🧱 Retaining Wall Stability

The wall can slide forward if the horizontal thrust exceeds the friction available under the base. Friction equals the base friction coefficient mu multiplied by the total vertical weight W. Sliding safety is mu times W divided by the horizontal thrust, and the simulator requires this to be at least 1.5 to pass.

The resultant vertical force should land within the middle third of the base, meaning the eccentricity e stays at or below B/6. If it strays outside, the heel of the base tries to lift in tension, which soil cannot provide, so contact reduces to a shorter strip and bearing pressure spikes. The tool labels this condition as lift and flags the eccentricity check as failed.

When the resultant stays in the middle third, pressure varies linearly and the peak is (W/B) times (1 + 6e/B). If the eccentricity exceeds B/6, the model switches to a reduced bearing width a = 3(B/2 minus e) and a triangular distribution with peak 2W/a. This is the value compared against the allowable bearing capacity of the foundation soil.

Enabling it adds a hydrostatic water pressure behind the wall using the unit weight of water, about 9.81 kN per cubic metre, building up as a separate triangle to half times gamma-water times H squared. Because water has no shear strength, it pushes with full pressure rather than the reduced Ka value, which sharply raises the thrust and is a common cause of real wall failures.

The surcharge represents a uniform load on the soil surface behind the wall, such as traffic, stockpiles or a building. It adds a rectangular pressure of Ka times q acting over the full retained height, giving an extra thrust of Ka times q times H whose resultant sits at mid-height rather than at the third point.

It captures the standard textbook approach used in introductory geotechnical design and gives realistic trends and factors of safety. It simplifies by assuming a vertical smooth wall back, horizontal backfill, dry conditions unless water is enabled and no passive resistance at the toe, so it is an excellent teaching tool rather than a substitute for a full design to Eurocode 7 or BS 8002.

The earth thrust grows with the square of the wall height, while the resisting weight grows roughly in proportion to base width and slab size. Doubling the height nearly quadruples the overturning moment, so a tall wall needs a much wider, heavier base. The Failing and Tall-wall presets let you see how quickly the safety margins collapse.

Widen the base, lengthen the heel so more soil weight bears down, thicken the base slab, or increase the toe length to push the resultant back toward the centre. Improving drainage to keep the water table down, reducing surcharge, or using better-draining backfill with a higher friction angle also lowers the thrust and lifts all three factors of safety.