SIZE AND MINIMUM SPACING REQUIREMENTS FOR DISPOSAL TRENCHES +---------+----------+----------------------+-------------+ | TRENCH | TRENCH | EFFECTIVE ABSORPTION | TILE LINES | | WIDTH-W | DEPTH-D | AREA IN SQUARE FEET | SPACING-S | |IN INCHES| IN INCHES| PER LINEAL FOOT | IN FEET | +---------+----------+----------------------+-------------+ | 18 | 18 to 30 | 1.5 | 6.0 | | 24 | 18 to 30 | 2.0 | 6.0 | | 30 | 18 to 36 | 2.5 | 7.5 | | 36 | 24 to 36 | 3.0 | 9.0 | +---------+----------+----------------------+-------------+ Wider spacing of the lines desirable where available area permits DISPOSAL-TILE TRENCH Disposal-tile lines--Maximum length for each line 100 feet. All lines to be equal in length. Disposal-tile lines to slope 2" to 4" per 100 feet, not over 6". Sewer-tile lines to slope 1/8" to 1/4" per foot. 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 2-1/2 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. Where the underdrain tile is not used, the absorption capacity of the field can be increased by providing a rock-filled trench across the lower end of the tiles for the full width of the field. The depth should be not less than 5 feet and the width not less than 3 feet. On account of the beneficial action of bacteria in the upper soil layers it is highly desirable to confine the effluent near the surface rather than to use underdrains. Purification becomes slower and less effective, the deeper the drains.