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Ultimate Foundation Design.

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I have to appoligize for my spelling error the other day.  It is Terzaghi not
Terghazi.  I was writing from memory, not looking at my text book until after
sending the e-mail.  I also want to mention Peck, who also worked on this
important area of structural design.  If the supporting soil is not understood
(at least an attempt at understanding), we would have leaning towers of pisa
everywhere. or at least sinking builldings like the John Hancock tower in
Boston.

Anyways back on the subject at hand.  You are correct, allowable bearing
pressure for foundation design is often given a large factor of safety,
typically 4 or more.  The strength reduction factor, phi, takes care of this.
For simplicity sake lets use a load factor of 1.7 for soil.  The resulting phi
factor would then be 4/1.7=0.425.  A reasonable value of phi would then be in
the range of 0.4 to 0.45.

On the issue of settlement,  The settlement is a function of the applied load
("working level").  Almost all of the settlement occurs immediatly due to
structural dead load.  Additional settlement will occur later due to live
load.  Here again, the segregation of the loads is important and the
"ultimate" resistance to settlement can be adjusted from the "allowable".

I am more inclined to SE, but GE is important.  Without it our structures will
collapse.

Of course, this is all my humble opinion.

Ted Beckwith
SFA, Inc.