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Stone Retaining Wall

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This is a gravity wall.  You analyze the lateral soil
pressure just as you would for a cantilever wall, then
make the wall thick enough and battered enough (i.e.
slope the front face back) so that at any section
analyzed (including the base), the wall section stays
in compression.

You also check it for sliding, although that usually
isn't a problem since you've got so much weight.  In
theory you should also check it for shear, but if it
doesn't slide on the relatively smooth shear plane of
the base soil, it probably won't slide through your
stone masonry.

Next, you check the bearing pressure on the underlying
soil, using the weight over the base area plus the
overturning moment over the base section modulus.

Last, in theory you check it for global stability,
i.e. you look at the possibility of an arc of soil
containing your wall, rotating under the impetus of
the eccentricity of load caused by your backfill being
higher than your forefill (to coin a term).  As far as
I know, this is a trial and error, highly iterative
calculation.  I have done this exactly once in my
career, and it was very painful and otherwise not too
instructive.  When Terzaghi was designing 90 foot tall
gravity walls (for locks) on clay subsoils with poor
shear strength, this calculation mattered.  For your 3
foot masterpiece, it obviously will not.  I've seen
photos of bridge foreslopes which failed in this
manner, maybe 20 feet tall with a 1.5:1 slope, in very
poor (very unconsolidated) clays, but for almost any
other subsoil it won't govern.

Probably more information than you wanted.  In answer
to your other questions, I don't know any rules of
thumb (I'd be interested), and I don't know enough
about stonework mortar to know.

Mike Hemstad
St. Paul, Minnesota 

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