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6.6 Filter layer

A layer between fine soil and coarse drainage stone. It lets water through but holds the soil back, so the stone does not silt up and stop working.

Two cross-sections side by side. Left: fine soil, then sand, then fine grit, then coarse stone. Right: fine soil, then a geotextile, then coarse stone. Arrows show water passing upward through the layers into the stone. A · granular B · geotextile 3 Drainage stone 16–32 mm washed 2 Filter A: 4–8 grit on 0–4 sand B: non-woven geotextile 1 Fine soil silt, clay, fine sand
Two ways to build a filter, not to scale. Water flows from the soil into the stone; the filter keeps the soil grains behind.

At a glance

Difficulty
2 of 4
Used in
Drainage trenches (6.4), drainage against walls (6.3), soakaways (6.5), permeable paving on silty ground (6.2)
Tools
Rake, shovel, knife for fabric, straight board
Parts
  1. 1  Fine soil
  2. 2  Filter (granular or fabric)
  3. 3  Drainage stone

What you need per m² of filter

MaterialSizeTypeAmount
B · Geotextile – Non-woven, opening size to suit the soil 1 m² + 300 mm laps
or A · two sand and grit layers:
A · Filter sand 0–4 mm Washed, ≤ 5 % fines 160 kg (100 mm)
A · Filter grit 4–8 mm Washed 140 kg (100 mm)

Why the 4–5 × rule?

fine enough

The gaps in the filter must be small enough to stop the soil. Its finest 15 % of grains (D15) may be at most 4–5 times the size of the soil's coarse grains (D85). Then the coarse soil grains jam in the gaps and hold the rest back.

coarse enough

The filter must also pass water much faster than the soil. Its D15 should be at least 4–5 times the soil's D15. That makes it roughly 16–25 times more permeable.

What holds what with typical gradings

Finer sideCoarser sideHolds?Check
Silt or clay0–4 mm sandYessand D15 ≈ 0.15–0.4 mm ≤ 0.7 mm
0–4 mm sand16–32 mm stoneNo16 mm > 4 × 1.2–2.4 mm
0–4 mm sand4–8 mm gritYes≈ 4 mm ≤ 4 × 1.2–2.4 mm
4–8 mm grit16–32 mm stoneYes16 mm ≤ 4 × ≈ 7 mm
Any soilGeotextileYes, if sizedopening ≤ B × soil D85

Sand values come from a standard concrete-sand grading (source 5). Grit and stone values are typical for those size names. Gradings vary, so ask your supplier for the sieve curve before relying on a granular filter.

Step by step

  1. 1

    Find out what the soil is.

    Rub a moist pinch between your fingers. Gritty is sand, smooth and floury is silt, sticky is clay. Silt and fine sand wash out most easily, so they need a filter most.

    Rubbing a soil sample
  2. 2

    Choose geotextile or a granular filter.

    A non-woven geotextile is the simplest and works on most soils. In very silty soil, over about 40 % silt, check that the fabric will not clog. A sand-and-grit filter does not clog, but needs two layers between fine soil and coarse stone.

    The two kinds of filter
  3. 3

    Geotextile: lay it flat on the soil and overlap joins by 300 mm.

    Keep it in close contact with the soil, without folds or gaps. Tip the stone on gently so it does not tear.

    Overlapping two widths of fabric
  4. 4

    Granular: lay the sand first, then the grit, then the stone.

    Never tip 0–4 mm sand straight against 16–32 mm stone: the sand washes into the stone. Make each layer at least 75 mm thick. 100 mm is easier to keep even.

    Sand, grit and stone in layers
Specifications

Granular filter rules

Retention: filter D15 ≤ 4–5 × soil D85. Permeability: filter D15 ≥ 4–5 × soil D15, and at least 0.1 mm. A filter should have at most 5 % passing 0.075 mm and no grains over about 50 mm.

SoilFines (< 0.075 mm)Largest filter D15
Silt and clayover 85 %9 × soil D85, at least 0.2 mm
Sandy silt and clay40–85 %0.7 mm
Sand and gravelunder 15 %4 × soil D85

Thickness

Around a drain pipe the filter should cover every opening by at least 75 mm. Make a flat filter layer thick enough to stay continuous everywhere.

Geotextile

Opening size (AOS or O90) ≤ B × soil D85. For fine-grained soil B is 1.8 for non-woven fabric and 1 for woven. For sand and gravel B is between 0.5 and 2 depending on how uniform the soil is. The fabric must also pass water at least as fast as the soil. Non-woven fabrics filter best. Overlap joins by about 300 mm.

Sources

  1. Giroud, Development of criteria for geotextile and granular filters (International Geosynthetics Society lecture): Terzaghi's retention and permeability criteria, filter D15 against soil D85 and D15, factor 4–5
  2. USDA NRCS, National Engineering Handbook Part 633 Chapter 26: gradation design of sand and gravel filters: D15 limits by soil category; “permeability about 16 to 25 times that of the base soil”; “maximum particle size allowed is 2 inches and the maximum percentage passing the No. 200 sieve is 5 percent”
  3. USDA NRCS, Conservation Practice Standard 606: subsurface drain: openings covered by “at least 3 inches of filter material”; geotextile clogging check where silt exceeds 40 %
  4. Iowa DOT Design Manual 200F-9: geosynthetic design, after FHWA NHI-07-092: AOS ≤ B × D85, B values by soil and fabric, permeability of fabric ≥ soil
  5. Oklahoma DOT, special provisions for portland cement concrete, 701.05(c): fine aggregate grading (9.5 mm 100 %, 1.18 mm 50–85 %, 300 µm 5–30 %, 150 µm 0–10 %), used for the sand D15 and D85
  6. Caltrans Geotechnical Manual: geosynthetics: “nonwoven geotextiles are most effective at filtration”
  7. Minnesota Stormwater Manual: infiltration media: filter fabric seams overlapped 12 in
  8. Penn State, CE 584: aggregates: aggregate bulk density 1,520–1,680 kg/m³
  9. FHWA-HRT-15-034, open-graded aggregates: loosest unit weight of No. 8 stone 85.7 lb/ft³ (1.37 t/m³)

Amounts per 100 mm layer, loose: sand 0.1 m × 1.6 t/m³ = 160 kg per m² (source 8); grit 0.1 m × 1.37 t/m³ ≈ 140 kg (source 9). Order a little extra for waste.

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