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Re: Retaining Wall Design Methodology

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Regarding Stan's questions about 3 soil-pressure-determination methods.

I use method 2), so that the heel is designed to support the pressure due to
the superimposed weight of soil as decreased by any upward foundation
pressure.  Usually, however, I will quickly make the judgement that the
effect of the upward foundation pressure is not significant and will ignore
its contribution in designing the heel.  For the height of walls I generally
need to design, I think that there is usually little practical difference
between method 1) and method 2), unless the footing is oversized.

I haven't encountered method 3).  It sounds like assumption of a rectangular
stress block somewhat like that used in strength design of concrete in
flexure.  Is that the way soil acts?  Is it supported by research?

The assumed triangular stress block of methods 1) and 2) matches the rotated
configuration of the footing, but the sudden drop of stress from maximum in
the soil under the edge of the footing to zero in the soil adjacent to the
edge doesn't seem probable.  Maybe a parabolic stress block [one that
extends beyond the edge of the footing] is more realistic.  Does anyone know
of problems because the wrong stress block was assumed, or has anyone saved
the client lots of money by assuming a different-shaped stress block?

Nels Roselund
Structural Engineer


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