Tutorial - Soil Absorption Systems

Common Drain Field Configurations

There are many different styles of drain fields ("soil absorption systems").  Some common configurations are presented below.

Leach Trenches

Leach trenches are shallow trenches. Typically, the trenches are less than 4-ft deep.  The bottom is filled with leach stone and a perforated pipe delivers flow along the entire length of the trench.  Learn more about Leach Trench Construction and why this is a preferred method for wastewater disposal.

Leach Field or Bed (a.k.a. Drain Field)

This is a shallow bed of leach stone, about 1-ft thick, with a network of perforated pipes installed in the top of the leach stone.  A layer of peastone is placed over the entire bed to prevent topsoil from mixing into the leach stone.


 Leach Pit

A single leach pit consists of a perforated concrete (or plastic) chamber placed on a layer of leach stone in a pit, about 3 times as wide as the chamber. The pit is then filled with leach stone up to the top of the chamber. A 3-inch layer of pea stone is then placed over the leach pit and leach stone to prevent topsoil from mixing with the leach stone.  When there is more than one leach pit, a distribution box (D-box) is used to split the flow to the different pits.


Leach Galleries (a.k.a. chambers)

These are similar to the leach pit except that galleries include a series (row) of chambers connected end-to-end.  Similar to the leach pit, the chambers are placed over a bed of leach stone, have leach stone installed along the sides to the top of the chambers, and peastone placed over the leach stone.  There are also gravel-less chamber systems in which the chambers are buried in a bed of clean sand without gravel.

Learn more about the installation and advantages of Leach Galleries & Chambers.


Common Features of Soil Absorption Systems

All of the above systems work in a similar fashion to disperse flow into the soil over a large area at a reasonable rate.  Each system provides further treatment of the wastewater by passing the flow through a naturally developed "bio-mat" in the soil below the system.  The "bio-mat" functions to filter or breakdown pollutants and kill pathogens.  All drain fields require some form of venting to function well.  Usually, venting occurs through the building roof vent.  In many cases, additional venting may be required in the yard, directly attached to the soil absorption system.

See also Vents.

Other options for soil absorption systems are presented under Alternative Drain Fields.

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