IBC Ch. 18 / Geotech

IBC Chapter 18 & Rankine Theory | Retaining Walls & Deep Piles

Cantilever retaining walls hold back soil wedges that exert horizontal Rankine active earth pressure against the stem. Under IBC Section 1807.2.3, retaining walls must achieve a minimum Factor of Safety of 1.5 against both overturning about the toe and horizontal sliding along the base, while deep piles transfer load through shaft skin friction and end bearing.

Code Equation

Governing Formula

K_a = \tan^2\left(45^\circ - \frac{\phi'}{2}\right), \quad \text{ FS }_{\text{ OT }} = \frac{\sum M_R}{\sum M_O} \ge 1.5, \quad Q_{\text{ ult }} = Q_s + Q_p

Standard: IBC Ch. 18 / GeotechDual US Customary & SI Metric

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Notation & Variables

K_a

Rankine active lateral earth pressure coefficient for level backfill

unitless

\phi'

Effective angle of internal friction of backfill soil

degrees

\sum M_R, \sum M_O

Sum of stabilizing resisting moments and overturning driving moments about the toe

kip-ft/ft or kN·m/m

Q_s, Q_p

Ultimate pile shaft skin friction and toe end bearing resistance

kips or kN

Worked Example

Compute active earth pressure coefficient Ka

For granular backfill with friction angle φ' = 30°: Ka = tan²(45° - 15°) = 0.333.

Calculate total horizontal thrust Pa

For a 12-foot wall with γ = 120 pcf: Pa = 0.5 × Ka × γ × H² = 0.5(0.333)(120)(12²) = 2,880 lb/ft acting at H/3 = 4.0 ft.

Verify overturning and sliding factors of safety

Ensure FS_OT = MR / MO ≥ 1.50 and FS_sliding = FR / Pa ≥ 1.50 per IBC §1807.2.3.

Governing Criteria

When does this govern?

Governs all basement walls, site retaining walls, bridge abutments, and driven/bored deep pile foundations.

Common Pitfall

Common Mistake

Relying on passive earth pressure in front of the wall toe for sliding resistance when the front soil may be excavated for utility trenches.

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