By: Javier Encinas, PE | January 15, 2017

A retaining wall is a structure exposed to lateral pressures from the retained soil plus any other surcharges and external loads. All overall stability failure modes must be thoroughly checked, but cantilever walls may be particularly sensitive to overturning problems. This article discusses the overturning calculations in either concrete or masonry cantilever retaining walls. Our software ASDIP RETAIN will be used to support the discussion.

A typical retaining wall is composed of four main components: the Stem, the Toe at the front of the wall, the Heel at the backfill side, and the Shear Key. The images below show the geometry of a typical cantilever retaining wall.

What are the typical loads on a retaining wall?

In addition to the retained backfill, retaining walls may be subject to surcharge loads at the top of retained mass. A surcharge may be a strip load. The stem may also have concentrated loads at the top.

When the stem extends above backfill the retaining wall may be exposed to wind load. When retaining walls are located in seismic zones the seismic effects are considered by utilizing Mononobe-Okabe approach. ASDIP RETAIN is structural engineering software for engineering professionals to quickly model retaining wall loads. The image below depicts typical external loads on retaining wall.

Each applied load has a particular effect on the wall. The backfill exerts a triangular lateral pressure calculated per the corresponding earth pressure theory. The surcharge produces a uniform rectangular pressure on the wall. The seismic pressure is trapezoidal, with the higher pressure at the top.

For a more in-depth discussion of the theories and overall stability modes please read my post Cantilever Retaining Walls: An Overview of the Design Process (Part 1). The picture below shows schematically the lateral pressure diagrams on a typical retaining wall.

How do you check the overturning failure mode?

The horizontal pressures on the backfill side will push the wall outward, which will tend to overturn around the end of the toe, as shown at the right. The overturning moment from the applied forces must be resisted by an opposite moment produced by the vertical forces, including the wall selfweight and the weight of the backfill over the heel.

The factor of safety against overturning is defined as the resisting moment divided by the overturning moment, and the minimum value should be 1.50.

As an example, the picture below shows the ASDIP RETAIN overturning calculations. Note that the load combinations are based on service loads, since the wall stability is being checked. In this example the safety factor is greater than 1.5 for the load combination shown.

You may be interested also in the post How to Design Cantilever Retaining Walls Using ASDIP RETAIN.