Septic soakaway bed / drainfield shape & layout guidelines:

This septic system design article discusses determination of a conventional drainfield shape or layout.

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.

Septic Field Shape or Layout Specifications: 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 (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.
• Local obstructions: boulders etc. Be sure to keep septic fields and trees or other sources of invasive roots well separated.
  
  See TREES or SHRUBS OVER THE SEPTIC FIELD or TANK

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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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-09-18 by Anonymous

thank!
But what if the mound is made with sand, then gravel. Will the sand be tamped before this or not? If so, how to do it correctly? Is it enough to spill sand with water or just tamp with an excavator bucket?

And is it possible to do without a junction box if there are 3 pipe lines of 20 feet each? How will it be correct?

On 2020-09-16 21:50:53.246179 - by (mod) -

Sasha

If the excavator digs the trench too deeply or unevenly and if the soil is not subsequently-compacted, the risk of subsidence as you describe is quite real. Moreso with flexible piping systems.

Properly the trench bottom is on virgin soil or on compacted soil or fill, along with being set on and surrounded by gravel (except in a "no-rock" system)

On 2020-09-16 by sasha

How to avoid sagging (deflection) of the pipe as a result of soil subsidence? This can happen with a new trench or mound system after backfilling with gravel and soil. I'm afraid the pipes will sag and the field will absorb unevenly gray water.

On 2019-09-04 - by (mod) -

If the drain line slopes 1/8" to 1/4" per foot AND is laid out so as not to have trouble from spots of too much slope it should be ok

For a distance that far - 280 ft - I'd expect the installer to have put some cleanouts along the route- good spots to take a look at what happens when you flush solids down the john.

On 2019-09-04 by Anonymous

My septic tank is 15' feet from house following leach is 280' feet away from Septic tank and farthest end of leach field is almost 7' foot drop in elevation. Wisconsin Leech field was built on slope. Downward end of field has a mound spanning the length of leach field. (Supposedly to deter runoff, but...)

On 2018-09-15 - by (mod) -

I don't know of a fixed number.

The maximum distance is going to depend on the site, size, layout, slope, and dips involved. After all your effluent has to drain by gravity so the slope is typically going to be a quarter of an inch or perhaps an eighth of an inch per foot.

On 2018-09-14 by Tony

What is the maximum distance a leach field can be from tank discharge

On 2018-06-15 by (mod) - How far from someones property ine can a septic drain field be

See SEPTIC CLEARANCE DISTANCES

On 2018-06-15 by Anonymous

How far from someones property ine can a septic drain field be

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Continue reading  at SEPTIC DRAINFIELD LOCATION or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• CLEARANCE DISTANCES, SEPTIC SYSTEM
• ODORS, SEPTIC or SEWER
• SEPTIC COMPONENT LOCATIONS
• SEPTIC D-BOX INSTALL, LOCATE, REPAIR
• SEPTIC SYSTEM DESIGN ALTERNATIVES - Advanced & alternative septic treatment methods
• SEPTIC SYSTEM DESIGN BASICS -
• SEPTIC DRAINFIELD INSPECTION & TEST - home
• SEPTIC DRAINFIELD LOCATION - home
• DRAINFIELD PIPE LOCATION, PRECISE
• EXCAVATE to LOCATE DRAINFIELD
• REASONS to FIND THE DRAINFIELD
• RECORDS to LOCATE the DRAINFIELD
• SURPRISING DRAINFIELD LOCATIONS
• UNLIKELY DRAINFIELD LOCATIONS
• VISUAL CLUES LOCATE the DRAINFIELD
• VISUAL CLUES LOCATE the SEPTIC TANK
• SEPTIC DRAINFIELD SIZE
• SEPTIC DRAINFIELD SHAPE
• SEPTIC TANK, HOW TO FIND
• SEPTIC VIDEOS - where the tank, D-box, and fields could or could not be found at a site

Suggested citation for this web page

SEPTIC DRAINFIELD SHAPE at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to SEPTIC SYSTEMS

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