Disposal Fields: Trenches, Mounds and Chambers
The disposal field is where treated effluent is dispersed into the soil. Its design depends entirely on the soil found in the test pit.
29 questions in this section
- What is an in-ground trench system?
- How wide and deep are the trenches?
- How long can a single trench be?
- How far apart are the trenches?
- What kind of stone goes in the trench?
- What slope should the perforated pipe have?
- Why is geotextile used?
- Why must distribution lines follow the contour?
- What is a distribution box?
- What if the distribution box shifts out of level?
- What is a raised mound system and why would I need one?
- What is scarification?
- What has to be removed before mound sand goes in?
- How is the imported sand placed?
- What are the specifications for imported sand?
- Why does the sand quality matter so much?
- What is D10 and what does it tell you?
- What is the uniformity coefficient?
- What is an apron and a taper on a mound?
- How much soil goes on top of a mound?
- How much cover goes over a trench field?
- Why does the cover have to breathe?
- What is a concrete leaching chamber?
- How are concrete leaching chambers installed?
- Why does the sand under concrete chambers need to be compacted?
- Where is the limiting factor distance measured from on a concrete chamber?
- What is a plastic infiltrative chamber?
- Do plastic chambers reduce the size of my field?
- Are there limits on plastic chamber runs?
What is an in-ground trench system?
The simplest design: trenches dug into the original ground, with clean crushed rock, perforated distribution pipe laid and graded on the rock, and more rock over the pipe. It is permitted in Category A, B or C soil.
How wide and deep are the trenches?
Minimum trench width 450 mm (18 inches). Maximum trench depth 900 mm (36 inches).
How long can a single trench be?
A maximum of 15 metres (50 feet).
How far apart are the trenches?
1.5 metres (5 feet) centre to centre for pipe and stone systems.
What kind of stone goes in the trench?
Clean crushed stone sized 10 mm to 50 mm (half an inch to 2 inches), with 200 mm (8 inches) under the pipe and 50 mm (2 inches) over it.
What slope should the perforated pipe have?
50 mm per 15 metres, which is 2 inches per 50 feet.
Why is geotextile used?
Geotextile placed over the crushed rock before backfilling helps prevent fine soil from infiltrating and clogging the trench gravel. Its use is recommended.
Why must distribution lines follow the contour?
Distribution lines must be laid parallel to the contour, across the natural slope, and perpendicular to groundwater flow. This spreads effluent evenly and stops it from running downhill and breaking out at the surface.
What is a distribution box?
A concrete or plastic box with one inlet and several outlets, used instead of a level header to ensure all distribution lines receive an equal share of effluent. It must be installed level, on a gravel base, with a baffle in front of the inlet, and it is recommended that it be insulated against frost.
What if the distribution box shifts out of level?
Adjustable flow equalizers may be used to ensure equal distribution even if the box does not remain perfectly level.
What is a raised mound system and why would I need one?
A mound is a disposal field raised above original grade with imported sand. It is required when limiting factors are present - bedrock, groundwater or a seasonal high water table, impermeable soil such as clay, or soil that drains too fast - and there is not enough natural separation below the pipe.
What is scarification?
Loosening and breaking up the soil in a way that prevents smearing of the surface while maintaining soil structure. The area under a proposed mound must be scarified parallel to the slope, using a cultivator, mould board plough or chisel plough. No equipment may be driven over the scarified area afterwards.
What has to be removed before mound sand goes in?
Stumps must be removed, and sod and root layers stripped, before imported sand is placed.
How is the imported sand placed?
In lifts not exceeding 300 mm, which helps ensure the sand does not settle after placement. It should be placed level with the top of the proposed trench, and the trench then excavated into the sand with the bottom level in both width and length.
What are the specifications for imported sand?
80 to 100 percent passing 9.5 mm and 4.75 mm sieves; no more than 8 percent passing the 75 micron sieve in the sand fraction, with less than 5 percent recommended; D10 between 0.15 and 0.50 mm; uniformity coefficient of 1 to 6.0; and permeability between 5x10-5 and 6x10-4 m/s.
Why does the sand quality matter so much?
Material with too many fines such as clay and silt cannot accept effluent without severe clogging. Material that is too coarse lets effluent pass through too fast to be properly treated. Sandstone typically does not qualify. Removing and replacing bad sand later is time-consuming and expensive.
What is D10 and what does it tell you?
The particle size that 10 percent of the sample by weight is finer than. The smaller the D10, the more silt and fine sand is in the material, which indicates lower permeability.
What is the uniformity coefficient?
Cu indicates the range of particle sizes in the sand. A higher Cu generally means lower permeability, because small particles fill the gaps between medium ones and medium particles fill the gaps between large ones.
What is an apron and a taper on a mound?
The apron is the sand buffer extending beyond the field: 6 metres (20 feet) on all downslope ends and 3 metres (10 feet) on all other sides. The taper slopes from the edge of the apron down to original ground at a 4:1 horizontal to vertical ratio. Together they minimize effluent breaking out at the sides.
How much soil goes on top of a mound?
0.3 to 0.6 metres (1 to 2 feet) of sandy loam or topsoil above the top of the pipe, with the top 100 mm at minimum being topsoil finished with sod or seed. That is in addition to the 50 mm of gravel over the pipe.
How much cover goes over a trench field?
A minimum of 30 cm (12 inches) of soil cover that allows air to permeate, including at least 10 cm (4 inches) of topsoil, finished with sod or seed to prevent erosion and aid evapotranspiration. The cover should not exceed 60 cm (24 inches) unless alternative aeration is provided.
Why does the cover have to breathe?
The treatment process in the soil is aerobic and needs oxygen. Too much cover, or cover made of dense material, smothers it. That is why the Guidelines cap the cover thickness.
What is a concrete leaching chamber?
A bottomless concrete unit used in place of perforated pipe and stone. The size approved for use in New Brunswick is 1.2 m x 2.4 m x 0.6 m deep. A perforated pipe must run through the length of the units and be capped at the end.
How are concrete leaching chambers installed?
Level, on compacted sand meeting the imported sand specification, with a minimum of 30 cm (12 inches) of coarse gravel sized 19 mm to 76 mm around the outside. Rows must be at least 6 metres (20 feet) centre to centre, the minimum field is 6 units, and the trench is at least 2 metres (6 feet) wide.
Why does the sand under concrete chambers need to be compacted?
Because concrete units are heavy. Without compaction they settle unevenly, which destroys the level distribution the system depends on.
Where is the limiting factor distance measured from on a concrete chamber?
From the bottom of the unit, not from the pipe invert. The bottom of the unit must be at least 1 metre (3.3 feet) from limiting factors.
What is a plastic infiltrative chamber?
An arched plastic chamber that replaces pipe and stone. Approved units in New Brunswick include Infiltrator Water Technologies Quick4 Standard and High Capacity, H-10 Standard and High Capacity, Standard BioDiffuser, and ARC 36 Standard and High Capacity. The approved list is subject to change.
Do plastic chambers reduce the size of my field?
Yes. Each approved infiltrative chamber receives a 50 percent reduction in the total length required for a perforated pipe and stone distribution system.
Are there limits on plastic chamber runs?
Yes. No single line may exceed 15 metres (50 feet), lines must run parallel to the contour, and the sand used must meet the manufacturer's specifications. The distance from the bottom of the trench to a limiting factor must be at least 1.0 metre (3.3 feet).
Have a question about your own property? General answers only go so far — your soil, slope, water table and setbacks are specific to your lot. Call (506) 233-6272 or send us the details.
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