-9).
C-42
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