Current through Register Vol. 50, No. 11, November 20, 2024
Section XIII-719 - Absorption Trenches [formerly Section III of Appendix A]A. [Formerly paragraph A:3.1 of Appendix A] Where soil conditions are satisfactory and sufficient land is available, septic tank effluent shall be disposed of in absorption trenches. This consists of a system of covered gravel (or other approved aggregate) filled trenches into which the septic tank effluent is applied so as to permit the liquid to seep into the soil. By action of microorganisms in the soil, the organic matter is converted into mineral compounds.B. [Formerly paragraph A.3.2 of Appendix A] A number of variables determine whether an absorption trench is feasible, including: soil porosity (permeability), ground water table, available space, and the rate at which septic tank effluent enters the soil (percolation rate). In general three conditions should be met. 1. The soil percolation rate must be within the acceptable range.2. The maximum elevation of the ground water table should be at least 2 feet below the bottom of the proposed trench system.3. Clay formations or other impervious strata should be at a depth greater than 4 feet below the bottom of the trenches.C. [Formerly paragraph A.3.3 of Appendix A] Unless these conditions are satisfied, the site is unsuitable for a subsurface sewage disposal system, and an alternative method must be utilized.D. [Formerly paragraph A.3.4 of Appendix A] The acceptability of soil for an absorption trench system and the required size of such a system is currently based upon the "Percolation Test" described below. 1. Three or more tests must be made in separate test holes spaced uniformly over the proposed absorption field site.2. Dig or bore a hole, with horizontal dimensions of from 4 to 12 inches and vertical sides to the depth of the proposed absorption trench. In order to save time, labor, and volume of water required per test, the holes may be bored with a 4-inch auger.3. Carefully scratch the bottom and sides of the hole with a knife blade or sharp-pointed instrument in order to remove any smeared soil surfaces and to provide a natural soil interface into which water may percolate. Remove all loose material from the hole.4. To conduct the test, fill the hole with clear water. This pre-wetting procedure should normally be accomplished on the day prior to the percolation rate measurement. This procedure is to insure that the soil is given ample opportunity to swell and to approach the operating condition of the wet season of the year. Thus, the test should give comparable results in the same soil whether made in a dry or in a wet season.5. With the exception of sandy soils, percolation rate measurements shall be made on the day following the procedure described under §719. D.4 above. Add water until the liquid depth is at least 6 inches, but not more than 12 inches from a fixed reference point. Measure the drop in water level over a 60-minute period. This drop is used to calculate the percolation rate. Section 1501. B.1 (Figure 1) shows methods of percolation rate measurement. If the drop in liquid depth in the first 30 minutes is less than one inch, it is unnecessary to continue the test for the full 60-minute period.6. The distance the water falls in 60 minutes in each of the three test holes is recorded. The average drop for the three holes is used to determine the total length of absorption trench from Table 1 below. Table 1 |
Absorption Trench Length Requirements for Individual Residences |
Average Water Level Drop in 60 minutes (in inches) | Length (in Feet) of Absorption Trenches Required per Bedroom* |
More than 12 | 72 |
12 | 83 |
11 | 87 |
10 | 91 |
9 | 96 |
8 | 100 |
7 | 104 |
6 | 110 |
5 | 117 |
4 | 127 |
3 | 142 |
Less than 3 | Not acceptable for absorption field |
NOTE: A minimum of 160 linear feet of field line shall be provided.
*-or per 150 gpd of design flow for non-residential applications.
E. [Formerly paragraph A:3.5 of Appendix A] Many different designs may be used in laying out an absorption trench system. The choice will depend on the size and shape of the available disposal area, the capacity required and the topography of the area.F. [Formerly paragraph A:3.6 of Appendix A] The septic tank effluent is applied to the absorption field through a system of level bottomed trenches. Conventional field lines are laid on a slope of 2 to 3 inches per 100 feet. Gravelless pipe and other distribution chambers must be laid as close as possible to a slope of 1 inch per 100 feet. A distribution box may be required for equal distribution of the effluent. Section 1501. B.2 (Figure 2 and 3) show a typical layout of a conventional absorption trench system for flat and sloping areas.G. [Formerly paragraph A:3.7 of Appendix A] To provide the minimum required backfill depth and earth cover, the depth of the absorption trenches must be a minimum of 18 inches. Additional depth may be needed for contour adjustment for extra backfill under the distribution line or for other design purposes. However, the total depth must not exceed 24 inches.H. [Formerly paragraph A:3.8 of Appendix A] Careful construction is important in obtaining a satisfactory soil absorption system. Section 1501. B.4 (Figure 4) shows details for absorption trench construction.I. [Formerly paragraph A:3.9 of Appendix A] Individual trenches shall not be greater than 100 feet in length and not less than 18 inches in width. The center line distance between individual trenches shall be at least 6 feet. In addition, the absorption trenches shall be located at least 10 feet from any dwelling or property line.J. [Formerly paragraph A:3.10 of Appendix A] The location of the absorption trenches shall comply with minimum distance requirements from water wells, water lines, etc., as contained in Part XII of this Code.K. [Formerly paragraph A:3.11 of Appendix A] In every case, at least two trenches shall be used.L. [Formerly paragraph A:3.12 of Appendix A] Trench bottoms must be level to promote even distribution, thereby minimizing premature failure of a portion of the trench. During excavation, attention must be given to the protection of the soil. Care must be taken to prevent sealing of the surface on the bottom and sides of the trench. Trenches should not be excavated when the soil is wet enough to smear or compact easily. All smeared or compacted surfaces must be raked to a depth of 1 inch and loose material removed before the backfill is placed in the trench.M. [Formerly paragraph A:3.13 of Appendix A] Conventional field lines shall consist of perforated non-metallic pipe meeting one of the following standards. PVC sewer pipe and fittings (Thin wall), ASTM D2729-93 |
Smooth wall polyethylene (PE) pipe, ASTM F810-93, for use in waste disposal absorption fields; |
SRP pipe and fittings, ASTM D2852-93. |
1. In every case, the minimum acceptable diameter is 4 inches. Although the trench bottom is level, conventional field pipes must be laid on a slope of between 2 to 3 inches per 100 feet to provide even distribution of the liquid throughout the trench.N. [Formerly paragraph A:3.14 of Appendix A] Where conventional field pipe is used, it must be surrounded by clean graded gravel or rock, broken, hard-burned clay brick or similar material. The bed material may range in size from 1/2 inch to 2.5 inches. The gravel must extend from at least 2 inches above the top of the pipe to at least 6 inches below the bottom of the pipe. The top of the stone should be covered with either untreated building paper, or similar pervious material to prevent the gravel from becoming clogged by the earth backfill [see §1501. B.4 (Figure 4)].O. [Formerly paragraph A:3.15 of Appendix A] Where gravelless pipe or distribution chambers are used, the fill must be porous soil or sand which allows the passage of water in all directions with a 6-inch layer below the pipe and filled 4 to 6 inches above grade and spread 3 to 4 feet on either side of the trench. Only gravelless pipe or other distribution chambers specifically approved for use in Louisiana by the state health officer may be used. The total length of gravelless distribution products required is the same as for conventional absorption trenches.P. [Formerly paragraph A:3.16 of Appendix A] For an absorption trench to work properly, it must have access to air, generally through the soil interstices of the backfill. Therefore, the absorption trench should be backfilled with 4 to 12 inches of pervious soil, hand-tamped and then overfilled with about 4 to 6 inches of earth. Care should be taken to avoid compacting of the backfill.Q. [Formerly paragraph A:3.17 of Appendix A] All of the above listed requirements, with the exception of the protection of water supplies, are aimed at preventing absorption trench clogging and premature failure. In addition, the septic tank should be inspected every six years after installation and pumped, as necessary, to prevent solid overflow to the soil absorption system and subsequent clogging and failure.R. [Formerly paragraph A:3.18 of Appendix A] Absorption trenches shall not be located: 1. beneath driveways, parking or other paved areas;2. in areas that may be subjected to passage or parking of heavy equipment or vehicles, or storage of materials;3. beneath buildings or other structures.La. Admin. Code tit. 51, § XIII-719
Promulgated by the Department of Health and Hospitals, Office of Public Health, LR 28:1349 (June 2002).AUTHORITY NOTE: Promulgated in accordance with R.S. 40:4(A)(6) and R.S. 40:5(9)(20).