Retaining Wall Forms

Written for Canada. Plans are metric, but many crews and suppliers work in feet and inches, so this site gives both: millimetres and metres, with feet and inches beside them. Codes are the National Building Code of Canada 2020 (NBC 2020) Part 9 as your province adopts it. A number marked Confirm has not been checked against every province, so check it with your own authority before you act on it.

Reviewed by a working tradesman with 17+ years in the field.

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Forms, steel and drainage for a wall that holds back soil, and why it needs more bracing than an ordinary wall.

The big idea

A retaining wall holds back soil, and soil pushes. The push grows with height, with wet ground, and with anything parked or piled behind the wall. The forms have to hold wet concrete while it sets, and the finished wall has to hold the hill for decades. Both jobs need more bracing, more steel and better drainage than an ordinary foundation wall.

Retaining walls are not covered by the house-sized rules in NBC Part 9 the way a foundation wall is. Tall walls are structural design work, and permit and engineer rules are set by your province and municipality.

When a permit or an engineer is needed

No single national height applies. Alberta's Safety Codes interpretation on retaining walls (2019 edition) gives a useful picture:

• Walls critical to the stability of building foundations are regulated by the building code.
• Walls over 1.2 m (4 ft) are regulated if they are next to public property, next to a building access, or on private property that the public can reach.
• A wall that supports soil with a slope flatter than the 45 degree line from the footing's outer edge may be a structural element and should be assessed by a registered engineering professional.
• Structural wood in retaining walls or cribbing over 1.2 m must be pressure-treated (9.3.2.9(4)(b)).
• A guard is required where the surface the wall supports carries foot traffic and the drop exceeds 600 mm (23 5/8 in).
• Wall height is the vertical difference between the ground on each side.

Types of retaining walls

• Gravity walls: resist the soil by their own weight and mass.
• Reinforced concrete cantilever walls: a stem on a footing, using rebar to resist bending.
• Basement walls: hold back soil on the sides of a building, braced by the floor system.
• Commercial and civil walls: engineered for high loads and tall heights.
• Stepped or terraced walls: several shorter walls so no one wall carries too much.

What the design has to handle

• Soil pushes sideways, and water makes it much worse.
• Wall thickness and steel go up with height.
• Frost can push on the wall too, so the backfill must drain.
• Surcharge (vehicles, buildings, stockpiles) adds load.
• Formwork and bracing must resist the wet-concrete pressure during the pour.

How hard does soil push?

The Alberta edition of the model code (9.4.4.6) designs walls that support drained earth for a fluid pressure of at least 480 kg/m³, or per Part 4. That is 480 × 9.81 ÷ 1,000 = 4.7 kPa for each metre of depth (about 30 lb/ft² per foot). For a wall of height H, the push on each metre of wall length is ½ × 4.7 × H².

Double the height, four times the push. And water: standing water 1.5 m deep behind the wall pushes ½ × 9.81 × 1.5² = 11.0 kN per metre (about 760 lbf per foot), more than twice what the drained soil does. That is why drainage is not optional.

How hard does the wet concrete push?

Fresh concrete acts like a heavy liquid at 23.5 kPa per metre of depth (see Foundation Wall Forms). In a 3 m wall poured fast, the pressure at the bottom reaches about 70 kPa (about 1,470 lb/ft²), and the push on each metre of form is ½ × 23.5 × 3² = about 106 kN (about 7,260 lbf per foot). That is roughly five times what the drained soil will put on the finished wall. The form is the hard part. The form design or the manufacturer's tables set the real design pressure.

Base preparation

A retaining wall is only as good as its footing.

• Excavate to the depth in the drawings, to undisturbed soil.
• Compact the base, and replace soft spots with compacted granular fill.
• Set footing forms to the exact width and elevation.
• Place footing rebar and the dowels the stem will tie to.
• Keep the excavation drained and the slope behind it safe (see Trench Safety).

Forming systems

• Panel systems: steel or aluminum panels with heavy ties and bracing.
• Lumber forms: plywood, studs, whalers and strongbacks, for smaller walls.
• Gang forms: crane-set panels for tall or long walls, handled by a trained crew.
• Hybrid systems: panels plus lumber for stepped or curved walls.

Tie spacing, bracing and pour rate come from the form design or the manufacturer's tables.

Wall layout and preparation

• Snap lines for both wall faces.
• Check the footing dowels for spacing and location.
• Mark drain pipe locations and weep holes if the drawings call for them.
• Mark step-downs and changes in height.
• Confirm the batter (backward lean) if the design has one.

Building retaining wall forms

1. Set the first panel perfectly. It controls plumb and alignment of the whole wall.
2. Pin and wedge the adjoining panels tight. No gaps.
3. Install ties at the engineered spacing. Retaining walls usually need more of them.
4. Add strongbacks and whalers. Spaced closer than for a standard wall.
5. Brace to the ground. With braces that can resist outward pressure, anchored where they will not move.
6. Check plumb and alignment. With a laser or string line along the whole wall.

Reinforcement

Placement has to match the drawings. See Rebar Placement for cover and splices.

• Vertical bars tied to the footing dowels, with more steel at the bottom where the pressure is highest
• Horizontal bars at the specified spacing
• Corner bars where the wall turns or steps
• Top bars for a cap or a slab connection
• Chairs and spacers to keep the cover

Drainage and waterproofing

Water behind a retaining wall is the leading cause of failure.

• Perforated drain pipe at footing level, sloped to a safe outlet, wrapped in filter fabric
• Free-draining granular backfill against the wall, as drawn
• Weep holes only where the design calls for them, kept clear
• Dampproofing or waterproofing after the forms come off, where the wall is part of a building
• Do not let the drain discharge where it can erode the toe or the neighbour's property

Pouring considerations

• Place in even lifts, at the pour rate the form design allows.
• Vibrate to remove air pockets.
• Watch ties and braces for stress, and stop if anything moves.
• Check plumb repeatedly during the pour.
• Keep the rebar cover throughout.

Common mistakes

• Not enough bracing, so the form moves outward
• Wrong tie spacing, giving bowing
• Rebar too close to the form face
• No drainage plan
• Wrong batter
• Weak whalers or strongbacks
• Backfilling too early, or with clay
• Forgetting that the soil behind the wall is also a load during construction