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. Time for water to fall 1 Allowable rate of sewage inch  (minutes) application in gallons per square foot per day, bottom of trench in tile field 1 4.0 2 3.2 5 2.4 10 1.7 30 0.8 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. P R O T E C T I N G   T H E   F I E L D .—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