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Re:Factor of safety for cantilever Pile Retaining Walls

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I see this all the time, and generally agree.  Most Geotechnical Engineers in
California are factoring down the "ultimate" passive pressure (Rankine
calculation) down to an "allowable" passive pressure, typically by a factor of
safety of 1.5.  This, of course, is in addition to the factor of safety you
probably apply as the Structural Engineer for sliding and overturning analyses. 
Seems redundant, but that is the current standard of practice for Geotechnical
Engineers in at least Southern California and most of the Bay Area.  The first
factor of safety could be justified by potential earth material strength
variations.  But, I bet if you reviewed 100 geotechnical reports from
California,  the recommended passive pressures would not fall outside of 250 to
350, or maybe rarely 450 pounds-per-square-foot, per foot of embedment, doubled
for isolated soldier piles.  Personally, I have never had a Structural Engineer
or any other wise client question these numbers (mine or any other GE's),
possible because often there is little cost impact associated with the passive
pressure magnitude.  Perhaps this is due to just habit and familiarity.

Tom Benson at Lowney Associates
251 East Imperial Highway, Suite 470
Fullerton, CA 92835-1063
(714) 441-3090
FAX: (714) 441-3091
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____________________Reply Separator____________________
Subject:    Factor of safety for cantilever Pile Retaining Walls
Author: seaint(--nospam--at)seaint.org
Date:       3/1/2001 5:40 PM

The geotechnical engineer includes a factor of safety of about 1.5 for the 
allowable passive lateral earth pressure used for designing embedment 
restraint of cantilever piles.  based on information from the geotechnical 
engineers I work with, an additional factor of safety on top of that value is
not required, and typically not added by engineers in the design of 
cantilever pile systems. 
paul franceschi, S.E.