Cantilever Retaining Wall Stability: Overturning and Sliding
A cantilever retaining wall stays in place through simple physics: the weight of the concrete stem and footing, plus the weight of the soil sitting on the heel, must resist the horizontal push of the retained earth trying to tip and slide the wall away. Two independent stability checks — overturning and sliding — verify that this balance holds with adequate margin.
Where the Earth Pressure Comes From
Rankine's active earth pressure theory models the retained soil as pushing against the wall with a triangular pressure distribution — zero at the top of the wall, increasing linearly to a maximum at the base. The active earth pressure coefficient, Ka = tan²(45° − φ/2), converts the soil's friction angle into a fraction of the vertical soil weight that acts horizontally against the wall. A looser, lower-friction-angle backfill produces a larger Ka and therefore more thrust; a well-compacted granular backfill with a higher friction angle reduces it. This is exactly why specifying and verifying backfill material matters as much as the wall's structural design.
Overturning: A Moment Balance About the Toe
Overturning stability compares two moments taken about the front edge of the footing (the toe): the resisting moment from the wall's self-weight and the heel soil's weight, each acting through its own centroid, against the overturning moment from the active earth pressure resultant, which acts at one-third of the wall height above the base. A factor of safety of 2.0 or greater on this ratio is standard U.S. practice, giving margin against uncertainties in soil properties and load estimation.
Sliding: Friction at the Base
Sliding stability compares the horizontal driving force — the earth pressure thrust — against the frictional resistance available at the base of the footing, which depends on the total vertical weight bearing on the soil and the friction coefficient at the concrete-soil interface. Because passive soil resistance in front of the toe is easily disturbed by excavation, freeze-thaw, or future grading changes, it's common and conservative to ignore it entirely in a preliminary check, as this calculator does, requiring a minimum sliding factor of safety of 1.5.
What This Check Doesn't Cover
Overturning and sliding are necessary but not sufficient. The same loads also produce a bearing pressure at the base of the footing that must be checked against the soil's allowable bearing capacity — including the effect of load eccentricity, which shifts pressure toward the toe. Global slope stability, where the entire wall-and-soil mass could fail along a deep slip surface, is a separate analysis entirely, especially relevant for tall walls or walls on marginal slopes. A complete retaining wall design addresses all of these together, not overturning and sliding in isolation.
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