several tests to determine the best location and average conditions should be made. From test results, the rate of sewage application to the total bottom area of the tiled trenches may be taken from the data below. Soil testing over 30 minutes is not suitable.

TRENCH WIDTH. - The minimum width of a trench on the basis of the types of soil is as follows:

Sand and sandy loam, 1 foot

Loam and sand and clay mixture, 2 feet

Clay with some gravel, 3 feet

FROST LINE. - Placing tile below the frost line to prevent freezing is not necessary. Tile placed 18 inches below the ground surface operated successfully in New England for many years. Subsurface tile should never be laid below groundwater level.

PIPE SIZE. - Design and construction should provide for handling and storage of some solid material, eliminating, as much as practical, the opportunity for clogging near pipe joints. Pipe 4 to 6 inches in diameter is recommended. The larger pipe gives greater storage capacity for solids and a larger area at the joint for solids to escape into the surrounding gravel.

LAYING THE PIPE. - To provide for free discharge of solids from the line to the filter trench, lay the pipe with 3/8-inch clear openings. The top of the space is covered with tar paper or similar material to prevent entry of gravel. Bell-and-spigot pipe is laid to true line and grade easily. Good practice calls for breaking away two thirds along the bottom of the bells at the joint and rising small wood-block spacers. The pipe is commonly laid at a slope of about 0.5 foot per 100 feet when taking the discharge directly from the septic tank and 0.3 foot per 100 feet when a dosing tank is used ahead of the field.

BEDS. - The tile is laid on a bed of coarse- screened gravel at 6 inches deep with 3 inches of coarse gravel around and over the pipe. Coarse- screened stone passing 2 1/2-inch mesh and retained on a 3/4-inch mesh is recommended. This gravel bed gives a relatively large percentage of voids into which the solids pass and collect before the effective leaching area becomes seriously clogged. The soil that fills the trench must not fill the voids in the coarse-screened gravel around the pipe. A 3-inch layer of medium-screened gravel over the coarse stone and 3 inches of either fine- screened gravel or suitable bank-run gravel over the medium stone is recommended.

LAYOUT. - The layout of the tile in the field should be designed carefully. Generally, the length of laterals should NOT exceed 100 feet. When tile is laid in sloping ground, distribute the flow so each lateral gets a fair portion. Flow must be prevented from discharging down the slope to the lowest point. Individual lines should be laid nearly parallel to land contours (fig. 2-14). Tile fields are laid out either in a herringbone pattern or with the laterals at right angles to the main distributor. The distance between laterals is three times the width of the trench. Distribution boxes to which the laterals are connected may be desirable. Trenches, 24 inches wide or more, are economical. When a trenching machine is practical on a large installation, base the design on the width of the trench excavated by the machine.

PROTECTING THE FIELD. - Once constructed, all traffic must be excluded by fencing or posting the tile field to prevent crushing of the tile. Planting shrubs or trees over the field is not good practice since the roots tend to clog the tile lines; grass over the lines aids in removing moisture and keeping the soil open.

Field-Type Latrines

Upon arrival at an advanced base, temporary facilities must be provided immediately for the disposal of human waste. A number of designs of field- type latrines are used for this purpose. A 16. by 32-foot wood-frame tent may be used to shelter the field-type latrine. 2-21