Statutes, codes, and regulations
Rhode Island Administrative Code
Title 250 - Department of Environmental ManagementChapter 150 - Water ResourcesSubchapter 10 - Wastewater And Storm Water
Part 6 - Rules Establishing Minimum Standards Relating to Location, Design, Construction and Maintenance of Onsite Wastewater Treatment Systems
Section 250-RICR-150-10-6.34 - Dispersal Trenches

250 R.I. Code R. 250-RICR-150-10-6.34

Download
PDF
Current through December 26, 2024
Section 250-RICR-150-10-6.34 - Dispersal Trenches
A. Effective Leaching Area - The effective leaching area shall be determined by the depth of stone meeting the requirements of § 6.33(K) of this Part that is placed below the distribution line in the trench and the width of the trench. The effective leaching area of dispersal trenches containing one half of a foot (1/2') of stone below the pipe invert shall be the total bottom area. Credit will be allowed for added sidewall absorption area gained by increasing the depth of stone in the trenches. Such credit shall be determined in accordance with § 6.34(B) of this Part which gives the square footage allowed per lineal foot of trench as the depth of stone increases. The maximum depth of stone allowed is one and one half feet (1 1/2'). The bottom of the dispersal trench shall have a maximum width of three feet (3'). The maximum depth of stone below the pipe invert shall be one half of a foot (1/2') when either of the following occur:
1. The seasonal high groundwater table is within two (2') to four feet (4') of the original ground surface; or
2. A restrictive layer is within four (4') to six feet (6') of original ground surface.
B. Effective Leaching Area

Depth of Stone Below Invert (Feet)

Area Allowed per Linear Foot of Trench (Sq. ft/ft)

24" Wide Trench

30" Wide Trench

36" Wide Trench

0.5

2.0

2.5

3.0

1.0

2.7

3.2

3.7

1.5

3.2

3.7

4.2

C. Distribution Lines
1. That portion of the distribution line from the distribution box to the beginning of the dispersal trench shall be a minimum of two feet (2') in length, four inch (4") diameter SDR 35 PVC, imperforated and laid with firmly connected watertight joints.
2. The invert of the distribution lines in the trench shall be two inches (2") lower than the invert of the outlet of the distribution box. The distribution lines in the trench shall be level. The first foot of the distribution line in the trench shall be imperforated SDR 35 PVC. Beyond the first foot, the distribution lines in the trench must consist of SDR 35 perforated PVC pipe with a minimum diameter of four inches (4"), or an equivalent pipe approved by the Director. The perforations shall be evenly spaced in two (2) rows, one (1) on each side of center, located at thirty degrees (30°) off vertical center in the lower half (1/2) of the pipe. The perforations shall be no smaller than three eighths of an inch (3/8") and no larger than five eighths of an inch (5/8") in diameter. Pipe joints shall be firmly connected. The ends of all distribution lines shall be inter-connected.
3. The maximum length of a dispersal trench shall be as follows:
a. Without dosing - fifty feet (50');
b. With a tipping distribution box - seventy-five feet (75'); or
c. With a pump - one hundred feet (100').
D. Stone - The stone surrounding the distribution lines shall meet the requirements of § 6.33(K) of this Part. The stone shall cover the full width of the trench, extend to the proper design depth, and extend two inches (2") above the top of the distribution lines. The stone shall be covered with a layer of synthetic filter fabric that meets the requirements of § 6.33(L) of this Part.
E. Leachfield Construction Where the Invert of the Distribution Lines is Below Original Grade (See § 6.65 of this Part, Figure 7).
1. The soil between the dispersal trenches shall remain undisturbed.
2. The Director may approve designs where the soil within the entire area of the leachfield is removed if the applicant shows that trench excavation is impractical, for example due to the presence of excessive boulders or stumps. If any B horizon soil remains, only tracked vehicles shall be allowed within this area to avoid compacting the soil. If the soil within the entire area of the leachfield is removed, the soil shall be replaced with properly compacted gravel meeting the requirements of § 6.33(M) of this Part to an elevation that will be two inches (2") above the top of the distribution lines. The trenches shall be excavated out of the compacted gravel.
3. A six inch (6") layer of gravel meeting the requirements of § 6.33(M) of this Part shall be placed below the stone in the dispersal trench. Where the bottom of the stone lies on or within a soil horizon that meets the description of Soil Category 1 from § 6.16(K) of this Part and such horizon is at least six inches (6") thick below the stone, the six inch (6") gravel layer is not necessary. However, if this Soil Category 1 horizon is described as extremely cobbly, the six inch (6") gravel layer shall be required.
4. The minimum distance between walls of adjacent dispersal trenches shall be five feet (5'), however, greater distances are recommended.
F. Leachfield Construction Where the Invert of the Distribution Lines is Above Original Grade. (See § 6.66 of this Part, Figure 8)
1. The leachfield and five feet (5') beyond the leachfield shall be stripped of all topsoil (A horizons). In order to avoid compaction of the B soil horizon, only tracked vehicles shall be allowed within this area.
2. Properly compacted gravel that meets the requirements of § 6.33(M) of this Part shall be placed throughout the excavation to an elevation that will be two inches (2") above the top of the distribution lines. Dispersal trenches shall be excavated out of the compacted gravel. There shall be a minimum six inch (6") gravel base layer meeting the requirements of § 6.33(M) of this Part below the stone.
3. The maximum depth of stone below the distribution line invert shall be one half of a foot (1/2'); and
4. The minimum distance between walls of adjacent dispersal trenches shall be ten feet (10').
G. Leachfield Construction on Sloping Sites - Where the dispersal trenches are to be constructed such that the invert of the distribution lines in the trenches will not all be at the same elevation, the invert of the distribution lines shall be below the original grade, the distribution lines in the trenches shall be laid level, and the leachfield shall be constructed in accordance with the following (See § 6.67 of this Part, Figure 9):
1. The distribution box shall provide equal dosing to each dispersal trench;
2. A tipping distribution box or pump shall be used;
3. Leachfield trenches shall be parallel to the contours of the existing grade where possible;
4. The ends of the distribution lines shall be connected by a relief line that is imperforated, SDR 35 PVC laid with watertight joints that is of the same diameter as the perforated pipe that it connects;
5. The minimum distance between walls of adjacent dispersal trenches shall be ten feet (10');
6. Gravel shall be placed below the stone in accordance with § 6.34(E)(3) of this Part;
7. Each dispersal trench must meet the adjacent side slope requirements of § 6.33(Q) of this Part;
8. The soil between the dispersal trenches shall remain undisturbed. If the presence of boulders or other obstacles make trench excavation impractical, the OWTS shall be constructed in accordance with §§ 6.34(E) or (F) of this Part; and
9. The Director may require that OWTSs with a design flow exceeding six hundred ninety (690) gallons per day submit additional information regarding wastewater loading and groundwater flow to ensure OWTS effectiveness on sloping sites.
H. Location Under Traffic Areas - The leachfield for a dispersal trench system shall not be paved or subject to vehicular traffic, including parking.
I. Summary of Dispersal Trench Construction Details:

Maximum length per line without dosing

50 feet

Maximum length per line with tipping distribution box

75 feet

Maximum length per line with pump

100 feet

Minimum diameter of distribution lines

4 inches

Maximum width of dispersal trench bottom

3 feet

Minimum distance between walls of adjacent trenches

5 feet/10 feet (Note 1)

Minimum cover over invert of distribution lines

1.5 feet

Maximum cover over invert of distribution lines

2.5 feet

Note 1: Ten feet (10') for those OWTSs on sloping sites and for those OWTSs where the invert of the distribution lines is above original grade

250 R.I. Code R. 250-RICR-150-10-6.34

Amended effective 11/25/2018
Amended effective 12/28/2021
Amended effective 7/1/2022