By: Javier Encinas, PE | March 13, 2017

Retaining walls are structures designed to bound soils between two different elevations, therefore they are mainly exposed to lateral pressures from the retained soil plus any other surcharge. Cantilever walls may be sensitive to sliding problems, particularly if founded on poor soils. This article discusses the sliding calculations in either concrete or masonry cantilever retaining walls. Our software ASDIP RETAIN will be used to support the discussion.

What are the pressures acting 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. 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 pressures should also be considered. The image below shows schematically the pressure diagrams on a typical retaining wall.

cantilever-retaining-wall-pressures

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. The action of these loads produces a bearing pressure under the footing, and a passive pressure at the front of the wall.

For a more in-depth discussion of the soil lateral pressure theories and overall stability modes please read my blog post Cantilever Retaining Walls: An Overview of the Design Process (Part 1).

How do you check the sliding failure mode?

The horizontal pressures on the backfill side will push the wall outward, which will tend to slide on its footing, as shown below. The driving force from the applied loads must be resisted by an opposite friction force at the interface of the footing base and the underlying soil, produced by the bearing pressure against the base.

In addition, the passive pressure against the front face of the wall and footing may be considered as well. It is unlikely that the natural soil will remain undisturbed during the construction, therefore it’s a common practice to conservatively ignore the top portion of the soil cover for the passive force calculation.

retaining-wall-sliding-failure-mode
passive-pressure-diagram

When the friction plus passive forces are not high enough to counteract the pushing force, a shear key can be designed under the wall footing. This structural element will bear laterally against the soil, allowing to extend the passive pressure diagram deeper, as shown in the image above. This is a very efficient way to increase the sliding resistance in a retaining wall. The position of the key along the footing is not very important, but many engineers prefer to place it just under the stem, so that the wall rebars can be extended down into the key.

The factor of safety against sliding is defined as the resisting forces (friction + passive) divided by the driving lateral force, and the minimum value should be 1.50.

As an example, the image below shows the ASDIP RETAIN sliding 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.

retaining-wall-sliding-calculations

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

Detailed information is available about this structural engineering software by visiting ASDIP RETAIN. You are invited to download a Free 15-Day Software Trial or go ahead and Place Your Order.

Best regards,

Javier Encinas, PE
ASDIP Structural Software

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