Retaining wall design is a structural decision, not an aesthetic one. Gravity, cantilever, crib and gabion walls each handle different loads, soil types and height ranges — and each fails differently when undersized.
Retaining Wall Types, Compared — practical guidance from the team running retaining wall work on the Garden Route. Below is how we choose between the four common wall types on Garden Route sites, what each one costs you in space and excavation, and the two things that put walls over more often than the wall type ever does.
The four wall types, and what actually separates them
A retaining wall holds back soil that would otherwise slump to its natural angle of repose. Everything else — appearance, materials, cost — follows from how the wall resists that push. There are only four common ways to do it, and the choice is usually made for you by the height, the soil and how much room you have behind the wall.
Gravity walls resist by sheer mass. Mortared stone, mass concrete or heavy interlocking blocks, wide enough at the base that the soil cannot tip them. Simple to build, no reinforcement design needed at modest heights, but the base width grows quickly — roughly 50 to 70 percent of the wall height — so they eat plan space.
Cantilever walls are reinforced concrete, shaped like an inverted T or L. The soil sitting on the heel of the footing is what holds the wall down. They use far less material than a gravity wall at the same height and are the standard answer once you pass roughly 1.5 m, but they need engineered reinforcement and proper formwork.
Crib walls are interlocking precast or timber cells stacked and filled with granular material. They behave like a gravity wall but build faster, tolerate some differential settlement, and can be planted. They want a batter — a lean back into the bank — so they are not the answer where you need a vertical face on a boundary.
Gabion walls are rock-filled wire baskets. Free-draining by nature, flexible enough to tolerate movement without cracking, and forgiving on poor ground. They are the usual choice on road cuts and river edges, and they are honest about what they are — nobody mistakes a gabion for a feature wall.
| Type | Usual height range | Space behind | Best where | Main weakness |
|---|---|---|---|---|
| Gravity (mass/block) | Up to ~1.5 m | Wide base | Garden terracing, short banks | Base width; tips if undersized |
| Cantilever RC | 1.5–4 m+ | Needs heel excavation | Boundaries, vertical faces, height | Engineered design and formwork cost |
| Crib | Up to ~3 m | Needs batter | Fast build, planted slopes | Cannot go truly vertical |
| Gabion | Up to ~4 m stepped | Moderate | Road cuts, watercourses, poor ground | Appearance; wire life in salt air |
On the block side, Terraforce and the segmental retaining wall systems are what most Garden Route walls are actually built from, with Technicrete and Bosun supplying the precast product ranges. Gabion baskets come through Maccaferri and the wire mesh suppliers. Whichever system is chosen, the geotextile and the drainage stone behind it are part of the design rather than an optional extra — Kaytech and the geosynthetics suppliers carry the separation and filter fabrics — and a wall built without them is a wall waiting for the first properly wet winter.
Height is the decision, not preference
Below about a metre, almost anything works and the choice comes down to looks and budget. Between one and one and a half metres, mass and block gravity walls are still viable but the base starts to get greedy. Above one and a half metres you are into engineered territory, and in practice that means a cantilever wall or a properly designed gabion.
There is a hard rule worth knowing before you plan anything: a retaining wall over 1.5 m, or any wall retaining a surcharge such as a driveway, parking or a structure above it, needs a rational design signed by a competent person under SANS 10400. That is not a formality. It is the difference between a wall that is insurable and one that is not.
Surcharge is the part most people miss. A 1.2 m wall holding a flat garden is a modest job. The same wall with a driveway running along the top is carrying vehicle loads through the soil into the back of the wall, and it needs to be designed for it.
Drainage is what actually fails
Most collapsed retaining walls on the Garden Route did not fail because the wall type was wrong. They failed because water built up behind them.
Saturated soil is heavier and pushes harder. Worse, water that cannot escape develops hydrostatic pressure against the back of the wall, and that load climbs fast — it can exceed the soil pressure the wall was designed for. A wall built for dry conditions and then drowned every winter is a wall on a countdown.
The fix is unglamorous and non-negotiable: a free-draining backfill zone of clean stone against the wall, a wrapped subsoil drain at the base falling to a discharge point that actually goes somewhere, weepholes through the face at regular centres, and a geotextile separating the stone from the fill soil so the drain does not silt up within two winters.
On the Outeniqua side, where winter rain is persistent rather than dramatic, the drain matters more than the wall thickness. We would rather build a slightly lighter wall with a proper drain than a heavy one with a blocked pipe.
Foundations and the ground you are building on
A retaining wall is only as good as what it stands on. The footing must bear on undisturbed material or engineered fill, below any topsoil, and below the depth at which the ground moves seasonally.
Garden Route soils vary enormously over short distances. Sandy coastal profiles behave very differently from the clays inland of George, and low-lying erven around Wilderness and Sedgefield can carry a high winter water table that changes the whole design basis. We dig trial holes before pricing anything structural, because the difference between founding at 600 mm and founding at 1.2 m is the difference between two quite different jobs.
Where the ground is genuinely poor, a gabion often beats a rigid wall. It can settle and deform without cracking, where a mass concrete wall in the same position will open a crack and then keep opening it.
What each type costs you beyond the wall itself
Comparing wall types on the face area alone is misleading, because the hidden costs differ more than the visible ones.
A gravity wall is cheap per square metre of face but wide at the base, so it consumes usable land and more excavation than it looks like it should. A cantilever wall is the opposite — narrow footprint, but reinforcement, formwork and an engineer’s design fee. Crib walls build fast and save on labour but need the batter, which quietly gives away plan space at every course. Gabions are competitive on material but heavy on transport if the rock is not local, and the wire is the life-limiting component in salt air.
Then there is the drainage, which is the same on all four and is not optional, and the backfill, which is not the spoil you just dug out. Pricing a wall without pricing the imported granular backfill is the most common way a retaining wall quote comes in low and finishes high.
When to repair and when to rebuild
A wall that is leaning, bulging in the middle of its face, or showing stepped cracking through the courses is telling you the structure is overloaded — usually by water, occasionally by a surcharge nobody planned for. That is not a patching job.
A wall that is sound but has spalled facing, blocked weepholes or a silted drain is worth saving. Clearing and reinstating the drainage is a fraction of the cost of a rebuild and often stops the deterioration.
The test we apply on site is simple: is the wall still doing its job and the damage cosmetic, or has it started to move? Movement means rebuild, and it means understanding why the first one moved before the second one goes up in the same place.
Frequently asked questions
What are the different types of retaining walls?
The four in common use are gravity walls, which resist by mass; cantilever reinforced concrete walls, which use the weight of soil on their footing heel; crib walls, which are stacked interlocking cells filled with granular material; and gabion walls, which are rock-filled wire baskets. Height, soil conditions and available space behind the wall decide which is appropriate far more than preference does.
What is the cheapest type of retaining wall?
For low walls under about a metre, mortared block or mass concrete gravity walls are usually cheapest because they need no engineered reinforcement. Gabions are often cheapest at height where suitable rock is available locally. But the cheapest wall that fails is not cheap, and the drainage behind any wall costs the same regardless of the type you choose.
How high can you build a retaining wall without engineering?
In practice, walls above 1.5 m or any wall retaining a surcharge such as a driveway or structure need a rational design by a competent person under SANS 10400. Below that, and with no surcharge, a well-detailed gravity or block wall is usually adequate. If in doubt, get the design — it costs a fraction of a rebuild.
Why do retaining walls fail?
Water, overwhelmingly. Saturated soil weighs more and pushes harder, and trapped water builds hydrostatic pressure against the back of the wall that can exceed what it was designed for. Blocked weepholes, a silted subsoil drain or backfill with too many fines are behind most of the failures we are called to look at. The second cause is an unplanned surcharge — a driveway or slab added above a wall that was never designed to carry it.
Do retaining walls need drainage?
Yes, without exception. A free-draining stone zone against the wall, a wrapped subsoil drain at the base falling to a real discharge point, weepholes through the face, and a geotextile separating stone from soil. On the Garden Route, with persistent winter rain, the drainage detail matters more to the life of the wall than the wall thickness does.
What is the best retaining wall for a slope?
On sloping or unstable ground, gabions and crib walls usually outperform rigid concrete because they tolerate movement without cracking. On a stable slope where you need a vertical face and maximum usable space at the top, a cantilever wall is the better answer. The ground conditions decide it, which is why we dig trial holes before pricing.
How long does a retaining wall last?
A properly designed and drained concrete or block wall should outlast the building it serves. Gabions are limited by the wire — in coastal salt air, galvanised mesh has a shorter life than PVC-coated, and specifying the wrong one near the sea is a false economy. Any wall with failed drainage has a much shorter life than its materials suggest.
Related reading
- Gravity, cantilever and gabion walls in detail
- Foundation types for George soils
- Slab thickness, MPa and reinforcement
- Retaining wall construction in George
- All our concrete services
Need a wall designed and built properly?
We’re based in George and we build retaining walls the length of the Garden Route. Walk the site with us and you’ll get a priced BOQ, a programme, and an honest view on whether you need an engineer.
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