The short answer: if your site cut leaves an exposed face taller than the soil can hold itself up at, you need a retaining wall. On Gold Coast clays and sandy soils, that's usually anything over about 600–900mm.
When you definitely need a retaining wall
- Cut face over 1m on standard residential soil
- Cut face over 600mm in sandy or loose soil
- Cut face supporting a structure, driveway, or paved area above
- Cut face within boundary setbacks specified by your engineer
- Cut face that would create a fall hazard (council/BCA rules)
When you might get away without one
Shallow cuts under 600mm in stable clay can often be battered (sloped) instead of retained. The batter angle depends on the soil — your geotech or engineer specifies it. The trade-off is footprint: you lose usable yard to the slope.
Retaining wall types we usually see on the Gold Coast
- Timber sleeper walls (cheapest, ~10-15 year life): from $250/m²
- Besser block / core-fill walls (mid-range): $350–550/m²
- Concrete sleeper walls with steel posts: $300–500/m²
- Engineered structural walls (over 1m): $500–900/m² + engineer fees
Anything over 1m in QLD typically needs engineer's design and council approval.
Plan retaining at the same time as the site cut
Far cheaper to combine site cut + retaining wall benching in one mobilisation than to come back later. We build retaining walls up to 1 metre and handle the earthworks for taller engineered walls too — one crew, one timeline.
What we do during the site cut to make retaining easier
- Cut the bench level and square — saves the retaining trade hours of trim work.
- Trench the footing to engineer's drawing while the machine's on site.
- Backfill behind the wall with free-draining RCA 20-40mm + geofabric.
- Form the agi-line drainage path before the wall goes up.
- Compact the bench so the wall sits on stable ground.