Section 62-03.1-03-09 - Absorption trenches1.Design. Absorption trenches shall be designed and constructed on the basis of the percolation test results or other soil data. Trench bottom area required is shown in the table in subsection 4. The bottom of the trench shall be dug so it is dead level throughout its length. The maximum depth to the bottom of absorption trenches may not exceed forty-eight inches [121.92 centimeters]. The trench bottom must be at least twenty-four inches [60.96 centimeters] above the mottled soil condition indicating a water table or from standing water in the borehole. 2.Filter material. The filter material shall cover the four-inch [10.16-centimeter] diameter pipe to a depth of two inches [3.08 centimeters] measured from the crown of the pipe and extend the full width of the trench and shall be not less than six inches [15.24 centimeters] deep beneath the bottom of the four-inch [10.16-centimeter] diameter pipe. The filter material may be washed rock or crushed stone ranging in size from one inch to three inches [2.54 centimeters to 7.62 centimeters]. The filter material shall be covered by red rosin paper40 , hay, straw, or approved filter fabric40 , as the laying of the pipe proceeds. Approved graveless systems may be used in lieu of rockfill providing an equivalent surface area of soil is utilized. 3.Spacing. Trenches must have a minimum spacing of undisturbed earth of six feet [1.83 meters] for eighteen-inch to twenty-four-inch [45.72-centimeter to 60.96-centimeter] trench widths and nine feet [2.74 meters] for trenches up to thirty-six inches [91.44 centimeters]wide. 4.Absorption field. The size and requirements for absorption fields shall conform to those given in the following table: Table - Recommended absorption trench area.
| Depth of Rock |
| Percolation Rate | Soil | Classification | Below | Distribution | Pipe |
| Minutes/Inch | | 6" | 12" | 18" | 24" |
| -Trench bottom area loading rate, gal/ft2/day |
| 1 to 5 | .Sand | 1.2 | 1.5 | 1.80 | 2.1 |
| 6 to 15 | Sandy loam | 0.8 | 1.0 | 1.20 | 1.4 |
| 16 to 30 | Loam | 0.6 | 0.75 | 0.90 | 1.05 |
| 31 to 45 | Silt loam | 0.5 | 0.63 | 0.76 | 0.89 |
| 46 to 60 | Clay loam | 0.45 | 0.57 | 0.68 | 0.79 |
| -Square feet of trench bottom/bedroom1 |
| .1 to 5 | | 125 | 100 | 85 | 70 |
| 6 to 15 | | 190 | 150 | 125 | 110 |
| 16 to 30 | | 250 | 200 | 165 | 145 |
| 31 to 45 | | 300 | 240 | 200 | 170 |
| 46 to 60 | | 330 | 265 | 220 | 190 |
1Based on sewage volume of 150/GPD/Bedroom
5.Absorption lines.a. Gravity distribution. Absorption lines shall be constructed of four-inch [10.16-centimeter] pipe. For approved plumbing materials, see table 14-1 of chapter 14 of the Uniform Plumbing Code. b. Pressure distribution. Absorption lines must be constructed of one and one-half-inch to two-inch [3.81-centimeter to 5.08-centimeter] rigid plastic pipe with one-fourth-inch [6.35-millimeter] holes drilled in the bottom of the pipes. The number ofperforations and spacing of perforations for different diameter pipes for pressure distribution laterals must not exceed ten percent of the average pressure head on the perforations. The pipe and connections must be able to withstand a pressure of at least forty pounds per square inch. The perforated laterals should be attached to a two-inch [5.08-centimeter] manifold pipe and should have the ends capped. The laterals should be spaced no further than forty inches [101.6 centimeters] on center and no further than twenty inches [50.80 centimeters] from the edge of the rock. Pipe must be installed level and capped at the ends. The manifold must be supported and backfilled by hand. 6.Grade. The absorption trench bottom must be level. N.D. Admin Code 62-03.1-03-09
General Authority: NDCC 43-18-09
Law Implemented: NDCC 43-18-09