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Re: passive pressure for retaining wall

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Bill Sherman wrote:

> ...The "alternate" design method presented by
> Amrhein is basically the method I
> typically use to analyze sliding resistance with
> a keyway - with at least one
> essential correction.  Since the passive
> pressure is considered to the bottom
> of the keyway, the active pressure must also be
> extended to the bottom of the
> keyway.  Although most design examples I've seen
> for sliding resistance with a
> keyway do not address this added active
> pressure, a proper free body diagram
> would include the soil pressure on both sides of
> the keyway.  Consider for a
> moment if the entire footing were thickened 1 ft
> rather than just a keyway
> extended down 1 ft - there would be no question
> that the active pressure
> diagram should be extended to the bottom of the
> footing in an opposing
> direction to the resisting passive pressure.
> The fact that only a portion of
> the footing is extended down as a keyway should
> not alter this load effect.
> (Can anyone explain why this added force is
> neglected in many design
> examples??)

Perhaps a converse of the Armhein Passive
Resistance can be used to try to justify this?
For example.  If your key is located approximately
2/3 of the way back from the toe to the heel, then
the soil in back of the key may have a significant
portion of  its overbearing load removed due to
the overturning effects, and therefore not exert
an active pressure consistent with its true depth.

Stan Johnson
BS, EIT, What about the added dead load due to
graffiti on the face of the wall   :))