Choosing the wall type is the easy part. This is what each one takes to build properly — the sequence, the details that matter, and the shortcuts that show up two winters later.
Building Gravity, Cantilever and Gabion Walls — practical guidance from the team running retaining wall work on the Garden Route. If you are still deciding between types, start with our comparison of retaining wall types. This piece assumes the decision is made and covers execution.
Gravity walls: mass, base width and the batter
A gravity wall works because it is heavy enough that the soil cannot push it over or slide it forward. Everything in the build serves that.
The base is founded on undisturbed material, stepped where the ground falls so each step stays level rather than following the slope. Base width is typically 50 to 70 percent of the finished height — that is the number that surprises people when the trench goes in.
Most gravity walls get a slight batter, leaning back into the retained soil by around one in twelve. It buys stability and it hides the small forward movement that almost every wall makes as the backfill settles. A wall built dead plumb that then leans two degrees looks like a failure. The same movement on a battered wall reads as vertical.
The common shortcut is building the wall first and backfilling in one lift at the end. Backfill goes in layers, compacted, as the wall rises — with the drainage stone placed at the same time. Dumping two metres of fill against a finished wall in one go is how you push a good wall over.
Cantilever walls: the heel does the work
A cantilever wall is a reinforced concrete stem on a footing shaped like an inverted T or L. The critical insight is that the wall is not held up by its own weight — it is held by the weight of soil sitting on the heel of the footing, the part that extends back under the retained ground.
That means the heel must actually be buried under the soil it is relying on. A cantilever wall backfilled short, or with the heel left exposed during construction and never properly covered, has lost the mechanism that keeps it standing.
Reinforcement runs vertically up the back face of the stem — the tension face — hooked into the footing. Getting that steel on the correct face is not a detail, it is the whole design. We have been called to walls where the reinforcement was placed on the front face; the wall was, structurally, unreinforced.
Cover matters here as much as position. On a retaining wall the back face is permanently against wet soil, so cover to the steel and a dense mix are what stop corrosion. This is a place where 30 MPa and proper spacers earn their money.
Gabion walls: rock, wire and patience
Gabions look casual and are not. A good gabion wall is hand-packed, not tipped. Rock is placed so the faces are tight and flush, voids are filled with smaller stone, and the baskets are filled in layers with internal bracing wires tensioned as you go.
A tipped gabion settles unevenly, bulges at the face, and loses the interlock that makes the mass behave as one unit. It is also visibly untidy, and on a wall that is deliberately showing its structure, that matters.
Baskets are laced together — every adjacent edge, vertically and horizontally — so the wall acts as a single mass rather than a stack of separate boxes. Lacing is the most skipped step and the most consequential.
The wire specification is the life of the wall. In coastal air, galvanised mesh corrodes faster than most people expect. PVC-coated or heavily galvanised mesh costs more up front and is the correct call anywhere near the sea on this coast. Specifying standard galvanised for a gabion within sight of the water is a false economy with a visible end date.
Drainage, on all three
The drainage detail is essentially the same regardless of wall type, and it is what determines whether the wall reaches its design life.
A zone of clean, free-draining stone against the back of the wall. A perforated subsoil drain at the base, wrapped in geotextile, laid to a fall, discharging somewhere real — not into the fill, not to a soakaway that floods, but to a stormwater point or a daylight outlet. Weepholes through the face at regular centres, typically every metre or so on the lowest course and staggered above. And geotextile separating the stone from the retained soil so fines do not migrate in and blind the drain.
Gabions are the exception in one respect: they are free-draining by construction, so the stone zone is inherent. They still need the toe drain and they still need the geotextile behind, or the retained soil washes through the rock over time and the wall settles.
Sequence and programme
The order of work is where retaining wall jobs go wrong on programme. Excavation and temporary support come first, and on anything above about 1.5 m the temporary works need thinking about as seriously as the permanent wall — an unsupported face can stand for a week and let go in an afternoon of rain.
Then foundations, then the wall in lifts, with drainage and compacted backfill rising alongside. Concrete walls need curing time before backfilling — striking formwork early and loading a green wall is a real failure mode.
On the Garden Route the weather is the programme risk. An open excavation through a week of Outeniqua winter rain is a different job to the one that was priced. We sequence retaining wall work to close the excavation as quickly as possible and we price temporary protection rather than pretending it will be dry.
What we check before signing off
That the drain runs. Water poured in at the top of the subsoil drain should appear at the outlet. It is a two-minute test and it is the single most useful check on the whole job.
That weepholes are clear and not grouted shut by the plastering crew — which happens more often than it should.
That the backfill is what was specified, compacted in layers, and not just the spoil from the excavation with the big rocks picked out.
And that the wall is plumb or correctly battered, with no bulge developing at mid-height. A bulge at mid-height on a new wall means the backfill went in too fast or too wet, and it will not settle back.
Frequently asked questions
How thick should a gravity retaining wall be?
The base is usually 50 to 70 percent of the finished height, tapering towards the top. A 1.2 m wall therefore wants roughly a 600 to 850 mm base. That base width is what stops it tipping, so trimming it to save on excavation removes the mechanism holding the wall up.
Which side does reinforcement go on a cantilever wall?
On the back face of the stem — the side against the retained soil. That is the tension face. Reinforcement placed on the front face leaves the wall effectively unreinforced, and it is a mistake we have been called out to more than once.
Do gabion baskets need to be laced together?
Yes. Every adjacent edge, vertically and horizontally. Lacing is what makes the wall behave as one mass rather than a stack of independent boxes, and it is the most commonly skipped step on a gabion job.
How long do gabion baskets last?
The rock lasts indefinitely; the wire decides the life. Standard galvanised mesh in coastal salt air has a much shorter life than PVC-coated. Anywhere near the sea on this coast, PVC-coated or heavy galvanised is the correct specification and the price difference is small against a rebuild.
Can you backfill a retaining wall all at once?
No. Backfill goes in compacted layers as the wall rises, with the drainage stone placed at the same time. Dumping the full height of fill against a completed wall in one lift applies a load the wall was never designed to take in that way, and it is a common cause of walls moving before they are even finished.
How long before you can backfill a new concrete retaining wall?
Long enough for the concrete to have developed adequate strength — in practice usually a week or more, and governed by the engineer’s requirement rather than the programme. Backfilling a green wall is one of the few mistakes that can fail a wall immediately rather than slowly.
What is a batter on a retaining wall?
A deliberate backwards lean into the retained soil, typically around one in twelve. It improves stability and it absorbs the small forward movement that most walls make as backfill settles, so the wall still reads as vertical afterwards rather than looking like it has failed.
Related reading
- Retaining wall types, compared
- Foundation types for George soils
- Earthworks: cut, fill and compaction
- Retaining wall construction in George
- All our concrete services
Building a wall that has to hold?
We build retaining walls across the Garden Route — designed for the ground, drained properly, and backfilled the way they were meant to be.
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