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Excavation to install a deep trench septic field at a New York home (C) Daniel Friedman at InspectApedia.comDeep Trench Septic Drainfield Design

This article explains the design and construction specifications of a deep trench septic leachfield or soakaway bed or drainfield.

Septic drainfields, also called leach fields, absorption beds, soil absorption systems, soakaway beds, and leaching beds, perform the functions of septic effluent treatment and disposal in onsite wastewater treatment systems, conventionally called "septic systems".

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Deep Trench Septic Absorption Systems

Below we summarize the basics of a deep 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 drainfield trench, and places below the trench the critical biomat as well as other septic field design areas and considerations. (Source US EPA who in turn obtained the drawing from Ayres Associates)

a. Site Requirements for deep trench septic systems

These are used on sites where a usable layer of soil is overlaid by three to five feet of impermeable soil.

Septic drainfield trench cross section - USDA - DJF

b. Design Criteria for deep trench septic systems

(i) There shall be at least four feet of usable soil

beneath the impermeable layer.

(ii) The required length of absorption trench

is determined from TABLE 4A based upon percolation tests conducted in the underlying soil.

c. Materials Used for deep trench septic absorption systems

Deep trench septic systems use essentially the same materials as found at

CONVENTIONAL SEPTIC LEACH FIELDS

d. Construction Specifications for deep trench septic absorption systems

(i) Septic drainfield / soakbed trenches are excavated at least two feet into the usable layer

and backfilled with aggregate or coarse sandy material containing a low percentage of fines (very small soil particles) more permeable than the underlying material to a level 30 inches below the original ground surface.

(ii) An absorption trench system as described in

CONVENTIONAL SEPTIC LEACH FIELDS is constructed in the upper 30 inches of the excavation.

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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

    Also including as
    APPENDIX A: WHY DO COARSE SOILS HAVE LARGER PORE SIZE & LESS SURFACE AREA? & WHY DO FINER SOILS HAVE SMALLER PORE SIZE & MORE SURFACE AREA
    and
    Cogger, Craig G., APPENDIX B: SEPTIC SYSTEM WASTE TREATMENT in SOIL, (1987) Washington State University Cooperatie Extension, EB1475,
  • [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.

    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.
  • [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.
  • [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.
  • 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

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