Cantilever Retaining Wall Design Step by Step — Full Worked Example (ACI 318-25, IBC 2024)

What this module designs

0:01The Cantilever Retaining Wall module designs a cast-in-place concrete wall, per one-foot strip, to ACI 318-25 and IBC 2024. Overturning, sliding and bearing; the stem, heel, toe and shear key; development, hooks and laps; and the seismic earth-pressure increment from ASCE 7-22. Let's design a wall on a hillside site. From the module list, add the Cantilever Retaining Wall. It opens completely empty — nothing is assumed.

Step 1 — Proportioning the wall: stem, footing, toe, heel

0:30Step one, geometry. A ten-foot stem, twelve inches thick, on an eighteen-inch footing: a three-foot toe in front, a six-foot heel under the backfill, and one foot of soil covering the toe. The footing width adds itself up.

0:54The section sketch appears the moment the wall can be drawn, dimensioned from what you typed. Backfill slope beta stays at zero for now, and the shear key stays off — the ground behind this wall is level.

The code quoted at the field (IBC 2024 Sec. 1807.2.2)

1:05The book icon beside a field is not a tooltip — it opens the code, quoted verbatim. Beside retained height sits IBC section 1807.2.2: a wall in Seismic Design Category D, E or F retaining more than six feet of backfill has to carry the additional seismic earth pressure. That is the rule that makes step two ask about earthquakes. Collapse the step and its summary turns blue: every dimension user-defined.

Step 2 — Where the lateral soil load comes from

1:31Step two, soil. The lateral soil load comes from one of three sources: IBC 2024 Table 1610.1, ASCE 7-22 Table 3.2-1, or the geotechnical report's equivalent fluid pressure. Both code tables are printed verbatim, and they disagree for several soil classes — so the module never blends them.

IBC 2024 Table 1610.1, reproduced verbatim

1:52We'll take the IBC table. Our backfill is a well-graded clean gravel, class GW: thirty pounds per square foot per foot of depth active, sixty at rest.

Active vs at-rest earth pressure

2:01Then the pressure condition — the module will not guess it. A wall free to rotate at the top gets active pressure. Restrain that top and you owe at-rest, which for this gravel doubles the load.

Surcharge: variable vs permanent, and why it matters

2:12Soil unit weight, a hundred and twenty pcf, and a hundred psf of surcharge. Its nature changes the arithmetic: a variable surcharge, traffic or parking, pushes on the wall but its weight is kept off the resisting side. A permanent one counts both ways.

Friction, passive and bearing from IBC Table 1806.2

2:28Sliding and bearing resistance come straight from IBC Table 1806.2. Pick the sandy gravel row and it fills the resistance fields: allowable bearing three thousand, lateral bearing two hundred per foot, friction zero point three five — and no cohesion, because a clean gravel has none. If the soil in front of the toe might be dug up during the life of the wall, one checkbox drops passive resistance out of the sliding check entirely.

2:53Here we keep it. Last, the seismic earth-pressure question. It has no default — you answer it, one way or the other, before the module will run. This site starts in Seismic Design Category B, so: not included.

Step 3 — Concrete, covers and the five bar sets

3:05Step three. Three thousand psi concrete, Grade 60 steel, normalweight — which is what sets lambda. Covers follow ACI Table 20.5.1.3.1: three inches at the footing bottom cast against earth, two inches on the formed faces exposed to earth.

3:28Five bar sets. Stem verticals on the earth face; stem horizontals for shrinkage and temperature; heel top bars, because the heel hangs; toe bottom bars, because the toe is pushed up; and footing longitudinals. Number six at eight inches in stem and heel, at twelve in the toe.

Live check of As,min, As,max and bar spacing

3:58The table checks every set live — area against As minimum and As maximum, spacing against the code cap. A red cell means you are outside ACI before you ever reach verification.

Step 4 — Overturning, sliding and bearing

4:10Step four runs it. Overturning, factor of safety seven point three. Sliding, two point zero. Bearing, thirteen hundred sixty-one psf against three thousand. Then stem, heel and toe strength, development and the minimums. Note the stability checks: factors of safety at nominal load per IBC 1807.2.3, not load combinations. Open any row and the calculation is underneath it: the formula, the arithmetic with the real numbers substituted, and the ACI or IBC section it comes from.

Sloped backfill 3H:1V — the code tables stop applying

4:41Now the site as it really is. The ground behind this wall does not stay level; it climbs at three horizontal to one vertical. Back to step one, and enter beta: eighteen point four three degrees. Step two objects at once. Table 1610.1 and Table 3.2-1 are level-backfill values, so a rising backslope needs the geotechnical report's own pressure. Switch the source: thirty-six psf per foot.

SLIDING FAILS — FS 1.31 against the 1.5 required

5:06And the wall fails. The pressure plane at the heel grew from eleven and a half feet to thirteen and a half, and sliding drops to a factor of safety of one point three one against the one point five required. Sliding is nearly always the check that goes first.

Three levers, and the shear key that fixes it

5:20A sliding failure has three levers: widen the base to pick up more soil weight, change the backfill specification to cut the driving pressure, or add a shear key. The key is the cheapest — passive resistance grows with the square of depth. Twelve inches wide, twenty-four deep. Sliding recovers to one point seven two and every static check passes. And the module adds a tenth check: whatever the key attracts, the key itself has to carry — flexure and shear at the junction with the footing.

Seismic earth pressure in SDC D

5:47Now the seismic side. This wall is in Seismic Design Category D retaining more than six feet, which is exactly when ASCE 7-22 wants a seismic earth-pressure increment. Go back and include it.

Mononobe-Okabe vs Seed-Whitman vs the geotech report

6:00Three methods are offered: the geotechnical report's own seismic pressure, the Seed-Whitman simplified increment, or full Mononobe-Okabe. Seed-Whitman is a level-ground, cohesionless method, so on this slope the module rules it out — leaving Mononobe-Okabe, or the report's own pressure, which the code prefers whenever the report states one. We have no report value, so Mononobe-Okabe. S_DS is zero point six six seven. The restraint answer sets k_h: a wall free to displace takes S_DS over two point five.

6:30Phi forty degrees, delta eighteen point four, and beta comes straight from step one — one wall, one slope. We include the wall's own inertia too.

SEISMIC SLIDING FAILS — FS 0.93 against 1.1

6:43Six seismic checks join the list and the earthquake takes over. ΔP_AE is thirty-four hundred pounds per foot. Seismic sliding comes back at zero point nine three against the one point one the IBC exception allows — the twenty-four inch key is not enough.

Deepening the key to 42 in

6:57Back to step one, and deepen the key from twenty-four inches to forty-two.

All sixteen checks pass

7:07All sixteen pass. Seismic sliding, one point one six. Seismic bearing, twenty-nine hundred fourteen psf under the ASD combination. On this wall the earthquake, not gravity, sized the key. Underneath, the module states its own assumptions: drained backfill, the sloped-backfill model, what the seismic increment leaves out, and that the soil in front of the toe is assumed permanent.

Step 5 — Pressure diagrams and the free body

7:29Step five draws it. Each load case alone — dead weights, earth pressure, surcharge, the seismic increment — then the service and strength combinations, each labelled with the check it governs. Pick a single load case and the section redraws that component by itself — here the seismic increment, standing on its own vertical plane beside the static block, at its own scale. Pick a combination instead and the same figure closes the loop:

7:52the soil bearing distribution appears, drawn to the very same scale as the pressure blocks so the shapes are honestly comparable, with the allowable bearing marked as a dashed reference line. And beside it the whole free body, every resultant with its lever arm — active thrust, surcharge push, passive resistance, base friction, the permanent weight and the bearing reaction.

The reinforcement detail drawing

8:14And the reinforcement detail is drawn to scale from the results themselves: hook lengths, lap splices, embedments and covers, with the ACI section behind every note. That is a complete cantilever retaining wall — stability, bearing, strength, detailing and seismic — ready to print straight into your calculation package.