1 What the tool does
A preliminary stability check for a reinforced-concrete cantilever or counterfort retaining wall, using Rankine active earth-pressure theory.
From your geometry, soil and loads it works out the three classic stability checks — overturning, sliding and bearing — plus material quantities and cost, and draws a live cross-section that redraws as you type.
It applies textbook equilibrium for a level or gently sloped granular (cohesionless) backfill. It deliberately does not:
- size the reinforcement (bar diameters / spacing),
- check global slope stability, seismic loads, or long-term settlement,
- model uplift water pressure under the base.
2 Reading the results
Three verdicts, each a factor of safety (FoS) — how many times stronger the wall is than the load trying to defeat it.
| Check | The question | Passes when |
|---|---|---|
| Overturning | Does it tip over about the front (toe) edge? | FoS = M_resist / M_overturn ≥ 2.0 |
| Sliding | Does it slide forward on its base? | FoS = (friction + passive) / push ≥ 1.5 |
| Bearing | Is the soil under the base overloaded? | q_max ≤ q_allowable |
The "middle third" (eccentricity)
The resultant of all the downward loads should land within the middle third of the base width — e ≤ B/6. Inside it, the whole base pushes down on the soil (good). Outside it, the base starts to lift at one edge (heel uplift) and the pressure spikes at the other — the wall is being loaded off-centre.
The pills read OK MARGINAL FAILS. The diagram has two views — Forces (earth-pressure arrows, the red/green sliding line, the bearing-pressure block, and any water) and Reinforcement (the bar layout) — plus a plan strip when counterforts are on.
3 Choosing a wall type
Height decides the family; site constraints decide the base shape.
| Height | Economical choice |
|---|---|
| up to ~2 m | Gravity / masonry / segmental block |
| ~2 – 6.5 m | RC cantilever — the workhorse |
| ~6.5 m and up | Counterfort / buttress |
| soft ground / no room | Gabion, MSE (reinforced soil), or piled walls |
Cantilever vs. counterfort
A cantilever wall's stem is a vertical cantilever fixed into the base. A counterfort wall adds triangular ribs on the soil side that tie the stem back to the heel — turning the stem into a slab that spans horizontally between ribs, so it can stay thin as it gets tall. See §5.
T-shape vs. L-shape base
B ≈ 0.5–0.7 × H; stem & base thickness ≈ H/10; heel longer than toe.4 Why height matters so much
Demand doesn't grow in proportion to height — it grows far faster.
Earth pressure builds with depth, so the total push grows with H² and the overturning moment with H³. Going from a 3 m to a 4 m wall is only 33% taller, but:
| Demand | Scales as | 3 m → 4 m |
|---|---|---|
| Horizontal push | H² | ×1.8 (+78%) |
| Overturning moment | H³ | ×2.4 (+137%) |
| Concrete & steel | — | ≈ ×1.75 |
So a 1 m taller wall isn't "a third harder" — it's roughly 2.4× harder to keep from tipping, and needs a wider base and a much stronger (or ribbed) stem. If you keep the base width in step with height (~0.6 H), the safety factors stay about the same; it's the size and material that climb.
5 Counterforts
Triangular ribs that let a tall wall keep a thin stem.
A tall cantilever stem is punished by that H³ moment. Add ribs at a spacing and the stem stops being a tall vertical cantilever — instead it spans horizontally between the ribs, so its design moment drops to about p·s²/10 (spacing-squared, not height-cubed). The ribs carry the load down to the heel as tension ties.
6 Reinforcement & tension faces
One rule reads every rebar drawing: the main steel follows the tension face.
Concrete is weak in tension, so the big bars go wherever the element is being stretched:
- Stem — earth pushes it toward the front, so it bends with the soil (back) face in tension. Main vertical bars go on the soil face.
- Heel — backfill weight pushes it down, tension on top → main heel bars near the top.
- Toe — ground bearing pushes it up, tension on the bottom → main toe bars near the bottom.
- Dowels / starter bars lap the stem's soil-face bars around the corner into the base, keeping the tension path continuous.
- The opposite faces get lighter nominal / temperature steel to control cracking and hold the cage.
The tool's Reinforcement view draws these positions live; for a counterfort it also shows the rib's sloping tension steel.
7 The shear key
A small tooth under the base that fixes sliding cheaply.
Sliding is often the tight check. A shear key — a downward projection under the base, usually 0.3–0.7 m deep — helps two ways:
- Passive resistance: to slide, the wall must now shove the soil in front of the key out of the way, which pushes back hard:
Pp = ½·Kp·γ·(Df+dk)². Because it grows with depth squared, a modest key adds a lot. - Better failure plane: the slip surface moves off the slick concrete into soil-on-soil, mobilising the full friction angle.
8 Drainage & the gravel layer
The single most important thing keeping a retaining wall standing.
If water builds up behind the wall it adds hydrostatic pressure on top of the earth pressure — enough to roughly double the load and fail an otherwise-fine wall. (Try setting a water table in the tool: 2 m of water alone can push sliding from pass to fail.) The whole drainage system exists to keep the water table at zero — which is what every "drained" calculation assumes.
The gravel drainage layer — requirements
| Requirement | Spec |
|---|---|
| Material | Clean, angular crushed stone, free-draining, durable, non-plastic. |
| Size | ~20–40 mm (¾″–1½″) |
| Fines | < ~5% passing 0.075 mm (No. 200 sieve) — fines are what clog it. This is the #1 spec. |
| Separation / filter | Wrap in non-woven geotextile, or size as a graded filter to the backfill (Terzaghi rules) so soil can't wash in and clog it. |
| Thickness | ≥ 300 mm blanket against the back of the stem, run the full height. |
| Base collector | Drain into a ~100 mm perforated pipe at the base, laid to fall ≥ 1% to a daylight outlet or sump. |
| Top / surface | Cap with a low-permeability layer or grade the surface to shed rain away from the wall. |
| Placement | Don't over-compact the drainage stone; compact the structural backfill beyond it in layers. |
The Terzaghi filter criteria (if using graded gravel instead of geotextile): retention D₁₅(gravel) < 4–5·D₈₅(soil) and permeability D₁₅(gravel) > 4–5·D₁₅(soil), well-graded.
Weep holes
Weep holes are a secondary path: they run from the gravel/soil side through the stem, sloping down, to daylight on the exposed (front) face, above the toe grade. Keep gravel behind them so they don't silt up. Direction matters — water always goes from behind the wall to the front.
9 Costing
Two ways to price the wall.
A quote often rises roughly with height (e.g. +33% from 3 → 4 m) while the raw material grows faster (~75%). Counterforts are usually why: they keep the stem thin, so the taller wall doesn't cost proportionally more.
10 Glossary
- Stem
- The vertical wall that holds back the soil.
- Base slab / footing
- The horizontal slab the stem sits on, spreading load to the ground.
- Toe
- The part of the base projecting in front (exposed side).
- Heel
- The part of the base projecting behind, under the backfill.
- Counterfort
- A triangular rib on the earth side tying stem to heel (tension).
- Buttress
- The same rib on the front side (compression) — rarer.
- Shear key
- A tooth under the base that boosts sliding resistance.
- Backfill
- The soil retained behind the wall.
- Surcharge
- Extra load on top of the backfill (building, road, slope) — adds to the push.
- Kₐ / Kₚ
- Rankine active / passive earth-pressure coefficients (from the friction angle φ).
- φ (phi)
- Soil friction angle — how well the soil resists shear.
- γ / γ′
- Soil unit weight / submerged (buoyant) unit weight below the water table.
- FoS
- Factor of safety — capacity ÷ demand.
- Eccentricity (e)
- How far off-centre the load resultant lands on the base; keep
e ≤ B/6. - Weep hole
- A hole through the stem that drains water from behind to the front.
11 FAQ
Can I build straight from this tool?
No. It's a preliminary planning aid. It doesn't size reinforcement bars, and it skips seismic, slope-stability and settlement checks. Get stamped drawings and calculations from a licensed engineer with your site's soil data before building.
Why does my wall fail sliding when overturning and bearing pass?
Sliding is commonly the tightest check — plain base friction often isn't enough. The usual fix is a shear key (or counting the toe's passive resistance), which the tool lets you add. Widening the base or roughening the underside also help.
Do counterforts fix sliding or bearing?
No — those depend on base width and weight. Counterforts cut the stem's bending moment so the stem can stay thin. You still size the base, shear key and drainage the same way.
Can I use an L-shape (no toe) on a boundary?
Yes, and it's common on property lines. But losing the toe pushes the load resultant out of the middle third, so bearing suffers — an L-shape usually needs a noticeably wider base than the equivalent T-shape to pass. See §3.
There's a building above the wall — what do I do?
Add it as a surcharge (kPa) on the backfill. It increases the active push and the overturning moment. A significant surcharge is also one of the few reasons a counterfort can be worth it below ~6 m.
Why did my quote only rise 33% but the material ~75%?
Because a contractor's ฿/m lump rate isn't a raw material sum, and because ribbing the stem keeps concrete/steel from ballooning as the wall gets taller. The quote is the number to budget with; the unit-rate buildup is a cross-check.
What's the most important thing to get right?
Drainage. Water behind the wall is the number-one cause of failure. A clean gravel layer, geotextile filter, a base pipe that drains to daylight, and weep holes keep the water table at zero — where all your safety factors pass. See §8.
12 Limits & disclaimer
The calculator applies textbook Rankine earth-pressure theory and standard equilibrium checks for a level or sloped granular (drained, cohesionless) backfill. It assumes the soil's given unit weight is saturated below any water table (γ′ = γ − 9.81), and it does not model uplift pressure under the base — so a water-logged case is at least as bad as shown.
It does not perform reinforcement design (bar sizing/spacing), global slope stability, seismic, or settlement analysis. All output is order-of-magnitude planning information.