GLAREUM.NET›Chapter 6›Drainage and water

6.3 Drainage against walls

A zone of clean stone behind a retaining wall or next to a basement wall. Water in the soil drains down through it to a pipe, instead of pushing on the wall.

Cross-section of a block retaining wall: blocks on a base, a 300 millimetre zone of clean stone behind them, fabric between the stone and the soil, a perforated pipe at the bottom and a soil cap on top. 300 mm 4 Soil cap less permeable soil 3 Geotextile between stone and soil 2 Drainage stone 300 mm wide · 8–16 mm 1 Drain pipe 100 mm perforated Retained soil
Cross-section of a 1 m block wall, drawn to scale. Parts are numbered in the order you lay them. The base under the blocks is covered in 9.1.

At a glance

Difficulty
3 of 4
Used behind
Block and stone retaining walls, basement and foundation walls
Tools
Spade, wheelbarrow, hand tamper or light plate compactor, spirit level, knife for fabric
Parts
  1. 1  Drain pipe
  2. 2  Drainage stone
  3. 3  Geotextile
  4. 4  Soil cap

What you need per m² of wall face, 300 mm zone

MaterialSizeTypeAmount
Drainage stone 8–16 mm Washed, crushed, ≤ 5 % fines 460 kg
or 4–20 mm Washed, crushed
Geotextile – Non-woven, joins overlapped 1.3 m²
Drain pipe – Perforated, at least 100 mm 1 m per m of wall

Why crushed and clean?

clean

With no fines the stone lets water fall straight to the pipe. Soil behind a wall that cannot drain holds water, and the water presses on the wall.

crushed

Angular stone locks together and stands up behind the blocks.

Step by step

  1. 1

    Dig at least 300 mm further back than the back of the blocks.

    Line the back and the bottom of that space with geotextile, leaving enough to fold over the top later.

    Fabric against the cut soil
  2. 2

    Lay the perforated pipe at the bottom, behind the base.

    Give it a steady fall to an outlet. Lead it out through the wall face or to the side at least every 15 m.

    Pipe laid at the foot of the wall
  3. 3

    Fill stone behind each course as you build, in layers of up to 200 mm.

    Compact each layer by hand or with a light plate. Keep heavy machines at least 0.9 m back from the wall.

    Stone filled up to the top of a course
  4. 4

    Fold the fabric over and finish with a soil cap.

    A top layer of less permeable soil makes surface water run off instead of straight into the stone.

    Soil cap over the folded fabric

Basement and foundation walls

Against a basement wall the drain runs along the footing. One building code gives these minimums:

Under the pipeat least 50 mm washed gravel or crushed rock
Over the pipeat least 150 mm of the same
Without a pipe300 mm beyond the footing, 150 mm above its top
Pipe levelat or below the floor it protects
Fabricfilter membrane over the stone

The US Department of Energy adds: ½–¾ in (12–19 mm) washed stone, a 100 mm pipe with the holes facing down, and fabric under, around and over the stone. 8–16 mm is the nearest metric size.

Specifications
PartSizeValueHow
1 Drain pipe≥ 100 mmoutlets ≤ 15 m apartPerforated, behind the base, with a fall to the outlets.
2 Drainage stone8–16 mm or 4–20 mm≥ 300 mm wideWashed, crushed, ≤ 5 % passing 0.075 mm. Lifts ≤ 200 mm.
3 Geotextile–laps 450 mmBetween stone and soil.
4 Soil cap––Less permeable soil.

Stone size

Road authorities specify US No. 57 stone (4.75–25 mm) behind block walls, and a university construction standard asks for 100 % passing 25 mm with at most 5 % passing 0.075 mm. 8–16 mm and 4–20 mm fit that. 16–32 mm is coarser than these specifications.

Fall

A US federal standard drawing and a university wall standard both slope the collector pipe at least 2 %. For drain trenches 0.5–1 % is used (6.4).

Fabric

One state road authority requires a separation geotextile between the stone and the soil, with laps of at least 450 mm. On clean sand or gravel soil it matters less. See 6.6 for choosing it.

Sources

  1. NCDOT Standard Specification 454: segmental gravity walls: “No. 57 stone for a horizontal distance of at least 12 inches”; geotextile between stone and soil, laps 18 in; 100 mm perforated pipe; 8–10 in lifts with hand equipment
  2. Iowa DOT SP-230205: modular block retaining wall: granular fill “a minimum of 12 inches beyond the wall”; maximum 8 in lifts; light equipment within 3 ft; subdrains at least 4 in
  3. Texas State University construction standard 32 32 23.13: segmental retaining wall: drainage aggregate “Clean crushed stone or granular fill”, 100 % passing 1 in, 0–5 % passing No. 200; “12 inches (minimum) of drainage aggregate” behind the units; hand-compaction lifts “not exceeding 6 to 8 inches”; outlets “at 50 foot (maximum) intervals”; collection pipe sloped at least 2 %.
  4. Maryland Department of the Environment: stormwater materials: AASHTO M43 grading of No. 57 stone, about ⅜–1 in
  5. FHWA Federal Lands standard drawing C257-50: wall collector pipe, 2 % minimum fall, outlets at most every 50 ft
  6. International Residential Code 2003 R405.1, Winnebago County copy: pipe “placed on a minimum of 2 inches (51 mm) of washed gravel or crushed rock” and “covered with not less than 6 inches (153 mm)”; gravel drain 1 ft beyond and 6 in above the footing; filter membrane
  7. US DOE Building America Solution Center (PNNL): footing drain pipe: “½-inch to ¾-inch washed gravel or stone”; “perforations down”; fabric under, around and over
  8. FHWA-HRT-15-034, open-graded aggregates: No. 57 stone loose 95.4 lb/ft³ (1.53 t/m³)

Amounts per m² of wall face: stone 0.3 m × 1.53 t/m³ ≈ 460 kg, loose. Fabric: back face plus the fold over the top and laps, about 1.3 m². Order about 15 % extra stone for compaction and waste (FHWA loose to dense is about 14 %).

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