Cantilever Retaining Wall Design Example — 10 ft Backfill per ACI 318-25 & IBC 2024

Retaining walls fail differently than beams. A beam that is 10% overstressed cracks; a retaining wall that is 10% short on sliding resistance moves. That is why the building code gives retaining walls their own acceptance criterion — a safety factor of 1.5 against overturning and sliding at unfactored loads (IBC 2024 §1807.2.3) — before any strength design begins. In this example the strength checks all pass with room to spare, but the sliding check comes down to the wire: without a shear key the wall slides at FS = 1.35, and a 12 × 12 in key is what carries it past 1.5.

Every number here was produced by the calculation engine that runs StructSuite's Retaining Wall module (/design/retaining-wall), and the agreement is locked by an automated regression test (tests/verification/retaining-wall-guide-example.test.ts) — the same trust chain used for all StructSuite worked examples.

#The wall

Geometry — 10 ft uniform-thickness stem (12 in), retained height 10 ft (backfill to the top of the wall), toe 3 ft, heel 6 ft, footing 18 in thick (B = 3 + 1 + 6 = 10 ft), 1 ft of soil over the toe, 12 × 12 in shear key under the stem.

Soil — SM silty-sand backfill, γ = 120 pcf, uniform surcharge q = 100 psf, coefficient of friction μ = 0.35, passive lateral bearing 200 psf/ft, allowable bearing 3,000 psf (values in the range of IBC Table 1806.2 presumptive class 4).

Concrete — f′_c = 3,000 psi, Grade 60 (f_y = 60,000 psi), normalweight 150 pcf. Covers per ACI 318-25 Table 20.5.1.3.1: 3 in at the footing bottom (cast against earth), 2 in at the stem earth face and footing top (formed, exposed to earth). Reinforcement: stem #6 @ 8 in (earth face), stem horizontal #4 @ 9 in, heel top #6 @ 10 in, toe bottom #6 @ 12 in.

The design assumes drained backfill (IBC §1610.1 otherwise requires full hydrostatic pressure) and no seismic earth pressure (required only for SDC D–F with more than 6 ft of backfill, from the geotechnical report — IBC §1807.2.2; ASCE 7-22 §11.8.3).

#1. Design lateral soil load — IBC 2024 Table 1610.1

A cantilever retaining wall is free to move and rotate at the top, so IBC 2024 §1610.1 (and ASCE 7-22 §3.2.1, in identical words) permits active pressure; walls restrained at the top must use at-rest. For SM backfill, Table 1610.1 gives:

Worth knowing: ASCE 7-22 Table 3.2-1 tabulates the same SM class at 45 psf/ft active — but the two books disagree on several other classes (SC is 60 psf/ft in the IBC and 85 in ASCE 7-22). StructSuite offers both tables verbatim; use the one your jurisdiction enforces.

The surcharge needs a lateral pressure coefficient. The equivalent fluid pressure already embeds one: K = w/γ = 45/120 = 0.375, so the surcharge adds a uniform K·q = 0.375 × 100 = 37.5 psf of lateral pressure over the full height (surcharge-induced lateral pressure is part of the load H — ASCE 7-22 §2.2; added per IBC §1610.1).

For stability, the pressure acts on the vertical plane at the heel over the full height from backfill surface to footing bottom: H = 10.00 + 1.50 = 11.50 ft.

• P_soil = w·H²/2 = 45 × 11.50²/2 = 2.976 kip/ft, acting at H/3 = 3.83 ft above the footing base
• P_surcharge = K·q·H = 0.375 × 100 × 11.50 = 0.431 kip/ft, acting at H/2 = 5.75 ft

#2. Overturning — FS ≥ 1.5 (IBC §1807.2.3)

IBC §1807.2.3 is explicit: the §1605 load combinations do not apply to this check. Stability runs at 1.0 × nominal loads, and variable loads are investigated set to zero — which cuts both ways. The surcharge's lateral push is driving, so it stays; its vertical weight over the heel would help resist, so it is dropped.

Resisting moments about the toe (per foot of wall):

Overturning moments: M_OT(soil) = 2.976 × 3.83 = 11.407 kip-ft/ft; M_OT(surcharge) = 0.431 × 5.75 = 2.480 kip-ft/ft.

• Case D + H (no surcharge): FS = 67.965/11.407 = 5.96
• Case D + H + surcharge (driving only): FS = 67.965/13.886 = 4.89 ← governs

FS_OT = 4.89 ≥ 1.5 ✓ — overturning is rarely the problem for a wall with a 6-ft heel full of soil.

#3. Sliding — the check the key rescues

The sliding safety factor is defined by §1807.2.3 as available soil resistance at the base ÷ net lateral force. Resistance may combine base friction, cohesion (capped at half the dead load, §1806.3.2), and passive lateral bearing (§1806.3.1):

• Friction = μ × ΣW = 0.35 × 11.460 = 4.011 kip/ft (Table 1806.2 footnote a: coefficient × dead load)
• Passive: the key extends the pressure depth to D_p = 1.00 (soil over toe) + 1.50 (footing) + 1.00 (key) = 3.50 ft, so P_passive = ½ × 200 × 3.50² = 1.225 kip/ft
• R = 4.011 + 1.225 = 5.236 kip/ft

Against the driving forces:

• Case D + H: FS = 5.236/2.976 = 1.76
• Case D + H + surcharge: FS = 5.236/3.407 = 1.54 ← governs

FS_sliding = 1.54 ≥ 1.5 ✓ — barely. Delete the key and D_p drops to 2.50 ft, P_passive to 0.625 kip/ft, and FS to 1.35 ✗. A 1-ft-deep key is cheaper than a wider footing, which is why it is the classic fix for a sliding-governed wall.

#4. Soil bearing pressure — kern-aware

With the resultant location x̄ = (M_r − M_OT)/ΣV from the toe, the eccentricity e = B/2 − x̄ decides the distribution: inside the kern (|e| ≤ B/6 = 1.667 ft) the base is in full contact with q = (ΣV/B)(1 ± 6e/B); outside it, a triangular partial-contact block applies.

For bearing (unlike stability) the surcharge's vertical weight is unfavorable — it raises the pressure — so it is included. q_max = 1,327 psf ≤ q_allow = 3,000 psf ✓ (D/C = 0.44). Both cases sit comfortably inside the kern; the wall bears on its full base.

#5. Stem flexure — a one-way slab per ACI 318-25 §13.3.6

ACI 318-25 treats the stem of a cantilever retaining wall as a one-way slab designed per Chapter 7 (§13.3.6.1), and for a uniform-thickness stem the critical section for both moment and shear is at the stem–footing interface (§13.3.6.3) — the joint opens under lateral load. The pressure on the stem acts over the retained height H_s = 10 ft only (the footing depth belongs to the stability model, not the stem).

Service moment at the interface:

M = w·H_s³/6 + K·q·H_s²/2 = 45 × 10³/6 + 0.375 × 100 × 10²/2 = 9.375 kip-ft/ft

Lateral earth pressure adds to the load effect, so H takes a load factor of 1.6 (ASCE 7-22 §2.3.1; ACI 318-25 §5.3.8):

M_u = 1.6 × 9.375 = 15.000 kip-ft/ft

Capacity with #6 @ 8 in at the earth face (A_s = 0.44 × 12/8 = 0.660 in²/ft; d = 12 − 2 − 0.375 = 9.63 in):

• a = A_sf_y/(0.85f′_cb) = 0.660 × 60,000/(0.85 × 3,000 × 12) = 1.294 in
• φM_n = 0.90 × 0.660 × 60,000 × (9.63 − 1.294/2)/12,000 = 26.664 kip-ft/ft

D/C = 15.000/26.664 = 0.56 ✓. Two more Chapter 7 requirements are not optional: the section must be tension-controlled (§7.3.3.1 — here ε_t = 0.01597 ≥ ε_ty + 0.003 = 0.00507 ✓), and A_s ≥ 0.0018bh = 0.259 in²/ft (§7.6.1.1 ✓), with spacing ≤ min(3h, 18) = 18 in (§7.7.2.3 ✓).

#6. Stem shear

§13.3.6.1.1 permits the pre-2019 simple form V_c = 2λ√f′_c·b_w·d for cantilever retaining wall stems — no size-effect factor, no axial term.

• V_u = 1.6 × (45 × 10²/2 + 0.375 × 100 × 10)/1,000 = 1.6 × 2.625 = 4.200 kip/ft
• φV_c = 0.75 × 2 × 1.0 × √3,000 × 12 × 9.63/1,000 = 0.75 × 12.652 = 9.489 kip/ft

D/C = 0.44 ✓ — checked at the interface, not d above it (§13.3.6.3).

#7. Heel — shear at the face, not at d

The heel hangs from the stem and carries the backfill down: w_u = 1.2 × (120 × 10 + 150 × 1.5) + 1.6 × 100 = 1,870 psf (upward bearing pressure under the heel is conservatively neglected). Both critical sections are at the back face of the stem — and note the shear subtlety: because the top of the stem–heel joint is in flexural tension, §7.4.3.2 does not permit moving the shear check to d from the face (§13.2.7.2).

• M_u = 1,870 × 6²/2 = 33.660 kip-ft/ft; V_u = 1,870 × 6 = 11.220 kip/ft
• Heel TOP bars #6 @ 10 in: A_s = 0.528 in²/ft, d = 18 − 2 − 0.375 = 15.63 in
• φM_n = 35.895 kip-ft/ft → D/C = 0.94 ← the governing strength check of the whole design
• φV_c = 0.75 × 2√3,000 × 12 × 15.63/1,000 = 15.405 kip/ft → D/C = 0.73

✓ PASS — with A_s = 0.528 ≥ 0.0018 × 12 × 18 = 0.389 in²/ft (§7.6.1.1).

#8. Toe — designed from the factored bearing distribution

The toe is loaded upward by the bearing pressure at strength level. The engine builds the factored distribution for both governing combinations (ASCE 7-22 §2.3.1 H rules):

Net of the factored toe self-weight and soil cover, moment at the stem front face and shear at d from the face (the toe end region is in compression, so §7.4.3.2 does allow d here):

• Governing (1.2D + 1.6H + 1.6L): M_u = 6.377 kip-ft/ft, V_u = 2.542 kip/ft
• Toe BOTTOM bars #6 @ 12 in (A_s = 0.440 in²/ft, d = 14.63 in): φM_n = 28.103 kip-ft/ft, φV_c = 14.419 kip/ft

D/C = 0.23 ✓ — the toe is nowhere near critical here, but it cannot simply borrow the heel's bars: it needs its own mat at the bottom face.

#9. Development — hooks, laps, and a 0.14-inch margin

Three development questions decide whether the bars actually work:

Stem dowels into the footing (§25.4.3.1, Table 25.4.3.2): ℓ_dh = (60,000 × 1.0 × 1.0 × 0.7 × 1.0)/(50 × √3,000) × 0.75 = 11.5 in ≥ max(8d_b, 6 in), with ψ_cc = 0.7 (side cover ⊥ the hook plane ≥ 2.5 in — automatic in a continuous wall — and 2 in over the 90° hook). Available = 18 − 3 − 0.75 = 14.3 in ✓. The 90° hook extension is 12d_b = 9.0 in, placed near the bottom of the footing with the free end toward the toe (R13.3.6.3).

Dowel-to-stem lap (§25.5.2.1, Class B): ℓ_d(stem #6, ψ_t = 1.0) = 32.9 in → lap = 1.3ℓ_d = 42.7 in of dowel projection above the interface.

Heel and toe mats (§13.2.8.3; Table 25.4.2.3): the heel TOP bars sit above 15.3 in of fresh concrete — more than 12 in — so they take ψ_t = 1.3 (Table 25.4.2.5): ℓ_d = 42.7 in vs 69.0 in available ✓. The toe bars need ℓ_d = 32.9 in vs 33.0 in available — a pass with 0.14 in to spare. A shorter toe would force a hook.

#10. Minimum & interface reinforcement

• Wall–footing interface (§16.3.4.2 → Table 11.6.1): A_s,min = 0.0015 × 12 × 12 = 0.216 in²/ft; the #6 @ 8 dowels provide 0.660 ✓
• Stem horizontal shrinkage + temperature (§24.4.3.2): 0.0018 × 12 × 12 = 0.259 in²/ft; #4 @ 9 provides 0.267 ✓, spacing 9 ≤ min(5h, 18) = 18 in (§24.4.3.3) ✓. A single curtain is expressly permitted in cantilever retaining wall stems (§11.7.2.3 exception).
• Footing longitudinal S+T: provide ≥ 0.0018 × 12 × 18 = 0.389 in²/ft along the wall.

#Results summary

The design is stability- and heel-governed — the classic cantilever wall outcome.

#FAQ

#When can I use active pressure instead of at-rest?

IBC 2024 §1610.1 and ASCE 7-22 §3.2.1 both permit active pressure for walls free to move and rotate at the top — which a free-standing cantilever retaining wall is. A basement wall braced by a floor diaphragm is restrained and must take at-rest pressure (60 psf/ft for this SM backfill instead of 45 — a 33% increase).

#Why don't the load combinations apply to the stability check?

IBC §1807.2.3 replaces them: stability uses 1.0 × nominal loads (0.7 × nominal earthquake where included) with a hard FS of 1.5, investigating variable loads set to zero. That last clause is why the surcharge pushes but never holds down: its lateral component is included as driving while its vertical weight over the heel is excluded from the resisting side. The 1.6H / 0.9D factored combinations return for the concrete strength design (ACI 318-25 §5.3.8).

#The IBC and ASCE 7-22 lateral soil load tables disagree — which governs?

Both are legal minimums in their own documents, and they genuinely differ (SC clayey sand: 60 psf/ft active in IBC Table 1610.1 vs 85 in ASCE 7-22 Table 3.2-1). Where the IBC is the adopted code it governs, but a geotechnical report per IBC §1803 supersedes either table. StructSuite shows both tables verbatim and records which one you selected.

#Do I need to add seismic earth pressure?

Only when the structure is assigned to Seismic Design Category D, E, or F and the wall supports more than 6 ft of backfill — and then the value comes from the geotechnical investigation, not from a code table (IBC §1807.2.2; ASCE 7-22 §11.8.3 requires the geotech report to determine it). Where earthquake loads are included, the minimum stability safety factor drops to 1.1 (IBC §1807.2.3 exception).

Run this example yourself in the StructSuite Retaining Wall module — the pre-filled version is at /examples/retaining-wall/cantilever-retaining-wall-10ft-backfill. Every number above is regenerated from the engine on every release and locked by tests/verification/retaining-wall-guide-example.test.ts.