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# Soil - Wall Friction Angle

## Soil - Wall Friction Angle

(OP)
How is the soil-wall friction angle determined, rigorously?

From my studies, I have been told to take is as ½ - ⅔ of the internal friction angle. But how does one get to this point. I have yet to find an explanation in any literature.

Thanks:)

### RE: Soil - Wall Friction Angle

It is an interface (soil-interface material) friction angle, rather than an internal (soil-soil) friction angle. You get it in the lab from interface friction tests, e.g. shearbox or ring shear.

### RE: Soil - Wall Friction Angle

There are charts for estimating various friction angles of dissimilar materials (NAVFAC, AASHTO) as it is not practical to actually test for these. Using 1/2 - 2/3 of soil phi angle for a concrete wall interface is customary but then this would not apply to a Rankine form of earth pressure analysis that is not based on wall friction, just Coulomb theories.

The dissimilar material friction factors (friction angle = atan(friction factor)) are used in various sliding calculations for many materials so they tend to not be defined in the Coulomb earth pressure sections of textbooks.

### RE: Soil - Wall Friction Angle

The log-spiral earth pressure graph in DM 7 shows adjustment factors for wall friction.

In certain instances, it's not necessary to use any reduction - for example on an MSE or a cantilevered retaining wall - one with a footing that extends below the retained soil.

1/2 to 2/3rds is about right though, when the design calls for such reduction.

f-d

ípapß gordo ainÆt no madre flaca!

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