Gravelless Chambers: Determination of Inside Bottom Width, Calculation of Effective Width, and Calculation of Soil Absorption Area
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Form #:  17-023
Guidance Documents
Revised: 11/2/17
SizeFile Name
41 KB 17-023 Gravelless Chambers.pdf


This guidance is to clarify the NDEQ’s accepted method for determining the “inside bottom width” of gravelless chambers, calculating the “effective width” of various makes and models of chambers, and calculating the soil absorption area based on the effective width.

1. The determination of inside bottom width and effective width for gravelless chambers is used when filter material (crushed rock, gravel, tire chips, etc.) IS NOT USED for backfill around the chambers, and the chambers are backfilled with the native soil material from the trench area. See Title 124, Chapter 14, Section 018.02B.
2. When filter material IS NOT USED in the trench, the “effective width” of the chambers is equal to the actual “inside bottom width” of the chambers multiplied by 1.5.
3. Calculation of the soil absorption area is then determined by multiplying the “effective width” of the trench by the total length of all trenches. For example:

Inside Bottom Width of Chamber
Multiply
By
1.5
Effective
Width of Chamber
Number
Of
Trenches
Length
Of Each
Trench
Total
Trench
Length
Soil
Absorption
Area
29 inches
or
(2.42 feet)
X 1.5 =
43.5 inches
or
(3.63 feet)
3
60 feet
(3 x 60)

180 feet
3.63 x 180 = 653.4 square feet


4. The soil absorption area calculated above must be at least as large as the required soil absorption area, as determined in Title 124, Chapter 14, Section 019. The required soil absorption area is based on the design flow (gallons per day) and the soil percolation rate (minutes per inch).
5. When filter material is used in the trench backfill, the actual trench width is used to calculate the soil absorption area. See Title 124, Chapter 14, Section 017.03.

In order to standardize gravelless chamber measurements and avoid confusion on field measurements, the NDEQ Onsite Wastewater Unit has conferred with the manufacturer’s representatives on a list of standard gravelless chamber width dimensions. These dimensions are to be used for the Inside Bottom Width of each make and model of chamber listed below. For any chambers not listed, please contact the NDEQ Onsite Wastewater Unit.


Gravelless Chamber
Make / Model
Inside
Bottom Width
Inches (Feet)
Effective
Width
Inches (Feet)
Infiltrator Quick4 Standard
27” (2.25’)
40.5” (3.375’)
Quick4 High Capacity
29” (2.42’)
43.5” (3.625’)
Quick4 Equalizer 36
19” (1.58’)
28.5” (2.375’)
Quick4 Equalizer 24 HD
13” (1.08’)
19.5” (1.625’)
Quick4 Equalizer 24 Low Profile
12” (1.00’)
18” (1.00’)
Infiltrator Quick4 Plus (Std.,HC, & LP)
29” (2.42’)
43.5’ (3.625’)
Quick4 Plus Equalizer 36 LP
19” (1.58’)
28.5” (2.375’)
Infiltrator High Capacity H-20
29” (2.42’)
43.5’ (3.625’)



ADS Arc 18
13” (1.08’)
19.5” (1.625’)
Arc 24
19” (1.58’)
28.5” (2.375’)
Arc 36 (Std., HC & LP)
29” (2.42’)
43.5’ (3.625’)
BioDiffuser 11” Standard
27” (2.25’)
40.5” (3.375’)
BioDiffuser 14” HC & 16” HC
29” (2.42’)
43.5’ (3.625’)
Bio 2
12” (1.00’)
18” (1.00’)
Bio 3
19” (1.58’)
28.5” (2.375’)



If the inside bottom width of the chambers is 29 inches X 1.5 (credit for using chambers without filter material) = 43.5 inches, divide by 12 to get feet = 3.625 feet. 100 (total trench length installed) X 3.625 = 362.5 total effective trench bottom area.