Cantilever concrete retaining walls are commonly used for residential purposes, often as integral basement walls. Usually the cantilever wall stem is of concrete block construction rising from an in-situ concrete foundation.
The following worked example is for a free-standing cantilever wall that is considered sufficiently flexible for active soil pressures to be used for design. Where used as integral basement walls they are often buttressed by return walls and floor diaphragms which may make them too stiff for active soil pressures to develop requiring higher design loads and a different design approach.
Possible modes of failure
Possible modes of failure for free-standing concrete cantilever retaining walls are illustrated in cartoon fashion in Figure X.1. A complete design should address each of these modes of failure where appropriate.
a) Wall stem structural failure: The wall stem fails in bending. Most likely location is at the base of the wall where the stem connects to the foundation.
b) Foundation bearing failure: A bearing failure of the soil under the toe of the foundation and a forwards rotation of the wall.
c) Sliding failure of wall: Possible mode for non-cohesive soils. Wall moves outwards with passive failure of soil in front of foundation and active failure of soil behind wall. Often a key is required beneath the foundation to prevent sliding.
d) Deep seated rotational failure: Possible mode for cohesive soils. Factor of safety controlled by increasing length of heel or depth of key. Factor of safety calculated using limiting equilibrium “Bishop” analysis or similar. Unlikely to govern design unless wall is embedded into sloping ground with sloping backfill or there is a weak layer at the toe of the wall.