Trench Width for Soil Absorption Systems With Filter Material Fact Sheet
This guidance document is advisory in nature but is binding on an agency until amended by such agency. A guidance document does not include internal procedural documents that only affect the internal operations of the agency and does not impose additional requirements or penalties on regulated parties or include confidential information or rules and regulations made in accordance with the Administrative Procedure Act. If you believe that this guidance document imposes additional requirements or penalties on regulated parties, you may request a review of the document.

Form #:  12-044
Guidance Documents
Revised: 12/12/16


In accordance with Title 124 - Rules and Regulations for the Design, Operation, and Maintenance of Onsite Wastewater Treatment Systems trenches must not be less than 18 inches or more than 36 inches wide for pipe laterals and no more than five feet wide for chambers. For soil absorptions systems where filter material is used, any trench wider than 36 inches for pipes and five feet for chambers will be considered a bed and have the required square footage of the soil absorption area or drainfield trench increased by the appropriate absorption bed multiplication factor listed in the following table (Table 14.1 of Title 124).

Soil Absorption Bed Multiplication Factor

Width of Bed (feet)
Multiplier
> 3 to 10
1.25
> 10 to 15
1.33
> 15 to 20
1.50
> 20
*Unacceptable

Note: The maximum width of a bed is limited to 20 feet

Calculating Effective Width of a Soil Absorption System Trench
  • Plastic Pipe
    • The width for calculating the trench bottom area shall be the width of the filter material covered trench.
  • Plastic Chambers
    • Plastic chambers meeting the requirements for gravelless chambers installed without filter material may be used for effluent distribution in a trench or bed with filter material and may be installed directly on the trench bottom.
    • The width for calculating the trench bottom area shall be the width of the filter material covered trench.
  • Bundled Expanded Polystyrene
    • Bundled expanded polystyrene synthetic aggregate contained in high-strength polyethylene netting in cylinders 9 to 12 inches in diameter specifically designed for use without filter material and meeting the requirements for bundles installed without filter material may be used for effluent distribution in a trench or bed with filter material and may be installed directly on the trench bottom.
    • The width for calculating the trench bottom area shall be the width of the filter material covered trench.
The filter material must completely encase the distribution pipes, chambers or synthetic aggregate bundles to a depth of at least two inches extending uniformly to the width of the trench. The filter material must also be covered with a permeable layer such as a Geotextile fabric, a two-inch layer of hay or straw or other similar permeable material that prevents the movement of fine soil particles into the filter material,.