From these considerations, with the siphon dose 20 gallons per person, it is usually a safe rule to provide 50 feet of 3-inch tile for each person served and to lay the lines 10 feet apart. Such provision gives a capacity within the bore of the tile lines about equal to the siphon dose, and as some sewage is wasted at each joint a reasonable factor of safety is provided. A spacing of 10 feet will, it is believed, permanently prevent the extension of lateral absorption from line to line, provided the area is fairly well drained. As between 3-inch and 4-inch tile the smaller size costs less and is better calculated to taper the dose to small proportions. Four-inch tile is less likely to get out of alignment or to become clogged; a length of 28 feet has the same capacity in the bore as 50 feet of 3-inch.
Fig. 27.—Methods of laying distribution system: Methods 1, 2, and 3 for flat or gently sloping land; method 4 for steep slopes (see also [Figure 30]); A, direction of slope; B, contour of field; C, sewer from tank, preferably size 5 inch, though 4 or 6 inch may be used, depending on the fall and the size of the siphon (see table, [fig. 24]); D, V-branch set to divide the flow exactly; E, reducer, to 4 inches; F, ⅛ bend, 4-inch; G, increaser, from 4 inches; H, increaser, 3 to 4 inches; I, reducer, 4 to 3 inches; J, distribution tile, 3-inch; K, distribution tile, 4-inch
Good-quality drain tile in 1-foot lengths or second-quality sewer pipe in 2-foot lengths may be used. The lines are generally laid in parallel runs, but may be varied according to the topography. Layouts 1, 2, and 3, [Figure 27], for flat or gently sloping land, run with the slope; layout 4, for steep slopes, runs back and forth along the contour in a series of long flat sweeps and short steep curves. The grade of the runs and sweeps should be gentle, rarely more than 10 or 12 inches in 100 feet. In layouts 1, 2, and 3, [Figure 27] especially, it is desirable that the last 20 feet of each run should be laid level or given a slight upward slope, thus guarding against undue flow of sewage to the lowest ends of the system.
The runs should be laid no deeper than necessary to give clearance when plowing and prevent injury from frost. Ten inches of earth above the top of the tile is sufficient generally throughout the southern half of the United States and 18 inches generally in the North, but if the field is exposed or lacks a thick heavy growth of grass, the cover should be increased to 3 to 6 feet near the Canadian line. Where frost goes down 5 to 7 feet, it is better to lay the tile at moderate depth and cover the runs with hay, straw, or leaves weighted down, removing the covering in the spring.
Making the joints of the distribution tile demands especial attention. For a short distance on the upper end of each run the tile should be laid with ends abutting; the joint opening should be increased gradually to one-eighth inch and this increased to one-fourth in the last 20 feet of the run. All joints should be protected against the entrance of loose dirt. Four methods are shown in [Figure 28]. The lower end of each run should be closed with a brick or flat stone; or, what is better, an elbow or T branch may be placed on the end and vented above the surface of the ground, improving the flow of sewage, the ventilation of pipes, and the aeration of the soil.
If the distribution tile must be laid in clay or other close, poorly drained soil, special treatment is necessary. A common method is to subsoil and underdrain the area thoroughly, as shown in [Figure 29]. It is not always possible to run the underdrain in lines between the distribution lines as shown in Figures [17] and [29], but it is a desirable thing to do, as the sewage must then receive some filtration through natural soil.
In some instances it is sufficient to lay the distribution tile on a continuous bed, 8 to 12 inches thick, of coarse gravel, broken stone, or brick, slag, coke, or cinders and complete the refill as shown in Figure [16] or [29].