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2.2 Backfill against foundations and walls

Filling the dig-out next to a foundation, basement wall or garden wall, so the ground does not settle and water runs away from the wall.

Cross-section of backfill against a foundation wall: a strip of clean drainage stone against the wall with a drain pipe at the footing, compacted crushed fill in layers beside it, and soil on top sloping away from the wall. 3 Soil cap sloped away, 5 % 2 Compacted fill 0–32 mm in layers 1 Drainage stone against the wall, see 6.3
Cross-section of backfill against a foundation wall.

At a glance

Difficulty
2 of 4
Time
1 day for 10 m of wall, 1 m deep
Tools
Shovel or mini digger, wheelbarrow, rake, plate compactor or rammer, spirit level
Parts
  1. 1  Drainage stone
  2. 2  Compacted fill
  3. 3  Soil cap

What you need per metre of wall, 1 m deep, 600 mm dig-out

MaterialSizeTypeAmount
Drainage stone 8–16 mm Washed, next to the wall 0.27 m³
Fill 0–32 mm Crushed, with fines 570 kg
or dug soil Only if free of topsoil, roots and heavy clay
Geotextile – Between the stone and the fill 2 m²

Why two materials?

0–32 mm

The fines fill the gaps between the stones, so it compacts hard and does not settle later. Use it for the bulk of the fill.

8–16 mm

No fines, so water drops straight through to the drain instead of pressing on the wall or freezing against it. It goes next to the wall. See 6.3.

Step by step

  1. 1

    Let the wall gain strength, and brace it.

    Wait at least 7 days in mild weather and 14 days in cold weather. A basement wall holding back more than about 1.2 m of soil needs the floor above in place, or bracing.

    A braced basement wall
  2. 2

    Lay the drain and the stone against the wall.

    Lay the drain pipe at the footing and start the strip of drainage stone, wrapped in geotextile. Chapter 6.3 covers this part.

    Drain pipe at the footing
  3. 3

    Fill in layers of 150–300 mm.

    Spread one layer, compact it, then spread the next. Use the thin end of the range with a light plate or rammer. Raise the drainage stone with the fill.

    Spreading a layer
  4. 4

    Use only hand compactors within 0.9 m of the wall.

    A walk-behind plate or rammer is fine. Keep rollers, diggers and loaded trucks back, because they push the wall over.

    Plate compactor next to the wall
  5. 5

    Finish with soil sloped away from the wall.

    Cap the top with soil so rain runs off rather than into the stone. Let the ground fall at least 150 mm over the first 3 m.

    The finished slope
Specifications
PartSieve sizeThicknessHow
1 Drainage stone8–16 mm300 mm wide, full heightWashed, in geotextile, raised with the fill. See 6.3.
2 Fill0–32 mmlayers of 150–300 mmEach layer compacted. Hand compactors only within 0.9 m of the wall.
3 Soil capdug soilabout 100 mmFalls at least 5 % away from the wall.

When to start

Backfill only once the concrete is strong enough. One building authority asks for 3 days above 23 °C, 7 days between 13 and 23 °C and 14 days between 4 and 13 °C. Walls holding back 1.2 m or more must be tied to the floor above or braced first.

Layer thickness

Loose layers of 150–300 mm are the usual maximum. Thicker layers stay loose at the bottom. The lighter the compactor, the thinner the layer: see the table in 2.3.

How firm

Where a design gives a figure, it is usually 95 % of the standard Proctor density. Without one, compact until the plate no longer leaves a mark and the layer does not move underfoot.

Frost

Fill that holds water can freeze to the wall and lift it. Free-draining stone against the wall and a dry, compacted fill prevent this.

Slope

Ground next to a building falls at least 150 mm in the first 3 m (5 %). Paving next to it falls at least 2 %.

Sources

  1. Prince William County, Foundation backfill policy: no backfill until the concrete is strong enough (3 days above 23 °C, 7 days at 13–23 °C, 14 days at 4–13 °C); walls holding 1.2 m or more of fill anchored to the floor above or braced; no organic material or high-plasticity clay in the backfill
  2. Iowa DOT Standard Specifications §2431, Segmental retaining wall: levelling pad at least 150 mm; backfill in layers of at most 200 mm at 95 % of standard Proctor density; only hand-operated compactors within 0.9 m of the wall face
  3. US DOT research report, Equipment and lift thickness optimization for structure backfill: typical maximum loose lifts 150–300 mm; thicker lifts leave the bottom under-compacted; plates usually need two to four passes
  4. US Bureau of Reclamation, Pipe bedding and backfill: compacted layers no more than 150 mm with tampers or rollers, up to 300 mm with surface vibrators on clean sand and gravel
  5. National Research Council Canada, Building Digest 26: Ground freezing and frost heaving: three requirements (freezing, fine soil, water), remove one to stop heave; coarse sand and clean gravel do not heave; backfill that freezes to a wall can lift it, so it should be selected and drained
  6. California Division of the State Architect, IR 18-2: Segmental retaining walls: free-draining gravel at least 300 mm thick behind the wall for its full height; compaction layers no thicker than 200 mm (after NCMA)
  7. City of Lawrence, Grading and drainage guidelines (IRC R401.3): ground falls at least 150 mm in the first 3 m from the wall (5 %); hard surfaces at least 2 %
  8. FHWA-RD-97-148, Granular base: natural aggregates weigh about 2.0–2.3 t/m³ in place
  9. EN 13242 Aggregates for unbound and hydraulically bound materials
  10. EN 13252 Geotextiles used in drainage systems
  11. EN 1997-1 Eurocode 7: geotechnical design

Amounts: dig-out 600 mm wide and 1 m deep, less 100 mm of soil cap. Half of it (300 mm) is drainage stone, 0.27 m³ in place. The other half is 0.27 m³ of compacted fill, 570 kg. Compacted crushed stone is taken as 2.1 t/m³, the middle of the 2.0–2.3 t/m³ range FHWA gives for natural aggregate in place. Order a little extra.

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