c. Construction. Trenches should be from 1 to 3 feet wide and at least 2 feet
deep in order to provide the minimum required gravel depth and earth cover. The
spacing between laterals is generally governed by practical considerations dependent
on the type of construction equipment, safety, and so forth, but the distance between
trenches should be at least twice the depth of gravel. For serial distribution on sloping
ground, trenches should be separated by at least 6 feet. Clean, graded gravel or rock
ranging in size from 1/2 inch to 2 1/2 inches should surround the pipe. The material
should extend from at least 2 inches above the top of the pipe to at least 6 inches below
the bottom of the pipe. If tile is used, the upper half of the joint openings should be
covered as shown in Figure 1-12. The top of the gravel should be covered with
untreated building paper or similar pervious material to prevent the gravel from
becoming clogged by the earth backfill. The top of a new absorption trench should be
hand-tamped and should be overfilled with about 4 to 6 inches of earth to prevent the
top of the trench from settling below the surface of the adjacent ground. Once a tile
field is constructed, fencing or posting to prevent crushing the tile should exclude all
heavy vehicle traffic. Planting shrubs or trees over the field is not good practice since
the roots tend to clog the tile lines; however, grass over the line assists in removing the
moisture and keeping the soil open. Freezing rarely occurs in a carefully constructed
system kept in continuous operation.
d. Absorption Area. The required absorption area is dependent upon the
results of the soil percolation test and the number of bedrooms served by the system.
(1) Residential requirements. Table 1-3 gives the absorption area
requirements for individual residences in square feet of trench bottom per bedroom for
percolation rates up to 60 minutes (the minimum acceptable rate for a soil absorption
system). The system should be designed to provide for the number of bedrooms that
can be reasonably anticipated, including the unfinished space available for conversion
as additional bedrooms. It is also desirable to provide sufficient land area for an entire
new system if the first one fails. To calculate the required absorption area, enter Table
1-3 at the percolation rate previously determined and find the absorption area per
bedroom in square feet. Multiply this figure by the number of bedrooms to find the total
absorption area required.
MD0161
1-30