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Re:RE: RE: Factor of safety for cantilever Pile Retaining Wa

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We usually like to see a statement concerning the factor of safety for the
passive pressures.  Other things we like to see is where the passive
pressure starts and what the maximum passive pressure is.  Sometimes the
maximum pressure will be included with a statement that the pressures can
be increase for wind and it won't be clear whether that is for the maximum
with no increase.  Most of the time it's not.    It's been a long time
since I've had a geotech tell me that I can double the passive pressures
for an isolated cantilevered pile.  

It is a good idea to have the geotech review your calculations and it
provide a letter that at least you followed his recommendations.  Don't
expect your client to provide your drawings and calcs to the geotech.  We
routinely send a set to the geotech the same time we send out the project
to the client.  Sometimes, eventually, the geotech will come back and say
that isn't what he wanted.  Usually it's a problem with boiler plate
content.  Then we either have to change our calcs and design, or the
geotech has to write a letter clarifying his criteria.  It's about 4 to 1
where the geotech ends up writing a letter clarifying his intent.  Good
thing to call the geotech if you can't interpret what he's trying to say.  

Some jurisdictions, especially in the SF Bay area, require review and
letters by the geotechs.

Neil Moore, S.E.




At 07:55 PM 3/1/2001 -0600, TBenson(--nospam--at)lowney.com wrote:
>I agree.  That is the issue.  Not all geotechnical reports state that a
factor
>of safety is used, when one often is.
>
>Tom Benson at Lowney Associates
>251 East Imperial Highway, Suite 470
>Fullerton, CA 92835-1063
>(714) 441-3090
>FAX: (714) 441-3091
>---------------------------------------------------------------------------
>Geotechnical & Environmental Engineering Services
>---------------------------------------------------------------------------
>
>
>____________________Reply Separator____________________
>Subject:    RE: RE: Factor of safety for cantilever Pile Retaining Walls
>Author: seaint(--nospam--at)seaint.org
>Date:       3/1/2001 9:25 PM
>
>Just commenting that what Geotechs are writing and Structurals are reading
>may be two different things here in California.
>
>George Richards of Borm Associates
>
>-----Original Message-----
>From: TBenson(--nospam--at)lowney.com [mailto:TBenson(--nospam--at)lowney.com]
>Sent: Thursday, March 01, 2001 4:48 PM
>To: seaint(--nospam--at)seaint.org; 'seaint(--nospam--at)seaint.org'
>Subject: Re:RE: Factor of safety for cantilever Pile Retaining Walls
>
>
>George,
>
>Not an assumption, just an observation regarding "most" (OK  if you prefer
>"the
>majority of") California geotechnical reports I have reviewed.  I have
>reviewed
>a few  ; ) .  If they are not using a factor of safety, then many are using
>very
>conservative soil strength assumptions (once again, for California only). 
>
>Is this George Richards of Borm Associates?  I agree "Not all Geotech's ..."
>
>Tom Benson (P.E., G.E.) at Lowney Associates
>251 East Imperial Highway, Suite 470
>Fullerton, CA 92835-1063
>(714) 441-3090
>FAX: (714) 441-3091
>---------------------------------------------------------------------------
>Geotechnical & Environmental Engineering Services
>---------------------------------------------------------------------------
>
>
>____________________Reply Separator____________________
>Subject:    RE: Factor of safety for cantilever Pile Retaining Walls
>Author: seaint(--nospam--at)seaint.org
>Date:       3/1/2001 7:33 PM
>
>I think most Geotech's do not know that we will add a second 1.5 factor of
>safety to their numbers.  I look at the friction angle and then ask.  By the
>way, not all Geotech's give you the numbers with a FofS in them. so don't
>assume.
>
>George Richards, P. E.
>
>-----Original Message-----
>From: TBenson(--nospam--at)lowney.com [mailto:TBenson(--nospam--at)lowney.com]
>Sent: Thursday, March 01, 2001 2:43 PM
>To: seaint(--nospam--at)seaint.org
>Subject: Re:Factor of safety for cantilever Pile Retaining Walls
>
>
>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
>---------------------------------------------------------------------------
>Geotechnical & Environmental Engineering Services
>---------------------------------------------------------------------------
>
>
>____________________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. 
>
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