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When I design retaining walls supporting some kind of structure, I usually use the at-rest earth pressure to limit possible deflections. However, a colleague of mine came up with a question about the design of one of his walls. At what point can we consider active/passive pressures on a retaining wall? In other words, we can consider it when the wall has “moved” for a given deflection but what is this value? 1 inch for each 10 feet of height?
To fully develop active earth pressure, you need to allow 1-in of movement on a 10-ft tall wall (think rotational movement). If you are not willing to allow that movement (or if the wall is braced at the top and it just can’t happen), then you should use at-rest earth pressure, which is about 1.5 times greater than active earth pressure.
Bear in mind, there also has to be movement at the toe to mobilize passive earth pressure.
Bear in mind also, if you are designing a sheeting system, Terzaghi and Peck found that the earth pressure envelop is either rectangular or trapezoidal, depending on whether you are restraining fine- or coarse-grained soils.
Some folks design cantilevered retaining walls using at-rest earth pressures on both the “active” and “passive” sides as they don’t want to count on any movement. This would then require stability from base shear or a gravity mass above the wall footing.
I’d hope that a geotechnical engineer would be able to explain this to a civil/structural designer, but then again…