Types of Retaining Walls

Retaining walls can be divided into cantilever or supported types.

Cantilever walls are dependent solely upon penetration into the soil for their support and clearly fixity of the toe is required to achieve equilibrium of the forces acting on the structure. As fixity of the wall toe requires longer and, in many cases, heavier piles to achieve the necessary penetration into the soil, this type of wall can only be economic for relatively low retained heights. It is also likely that deformations will be large for a cantilever solution.

Variations in soil properties, retained height and water conditions along a wall can have significant effects on the alignment of a cantilever wall and care must be taken when designing them for permanent structures, although provision of a capping beam will often alleviate alignment problems.

Supported walls, which can be either tied or strutted, achieve stability by sharing the support to be provided between the soil and the supporting member or members. In this situation the soil conditions at the toe of the wall are not as critical to the overall stability of the structure as in the case of a cantilever wall. The provision of longitudinal walings to transfer the soil loadings into ties or struts also caters for variations in displacement along the structure.

The maximum height to which a cantilever wall can be considered to be effective will generally be governed by the acceptable deflection of the wall under load. This comment doesn't just apply to sheet pile walls where the relative flexibility of the wall is often seen as a drawback because the overall deflection of the wall is a combination of bending of the wall structure and movement in the soil which will occur irrespective of the type of wall to be built.

However as a rough guide, it is unlikely that a cantilever wall will be more cost effective than a tied or propped wall when the retained height exceeds about 4.5 to 5 metres because the pile section needed for an unpropped wall of that height will be both long and heavy to resist the applied bending moments. Similarly a wall supported by a single tie or prop will generally only be cost effective up to a retained height in the order of 10 metres.

When more than one level of supports is used, wall stability becomes a function of the support stiffness and the conventional active/passive earth pressure distribution does not necessarily apply.