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RE: soils pressure

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> Doesn't a 1" diameter of water 12' tall exert the same pressure
> as a 1 foot diameter column? Isn't it the same for soil? doesn't
> a 12' tall column of soil exert the same pressure if it's 2'
> thick or 10' thick column?

To a certain depth your reasoning is correct.  But with increasing depth and
soil pressure, the friction forces between the granular fill and the bedrock
face and concrete wall increase.  Eventually, the combined friction force
per vertical foot is equal to the weight of fill per vertical foot.  From
that point on, the lateral driving force does not increase.  The thicker the
fill column, the deeper you have to go to generate enough friction.

This friction is usually ignored in conventional retaining wall design
because the force is relatively small in relation to the weight of the
retained fill, and because the rear face of most retaining walls are
vertical.  In situations like this, however, the reduced loads can be
significantly (financially) beneficial.

Jason Kilgore
Leigh & O'Kane, LLC
Kansas City, Missouri

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