By: Javier Encinas, PE | July 25, 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, including the bearing capacity of the supporting soil. This article discusses the calculation of the soil bearing bearing pressures under cantilever retaining walls. Our software ASDIP RETAIN will be used to support the discussion.

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.

Each applied load has a particular effect on the wall. As an example, 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.

cantilever-retaining-wall-pressures

How do you calculate the soil bearing pressure?

The horizontal pressures on the backfill side will produce an overturning moment with respect to the base of the footing. This overturning moment must be resisted by an opposite resisting moment produced by the vertical forces, including the wall selfweight and the weight of the backfill over the heel.

When the eccentricity falls within the kern = L / 6, the entire footing is under compression and the bearing diagram is a trapeze, as shown in the example above. In this case the maximum bearing pressure is Rv / L + 6 * Rv * e / L2.

When the eccentricity falls outside the kern, the footing is under partial bearing and the diagram is a triangle. In this case the maximum bearing pressure is Rv / (0.75 * L – 1.5 * e).

soil-bearing-failure

As an example, the picture below shows the ASDIP RETAIN bearing calculations. Note that the controlling load combination is based on service loads, since the wall stability is being checked.

cantilever-retaining-wall-pressures
bearing-pressure-calculation

Takeaway

Bearing over-pressure is one of the stability failure modes that needs to be checked as part of the design of a cantilever retaining wall. ASDIP RETAIN calculates the bearing pressure for any resultant eccentricity, and finds the controlling service load combination.

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).

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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