Concrete for Retaining Wall Footings: Volume, Grade and Access Guide

Quick Answer

A typical backyard retaining wall in Sydney, whether it is a 10–15 m block wall along a boundary or a sleeper wall on steel posts, uses between 1 m³ and 3 m³ of concrete in the footing. That puts most of these jobs in small-load territory, where the delivered price per cubic metre can be 15–73% above the headline rate because of short-load fees. That is too much for bags and too little for a standard truck to be cheap.

Three decisions set the cost and the risk of the pour:

1. Quantity. Order what the drawing needs plus a sensible allowance. Running short mid-pour leaves a cold joint in a structural footing.
2. Specification. Order the grade, slump and aggregate the engineer specified. Don't swap in a cheaper mix on the day.
3. Access. Retaining walls usually sit at the back or low side of a sloping block. Whether a standard truck, a mini-mix truck, a pump or a barrow crew gets the concrete there often moves the job cost more than the grade does.

How Much Concrete Does a Retaining Wall Footing Need?

Retaining walls use one of two footing types. Your engineer's drawing will show which one, with dimensions in millimetres. Sleeper-wall piers are commonly 300–450 mm in diameter, and as a rough guide they go about as deep as the wall is high. The drawing still decides.

Convert millimetres to metres before multiplying: 600 mm is 0.6 m. Then add a waste allowance. For footings, 10% is a sensible default because trenches are rarely dug perfectly square, and soft or crumbly trench walls take more concrete than the drawing shows. Use 5% for a clean, formed footing and 15% for rough or sandy excavation.

Round up to the next 0.2 m³, because most Sydney plants batch in 0.2 m³ steps. Rounding a full cubic metre above what you need is a false saving: returned concrete is typically charged at around $350/m³ ex GST.

Footing Estimator: Enter Your Engineered Sizes

Enter the footing sizes from the drawing. The estimator works out the order volume and an indicative delivered price using the same Sydney small-load rates as our small-load price table. Then enter your address to see live supplier prices for that exact volume and grade.

For other shapes, such as square pad footings or a slab behind the wall, use the full concrete volume calculator.

What Grade and Mix for Retaining Wall Footings?

A retaining wall footing holds back soil and water, so it is a structural element. Under the NSW exempt development rules, a retaining wall needs no approval only if it is no more than 600 mm high, at least 1 m from any boundary and meets the other listed conditions. Bigger walls need complying development or a DA, and most need an engineer's design. That design is usually based on AS 4678 Earth-retaining structures and AS 3600 for the concrete itself. Block walls also follow the Concrete Masonry Association of Australia retaining wall design manuals.

On a 3 m³ footing, stepping up from N25 to N32 adds only about $90 of material. The grade is rarely where the money goes on a small retaining wall pour. Getting it wrong can mean a failed inspection.

What to Tell the Plant When You Order

• Grade: N25, N32 or N40, exactly as drawn. Check for an exposure classification note (for example B1 or B2) as well.
• Slump: 80 mm is common for chute-placed footings. Ask for 100 mm if a line pump is placing it. Never add water on site to make it flow; see why adding water weakens concrete.
• Aggregate: 20 mm for most footings. Use 10 mm only if the pump or the engineer requires it.
• Volume and timing: the rounded order quantity, and a delivery time after the steel inspection is signed off.
• Access notes: street width, driveway slope, overhead wires and distance from the truck to the trench.

The engineer also sets reinforcement, bar laps and cover, which is the concrete between the steel and the ground. Footings cast straight against soil need more cover than formed concrete, so bar chairs must hold the cage at the height shown on the drawing. Starter bars for a block wall must be set in the footing pour. They can't be added afterwards.

Worked Examples With Sydney Pricing

Example 1: 12 m Besser block boundary wall

An engineer specifies a 1.2 m high block wall on a strip footing 600 mm wide × 400 mm deep, N32, with starter bars at 400 mm centres. The wall runs 12 m along the rear boundary of a sloping block in Baulkham Hills.

1. Net volume: 12 × 0.6 × 0.4 = 2.88 m³
2. Add 10% waste: 3.17 m³, then round up to 3.2 m³
3. Concrete: 3.2 m³ × $385/m³ (N32 mid-range) = $1,232
4. Cartage $110 + short-load fee $50
5. Indicative delivered: $1,392 ex GST (about $435/m³)

The trench sits 18 m behind the kerb, past the house. A chute reaches about 3–4 m, so this pour needs a line pump. On a typical Sydney line pump job that adds $500–1,000, which is more than a third of the total. Book the pump and the concrete for the same time slot.

Example 2: sleeper wall on 12 steel posts

A 900 mm sleeper wall in a Sutherland Shire backyard has 12 steel posts set in 450 mm diameter piers, 900 mm deep, in N25.

1. Per pier: π × 0.225² × 0.9 = 0.143 m³
2. 12 piers: 1.72 m³. Add 10%: 1.89 m³, round up to 2.0 m³
3. Concrete: 2.0 m³ × $355/m³ = $710
4. Cartage $110 + short-load fee $100
5. Indicative delivered: $920 ex GST ($460/m³)

Bagged concrete would need about 210 × 20 kg bags for the same volume. At $8–12 a bag, that is $1,680–2,520 before labour, so ready-mix is cheaper and more consistent from about 0.4 m³ upwards. See ready-mix vs bagged concrete for the full comparison.

Standard Truck, Mini-Mix, Pump or Barrow?

Retaining walls are one of the most access-sensitive small pours. The wall is often at the back of the block or on the low side of a slope, and spoil from the excavation may be stockpiled where a truck would stand. Decide the delivery method before you book.

Unloading time matters on small loads. Plants typically allow a 20–30 minute unloading window, and waiting time after that runs about $2–5 per minute. Barrowing 3 m³ up a slope can easily exceed that window. Use the mini-mix access checklist and pump guide if you are unsure.

Retaining Wall Footing Pour-Day Checklist

1. Get the steel and trench inspected by the engineer or certifier, then book the concrete. Don't book first and hope the inspection passes.
2. Pump out any water in the trench and remove loose soil from the base. Rain can collapse sandy trench walls overnight.
3. Check bar chairs, laps and starter bar positions against the drawing.
4. Set the ag pipe and drainage aggregate behind the wall to the drawing. The footing is only one part of the wall.
5. Have the crew, vibrator or rodding tools, and screed ready before the truck arrives. See what to expect on delivery day.
6. Keep the footing damp while it cures. Concrete typically reaches around two-thirds of its 28-day strength after 7 days of moist curing (see the CCAA curing guidance), so confirm with the engineer before laying blocks or backfilling.

Core-filling a block wall is a separate, later pour. Suppliers allow about 0.8 m³ of concrete per 100 × 200-series blocks. At 12.5 blocks per m², that is roughly 0.1 m³ per m² of wall face. That is often a second small load, and usually a line pump job with a 10 mm aggregate mix. Check the block manufacturer's figure and the engineer's core-fill specification before ordering.