GLAREUM.NET›Chapter 6›Drainage and water

6.4 Drainage trench

A French drain: a trench with a perforated pipe in clean stone, wrapped in fabric. It collects water from wet ground and carries it away to an outlet or a soakaway.

Cross-section of a drainage trench: fabric lines the trench, washed stone below and around a perforated pipe, the fabric folded over the stone, and topsoil on top. 350 mm 600 mm 4 Topsoil or turf about 250 mm 3 Fabric folded over non-woven geotextile 2 Drainage stone 16–32 mm washed 1 Perforated pipe 100 mm · holes down fall 0.5–1 %
Cross-section, drawn to scale. Parts are numbered from the pipe outwards. In a lawn the topsoil cap hides the drain; to also catch surface water, fill the stone up to the surface instead.

At a glance

Difficulty
2 of 4
Used for
Wet lawns and gardens, the uphill side of a building or path, collecting water from a wall drain (6.3)
Tools
Spade or trenching machine, string line, spirit level or laser, wheelbarrow, knife for fabric
Parts
  1. 1  Perforated pipe
  2. 2  Drainage stone
  3. 3  Geotextile
  4. 4  Topsoil cap

What you need per metre of trench

MaterialSizeTypeAmount
Drainage stone 16–32 mm Washed, rounded or crushed 175 kg
or 4–20 mm Washed, crushed
Geotextile – Non-woven, about 270 g/m² (8 oz) 1.7 m²
Drain pipe – Perforated, at least 100 mm 1 m

Why clean stone?

16–32 mm

Large, clean gaps let water reach the pipe from all sides and carry it along even if the pipe silts up a little.

fabric

Without a filter, fine soil washes into the stone and the trench slowly stops working. The fabric wrap is the filter (6.6).

Step by step

  1. 1

    Plan the route to a free outlet.

    The water must go somewhere lower: a ditch, a slope where the pipe can come out in the open, or a soakaway (6.5). Mark the line with string.

    String line along the route
  2. 2

    Dig the trench at least 350 mm wide and 560–600 mm deep.

    Give the bottom a steady fall of 0.5–1 %, which is 5–10 mm per metre. Check with a level as you go.

    The dug trench
    The dug trench Illustration
  3. 3

    Line it with fabric and lay 100 mm of washed stone.

    Leave enough fabric at the top on both sides to fold over and overlap by at least 300 mm.

    Fabric lining with the first stone
  4. 4

    Lay the pipe with the holes facing down.

    Water rises into the pipe from below and runs away along its bottom.

    Pipe on the stone, holes down
  5. 5

    Cover the pipe with 100–150 mm of stone and fold the fabric over.

    Lap one side over the other. Fill the rest, at least 230 mm, with topsoil and turf, or with more stone if the trench should also catch surface water.

    Fabric folded over before the topsoil
Specifications
PartSizeValueHow
Trench–≥ 350 mm wide, ≥ 560 mm deepSteady fall of 0.5–1 %.
1 Pipe≥ 100 mmholes downPerforated, on the bedding stone.
2 Stone16–32 mm100 mm under, 100–150 mm overWashed, clean.
3 Geotextile–top lap ≥ 300 mmLines the whole trench, folded over the stone.
4 Cap–≥ 230 mmTopsoil and turf, mulch or more stone.

Stone size

The sources name clean ¾–2 in rock (19–50 mm) and crushed No. 57 stone (4.75–25 mm). 16–32 mm sits between the two, and 4–20 mm matches No. 57.

Depth

One US federal guide puts a French drain at least 18 in (450 mm) below ground, in a trench at least 22 in (560 mm) deep. Simple yard drains can be shallower, 200–600 mm, where the water is near the surface.

Sources

  1. US DOE Building America Solution Center (PNNL): swales, drains and site grading: “Dig trench at least 22 inches deep at minimum and 14 inches wide”; “Lay 4 inches of clean rock (¾ to 2-inch)”; “4 to 6 inches over the pipe”; “perforations down”; 8 oz fabric lapped over the top; 9 in or more of stone, topsoil or mulch on top
  2. Washington State University Extension: improving drainage: “the trench should be sloped between 0.5% and 1%”
  3. Delaware DNREC: French drain for a yard: simple yard drains are “shallow (8 to 24 inches deep)”
  4. Michigan DOT construction manual: underdrains: “The geotextile blanket should overlap the top of the trench a minimum of 1 foot”
  5. WSDOT Standard Specifications, Division 7: “the perforated pipe shall be laid with the perforations down”
  6. Penn State Center for Dirt and Gravel Road Studies: stone underdrains: fabric folded to a double layer on top of the drain
  7. Ask Extension (Cooperative Extension): French drain stone: “crushed #57 stone (about 3/4" to 1" in size)”
  8. Maryland Department of the Environment: stormwater materials: No. 57 stone grading, at most 5 % fines
  9. FHWA-HRT-15-034, open-graded aggregates: No. 57 stone loose 95.4 lb/ft³ (1.53 t/m³)

Amounts per metre: stone 0.35 m wide × 0.35 m deep (100 under, 100 pipe, 150 over) = 0.123 m³, minus the pipe (0.008 m³) ≈ 0.115 m³ × 1.53 t/m³ ≈ 175 kg, loose. Fabric: bottom, two sides and the top with a 300 mm lap ≈ 1.7 m². Order about 15 % extra stone for settling and waste (FHWA loose to dense is about 14 %).

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