Understanding Wall Failures: Overturning & Sliding

Retaining walls fail when the horizontal forces pushing against the back of the wall overcome the vertical gravity forces holding it down. The soil behind the wall exerts Active Earth Pressure, attempting to push the wall horizontally (Sliding) and rotate it forward around its toe (Overturning).

To counteract these failures, the geometry of a cantilever retaining wall utilizes an extended "Heel" slab. By extending the base slab backward under the retained earth, the immense weight of the soil itself acts as ballast, holding the wall down and resisting both overturning and sliding. Structural engineering standards typically require a Factor of Safety (FOS) > 1.5 for Overturning and FOS > 1.5 for Sliding.

The Impact of Surcharge Loads

A retaining wall rarely just holds back dirt. It often holds back highways, parking lots, or adjacent buildings. These external weights applied to the top surface of the retained soil are called Surcharge Loads ($q$). Surcharge dramatically increases lateral earth pressure, pushing the wall harder, while simultaneously acting as vertical ballast on the heel slab. Miscalculating surcharge is a primary cause of retaining wall blowout failures.

⚠️ Why This is for "Reference Only"

Hydrostatic Pressure: This stability matrix assumes the wall has adequate weep holes and drainage systems, treating the retained soil as drained. If water builds up behind the wall, hydrostatic pressure can easily double the lateral force, guaranteeing catastrophic failure. Bearing Capacity: This tool calculates overturning and sliding; it does not evaluate the bearing capacity of the soil beneath the base slab. Always consult a licensed structural/geotechnical engineer for final stamped designs.