What is a "cut and fill" septic drainfield and is it constructed?
Here, based on expert sources cited at the end of this article we [DF] explain the design and give construction specifications for this type of septic system effluent absorption bed.
More generally, 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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Below, based on the New York State septic code
NEW YORK STATE 75-A.8 SUBSURFACE TREATMENT of EFFLUENT
we summarize the basics of a cut & fill septic drainfield design.
A cut and fill septic system is an absorption trench system installed on sites where impermeable soil overlays a permeable soil.
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)
Cut and fill septic systems may be used where all the following conditions are found:
(i) A soil with a percolation rate slower than 60 minutes per inch,
such as clay or clay loam, overlays a usable soil with a percolation rate faster than 60 minutes per inch;
(ii) At least three feet of usable soil is available beneath the tight soil;
(iii) All minimum vertical and horizontal separation distances can be maintained as described
at CLEARANCE DISTANCES, SEPTIC SYSTEM.
(i) It shall provide for the removal of the overlaying unusable soil and replacement by soil having a percolation rate comparable with the underlying soil.
(ii) An absorption trench system is designed as described
in CONVENTIONAL SEPTIC LEACHFIELDS.
(iii) The required length of septic effluent absorption trench is based upon the percolation of the underlying soil or the fill material, whichever has the slower percolation (lower permeability).
Cut and fill trench septic systems use essentially the same materials as found
at CONVENTIONAL SEPTIC LEACH FIELDS
(i) The cut and fill septic system trench area excavated and filled must provide at least a five foot buffer
in each direction beyond the trenches.
(ii) The material placed above the cut and fill septic trenches
shall have a percolation rate faster than 60 minutes per inch.
(iii) Original surface material
shall not be used as backfill above the cut and fill septic system trenches.
(iv) The surface area of the cut and fill septic system
must be mounded and graded to enhance the runoff of rainwater from the system and seeded to grass.
...
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SEPTIC DRAINFIELD DESIGN - CUT & FILL at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.
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