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Septic Drainfield Layout & Shape InspectAPedia®  -

• How to determine the septic drainfield shape or layout
• Septic system design guidelines

This chapter discusses determination of a conventional drainfield 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. Advanced septic treatment methods are discussed in separate chapters. 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 reviewers welcomed.

This document is a chapter of Inspecting, Testing, & Maintaining Residential Septic Systems an online book on septic systems.

© Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

How to Determine Septic Absorption Field Shape or Layout

FIELD SHAPE - Septic Leach Field or Septic Absorption Field Design

The 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 not "up and down" the hill (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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See the following septic design articles for more details:

• Septic System Design:articles on how to design a septic system
• Design Basics for Septic Systems: Choosing Septic Tank Size, Leach Field Size - basic septic system volume and absorption system design guides.
• Capacity: Determining Needed Septic Capacity - Septic Tank Size vs. Usage: What septic size is required?
• Capacity: Determining Needed Septic Capacity - Based on Water Usage: Typical residential water usage for people, appliances, and activities.
• Clearances & Distances from septic system components to other site features, property boundaries, waterways, etc.
• Distribution Box design, inspection, testing, and maintenance of the septic system D-box
• Leach Field or Septic Drainfield Size specifications for leach fields, absorption beds, gravelless systems, seepage pits
• How Big Should the Leach Field Be?: a practical example using sample calculations and a table of soil percolation rate vs. field size
• Sketches of the Septic System Components for Private Sewage Disposal Systems - Septic Drawing Library


• Design Alternatives for Septic Systems: Advanced & Alternative Septic System Designs for Onsite Wastewater Disposal - design engineers, consultants, products, books for special problem situations, difficult soils, old system repairs. Aerobic septic systems, chemical, composting, incinerating & waterless toilets, Evaporation-Transpiration (ET) Septic Systems, septic media filters, greywater systems, holding tank septic systems, mound septics, raised bed septics, pressure dosing septic systems, sand bed filters, peat beds, constructed wetlands, septic disinfection systems.
  
  
• The Plumbers's Handbook, Howard C. Massey, Craftsman Book Company; Rev Sub edition (April 1998), ISBN-13: 978-1572180567 includes septic system design basic sketches and specifications.

More Information on Building Diagnostic Inspections and Repairs

• The Septic System Information Website home page for this topic
• Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
• The Home Buyer's Guide to Septic Systems
• Septic Tank Pumping Guide: When, Why, How to pump the septic tank
• Home & Outdoor Living Water Requirements
• Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
• How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
• Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
• Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.

Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include

• Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
• Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
• Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
• Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)

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