Septic Drainfield Layout & Shape

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Septic soakaway bed / drainfield shape & layout guidelines: This septic system design article discusses determination of a conventional drainfield shape or layout. This article is a section of our online book SEPTIC INSPECTION & TEST GUIDE whose chapters are shown at the left of this page. Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and comments from readers are welcomed. Contributors are listed at the end of each article.

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How to Determine Septic Absorption Field Shape or Layout

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

We discuss several different conventional soil absorption systems: absorption fields: conventional trench, deep trench, shallow trench, cut-and-fill, and gravelless septic systems. Then we discuss septic absorption beds, and seepage pits. Basic septic system designs are discussed beginning at SEPTIC SYSTEM DESIGN BASICS. Advanced & alternative septic treatment methods are discussed beginning at SEPTIC SYSTEM DESIGN ALTERNATIVES.

Septic Field Shape or Layout Specifications: Septic Leach Field or Septic Absorption Field Design

Typical drainfield layout - USDA - DJFThe shape or footprint, of a septic system drainfield or leaching bed is determined by the following considerations:

  • The size and shape of the land available in which septic components can be placed.
  • The slope of the land in the area where septic components will be placed. For example, septic drainfield trenches generally run parallel to the slope of a hill (Sketch at left - USDA) not "up and down" the hill (also as shown in the illustration at the top of this page; if drainfield lines were to point straight down hill, all of the effluent would rush to and leak out of the end of the trench).
  • The soil characteristics (for example the soil percolation rate - the rate at which the soil can absorb septic effluent)
  • The anticipated average and maximum daily wastewater flow (how much effluent needs to be disposed-of)
  • The type of septic system (since different effluent handling methods need different total effluent disposal areas and different total linear feet, such as comparing a conventional a drainfield trench with a gravel-less system
  • The exact locations of property boundaries, and the locations of any nearby wells, streams, lakes, driveways, buildings, or any other site features that require a separation distance between septic system components and that site feature.
  • The local building codes which may specify certain septic component distances, set-backs, capacities, as well as the requirement for a reserve area on the site to permit future septic system expansion, repair, or replacement.

Once we know the total area of effluent disposal system needed, such as length in feet of gravel-filled perorated pipe trenches (or galleys or gravelless or whatever) then the septic system installer will excavate to prepare the drainfield in a pattern or shape that divides that length into sub-lengths of parallel (or other) trenches or pits or whatever, and the location, routing, etc. of these will be determined by the lot size and shape so that the necessary septic system components fit AND are separated by the reacquired distances from one another and from site boundaries and other site features like the well or buildings, driveways, and the like.

The specific shape and dimensions for a specific property, soil characteristics, and septic system type can't really be specified precisely here since we know the detailed answers to the septic system drainfield design considerations listed above. But we have posted addition articles on septic system drainfield design that give the general septic system specifications that should be helpful.


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