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RE: Strength Mobilization Factor - TEDDS

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Sorry, I based my response on my "recollection" but after re-reviewing EM 1110-2-2502, my original response was inaccurate.  The soil mobilization factor, SMF, is applied to the driving and resisting soil pressures rather than to the soil resisting values at the base of the wall.  The lateral forces due to soil pressure are determined based on "design tan(phi) = SMF * unfactored tan(phi)".  Although Coulomb's equation for active earth pressure is used, this results in a lateral earth pressure that is intended to approximate at rest earth pressure (see Sections 3-11 and 3-12).  Per section 3-13, the SMF is used to determine driving forces but not resisting forces when analyzing overturning, bearing, and structural design of the wall.

Per section 4-11, the friction at the base of wall is reduced by the factor of safety, FS: "design tan(phi) = unfactored tan(phi) / FS".  (This is similar to "design tan(phi) = SMF * unfactored tan(phi)".)  Example 1 in Appendix N indicates that the SMF is applied to the driving force simultaneously with applying the FS to the base friction resistance for evaluation of sliding.  In effect, the factor of safety against sliding is determined for effective at-rest driving pressures and reduced passive resisting pressures for USACE designs.

Traditional commercial retaining wall design has been based on active earth pressure for driving forces (assuming a yielding wall) and passive pressures for resisting forces - less conservative assumptions than for USACE designs.  However, stiffer walls should use at-rest driving earth pressures; and reduced passive pressures are often used where lateral movement must be controlled.

You may want to review the formulas in TEDDS to confirm how the SMF and FS are applied.  You may want to vary the SMF depending upon the conservatism you wish to apply to driving and resisting lateral earth pressures.

Bill Sherman
CH2M HILL / DEN
720-286-2792
 

> -----Original Message-----
> From: William.Sherman(--nospam--at)CH2M.com [mailto:William.Sherman(--nospam--at)CH2M.com] 
> Sent: Monday, August 03, 2009 10:03 AM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: Strength Mobilization Factor - TEDDS
> 
> --------------------------------------------------------------
> ------------
> Your following message has been delivered to the list
>   seaint(--nospam--at)seaint.org at 09:03:20 on 3 Aug 2009.
> --------------------------------------------------------------
> ------------
> 
> 
> My understanding is that the strength mobilization factor, as 
> used by USACE (US Army Corps of Engineers, EM 1110-2-2502), 
> is the inverse of the safety factor, i.e. 0.67 = 1/1.5.  In 
> effect, the soil resistance values are reduced by a factor of 
> 0.67 to provide an effective factor of safety of 1.5 for 
> static load conditions.
> 
> Bill Sherman
> CH2M HILL / DEN
> 720-286-2792
>  
> 
> > -----Original Message-----
> > From: Jeremy White [mailto:admin(--nospam--at)structuralae.com]
> > Sent: Monday, August 03, 2009 9:46 AM
> > To: seaint(--nospam--at)seaint.org
> > Subject: Strength Mobilization Factor - TEDDS
> > 
> > Does anyone have a resource that gives a good explanation of the 
> > strength mobilization factor for soil and how that relates to the 
> > safety factor?
> > 
> > Specifically, I am using TEDDS software for a retaining 
> wall analysis 
> > and the software asks for the soil mobilization factor. The default 
> > value is 0.67 as recommended by ASCE (apparently). I don\'t 
> understand 
> > the results I\'m getting and I feel I need more information 
> on how the 
> > strength mobilization factor applies.
> > 
> > - Jeremy
> > 
> > 
> > 
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