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Sketch of a septic drainfield trench cross sectionShallow Trench Septic Drainfield Design

This article explains the design and construction specifications of a shallow trench septic leachfield or soakaway bed or drainfield. You use this type of drainfield when there is usable but thin soil cover in the septic drainfield area.

Shown above and as a larger image below, a shallow bed septic design can be installed when there is 2 to 4 ft. of usable soil below the biomat layer.

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Shallow Trench Septic Drainfield Systems

Sketch of a septic drainfield trench cross sectionHere we summarize the basics of a shallow trench septic drainfield design.

In the most common design of drainfield, perforated pipes are buried in gravel-filled trenches to form the drainfield.

[Click to enlarge any image]

This sketch shows a cross section of a typical shallow-bed septic drainfield trench.

(Source US EPA who in turn obtained the drawing from Ayres Associates, in turn modified by InspectApedia.)

a. Site Requirements for shallow septic system absorption trenches

These systems are used where there is at least two feet but less than four feet of usable soil and/or separation to boundary conditions.

b. Design criteria for shallow septic absorption trenches

(i) A minimum two foot separation

must be maintained between the bottom of each septic soakbed / drainfield trench and all boundary conditions.

(ii) The bottom of each septic drainfield trench trench

must not be above the original ground surface.

(iii) Septic trench fill material:

Material of the same permeability as the underlying original soil shall be used as fill material. The depth of the fill shall not be greater than 30 inches above the original ground elevation.

(iv) An absorption trench system as described in CONVENTIONAL SEPTIC LEACHFIELDS is designed using the percolation of the underlying original soil.

c. Materials Used for shallow septic absorption trench systems

Shallow trench septic systems use essentially the same materials as found

at CONVENTIONAL SEPTIC LEACH FIELDS

d. Construction of shallow septic system absorption trenches

(i) Heavy equipment shall be kept out of the absorption area.

(ii) Fill material is carefully placed within the absorption area.

(iii) The edge of the fill material shall be tapered at a slope of no greater than one vertical to three horizontal.

On sloped sites a diversion ditch shall be placed on the uphill side to prevent runoff from entering the fill.

(iv) The absorption trench system is constructed in the fill material, extending into the existing natural soil.

 

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Citations & References

In addition to any citations in the article above, a full list is available on request.

  • NYS DOH, RESIDENTIAL ONSITE WASTEWATER TREATMENT SYSTEMS HANDBOOK [PDF] (2012) - original source: http://www.ongov.net/health/env/documents/DesignHandbook10_24_2
  • WA DOH, BASIC PRINCIPLES of ONSITE SEWAGE [PDF] (1992) Washington State Department of Health, Office of Water, Washington D.C. USA
  • [1] US EPA ONSITE WASTEWATER TREATMENT SYSTEMS MANUAL [online copy, free] Top Reference: US EPA's Design Manual for Onsite Wastewater Treatment and Disposal, 1980, available from the US EPA, the US GPO Superintendent of Documents (Pueblo CO), and from the National Small Flows Clearinghouse.

    Original source http://www.epa.gov/ORD/NRMRL/Pubs/625R00008/625R00008.htm Onsite wastewater treatment and disposal systems, Richard J Otis, published by the US EPA. Although it's more than 20 years old, this book remains a useful reference for septic system designers. U.S. Environmental Protection Agency, Office of Water Program Operations; Office of Research and Development, Municipal Environmental Research Laboratory; (1980)
  • [2] "International Private Sewage Disposal Code," 1995, BOCA-708-799-2300, ICBO-310-699-0541, SBCCI 205-591-1853, available from those code associations.
  • [3] "Manual of Policy, Procedures, and Guidelines for Onsite Sewage Systems," Ontario Reg. 374/81, Part VII of the Environmental Protection Act (Canada), ISBN 0-7743-7303-2, Ministry of the Environment,135 St. Clair Ave. West, Toronto Ontario M4V 1P5 Canada $24. CDN.
  • [4] Manual of Septic Tank Practice, US Public Health Service's 1959.
  • [5] "Installers Manual for Conventional Onsite Domestic Wastewater Treatment and Disposal Systems", Department of Environmental Conservation, Division of Environmental Health Drinking Water and Domestic Wastewater Program, Alaska Department of Environmental Conservation, 1 Aug 2000, Anchorage Offices, 555 Coredova, Anchorage AK 99501, Tel: 907-269-7500. retrieved 17 July 2012, original source: http://dec.alaska.gov/water/wwdp/onsite/pdf/Certified_Installer%27s_Manual.pdf [Copy on file as Alaska_Certified_Installer's_Manual.pdf].
    Notice: [Quoting]

    This document contains information regarding the installation of onsite sewer systems for single-family and duplex residences. It must be used by Certified Installers and homeowners who are subject to 18 AAC 72. Additional requirements are included in 18 AAC 72. If there is a conflict between the provisions of this manual and 18 AAC 72, 18 AAC 80, or other state regulations, the regulations language controls. AEDC offices.
    The regulations cited above for Alaskans can be found at https://dec.alaska.gov/commish/newsroom/23-15-updated-onsite-wastewater-regulations/
  • [8] Test Pit Preparation for Onsite Sewage Evaluations, State of Oregon Department of Environmental Quality, Portland OR, 800 452-4011. PDF document. We recommend this excellent document that offers detail about soil perc tests, deep hole tests, safety, and septic design.
  • [9] Wells and Septic System, Alth, Max and Charlet, Rev. by S. Blackwell Duncan, $ 18.95; Tab Books 1992.
  • [17] Thanks to reader Michael Roth for technical link editing 6/29/09.
  • [19] Septic Tank/Soil-Absorption Systems: How to Operate & Maintain [ copy on file as /septic/Septic_Operation_USDA.pdf ] - , Equipment Tips, U.S. Department of Agriculture, 8271 1302, 7100 Engineering, 2300 Recreation, September 1982, web search 08/28/2010, original source: http://www.fs.fed.us/t-d/pubs/pdfimage/82711302.pdf.
  • The NSFC Products List has an excellent list of design manuals/modules National Small Flows Clearinghouse (NSFC) now (2019/12/13) hosted at http://www.nesc.wvu.edu/ Tel: 304-293-4191 e-mail info@mail.nesc.wvu.edu. telephone 800-624-8301
  • In addition to citations & references found in this article, see the research citations given at the end of the related articles found at our suggested

    CONTINUE READING or RECOMMENDED ARTICLES.

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