Figure 1-9. Septic tank system.
b. Percolation Tests. In the absence of ground water or subsoil information,
subsurface sand explorations are necessary to determination the absorptive capacity of
the soil and serve as a basis of design for the liquid absorption. A percolation test of the
soil at the actual site of where the disposal field will need to be conducted. The
information gathered will govern the required area needed for the disposal (absorption)
field. The recommended procedure, developed by the Robert A. Taft Sanitary
Engineering Center, follows.
(1) Number and location of tests. Six or more tests should be made in
separate test holes spaced uniformly over the proposed absorption field site.
(2) Type of test hole. Dig or bore a hole from 4 to 12 inches in diameter,
with vertical sides, to the depth of the proposed absorption trench. This depth will
depend upon several factors such as differences in elevation between the house sewer
and the disposal field, necessary grade for connecting lines, and the depth of the
pervious soil layer. Some soils have a dense, impervious clay topsoil that must be
penetrated to achieve an acceptable percolation rate. Exploratory excavations must
determine this condition if a knowledge of the soil conditions does not exist.