Table 1.—Determining tile-disposal field requirements from percolation tests[6]
| Minutes required for water to fall 1 inch | Effective absorption area required, per person, in bottom of disposal trenches | Minutes required for water to fall 1 inch | Effective absorption area required, per person, in bottom of disposal trenches |
| Square feet | Square feet | ||
| 2 or less | 26 | 10 | 52 |
| 3 | 30 | 15 | 63 |
| 4 | 36 | 30 | 90 |
| 5 | 40 | 60[7] | 120 |
[6] A minimum of 150 square feet should be provided, equal to 100 feet of 18-inch trench.
[7] If more than 60 minutes, use special design with seepage pits or sand-filter trenches.
Figure 9 suggests methods of arranging the tiles in disposal fields under varying conditions and the length of tiles needed.
DISPOSAL METHODS IN TIGHT OR WET SOILS
If the soil is heavy clay or has tight formation, yet shows some porosity from percolation tests, the efficiency of the field may be increased by placing below the tile lines 12 to 15 inches of additional filter material (washed gravel, crushed stone, slag, clean cinders, or clean bank-run gravel 3/4 to 21/2 inches in size). When the surface soil is tight and is underlain by porous soil, sufficient drainage is sometimes obtained for the smaller installations by omitting the tile field and providing a dry well at the end of the effluent sewer, provided the water table will not be contaminated. Larger systems under such soil conditions should have a tile field, and absorption can be increased by boring 6- or 8-inch holes down to the porous stratum and filling them with gravel or sand; the holes should be 4 to 6 feet apart. Another and perhaps the best practice is to excavate the tile trenches 4 to 6 feet and install a lower tile line, as shown in [figure 10]. This latter method is especially desirable if the upper tight stratum is especially thick, or if there is no porous lower stratum, or if in irrigated regions and where the disposal field is limited in area.