It is generally found that a sufficient length of sub-surface tiling should be laid to provide for not more than one to three gallons of effluent per day for each linear foot of tiling. In sandy soils there should be at least thirty to forty feet of tiling for each person served by the sewer, with six feet of space between the lines of tiling. This length per person should be increased up to seventy or eighty feet for the more compact sandy or gravelly loams, or the lighter clay loams, with the lateral tiling spaced four feet apart. It is not considered feasible to attempt to dispose of sewage by sub-surface irrigation in soils which will not care for effluent when the greater lengths of tiling per person, as stated above, will not prevent the appearance of effluent on the surface. If, however, after the installation of a sub-surface system in a rather heavy soil, it is found that proper seepage of the effluent does not occur, the lateral branches may sometimes be lengthened and the system then found to operate satisfactorily.
Fig. 32.—Sub-surface Tiling with Broken Stone or Gravel Surrounding Pipe.
The lines of lateral tiling should be laid with the invert, or bottom of the pipe, inside, from six inches to one foot below the surface, as shown in Fig. [27]. They should be parallel with the contours or at right angles with the slope of the field, and should have a gradient or fall of one-sixteenth of an inch to the foot when laid in sandy soil or sandy loam, and of not more than one thirty-second of an inch to the foot when laid in the heavier loams. To obtain such gradients for the sub-surface tiling it is sometimes necessary to lay out the trenches along irregular or curved lines, as shown in Fig. [33]. The tiling should be laid near the surface, as stated, and never deeper than twelve inches. The temperature of the sewage will prevent its freezing even in very severe winter weather, especially when the ground is covered with snow.
Fig. 33.—Sub-surface System on Irregular Ground.
To provide for diverting the flow from the siphon chamber first into one of the two portions into which the sub-surface system is divided, and then, after an interval of three days or a week, into the other portion of the system, at the point where the main carrier is to branch, a ten-inch iron pipe casting (see Fig. [34]), with its lower portion forming the body of a double Y-branch of six-inch or eight-inch pipe, may be placed, having a swinging blade or gate attached inside in a vertical position. When, for example, the effluent has been passed for a week into section B of the sub-surface system, the gate C, shown in Fig. [34], may be swung to the dotted position and the effluent, at each discharge of the siphon chamber, will then pass through the branch carrier A to section A of the sub-surface system; or a double Y-branch of iron pipe (see Fig. [35]) or a cross may be placed at this point on the main carrier when there are to be three sections of the sub-surface system, and valves may be placed on the three branches of the main carrier thus formed to permit of alternately shutting off the flow to the various sections of the sub-surface tiling system (see also Fig. [30]). Perhaps the simplest and most serviceable device, however, for alternately resting different portions of the irrigation field is a diverting manhole with stop planks or wooden sluices sliding in grooves in the concrete walls or in a wooden frame, as shown in Fig. [27]. (See also Fig. [43], Chapter V.)
Where the ground-water level is not very deep below the surface, or a clay or hardpan stratum occurs at a depth of a few feet, it is advisable to underdrain the irrigation field by lines of open-jointed tiling laid at right angles to the lateral distributing tiling and spaced about fifteen feet apart. (See Fig. [36].)
Fig. 34.—Special Casting of Double Y-Branch with Swinging Gate.