Fig. 29.—Methods of laying distribution system: Methods 1, 2, and 3 for flat or gently sloping land; method 4 for steep slopes (see also [fig. 32]); 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. 26]); D, V-branch set to divide the flow exactly; E, reducer, to 4 inches; F, 1⁄8 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.
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 2½ or 3 feet near the Canadian line. What is better, the tile in all instances may be laid with a 10-inch cover and in cold weather the runs may be covered with hay, straw, or leaves weighted down, which may be removed in the spring.
Fig. 30.—Four methods of protecting open joints in distribution lines—an all-important work. Sketches show cross-section and longitudinal views; the depth from the surface of the ground to the top of the tile is about 10 inches.
1. A, Subsoil ground; B, 3 or 4 inch drain tile; C, strip of tarred paper about 6
inches wide and extending three-fourths the distance around the tile, allowing sewage
to escape at the bottom; D, coarse sand, gravel, broken stone or brick, slag, cinders, or
coke, the coarsest material placed around the tile (where the ground is naturally very
porous and well drained, special filling in the trench may be omitted); E, natural soil.
2. Drain tile covered with a board laid flat, leaving the entire joint open.
3. Drain tile laid in stoneware gutter pieces and the joint covered with stoneware
caps; gutter and cap pieces are inexpensive commercial products; their radius is longer
than that of the outside of the tile, thus leaving open most of the joint space; the gutter
aids in keeping the tile in line.
4. Vitrified sewer pipe with hubs facing downhill; the spigot end should be centered
in the hub with a few small chinks or wedges.
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 30]. 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 31]. It is not always possible to run the underdrain in lines between the distribution lines as shown in figures [19] and [31], but it is a desirable thing to do, as the sewage must then receive some filtration through natural soil.