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Re: Sliding factor of safety for retaining walls

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As noted below, many textbooks include the passive soil resistance in the 
total resisting forces and divide by the total driving forces to determine the 
factor of safety.  AASHTO also specifies this method for retaining walls.  
However, the Corps of Engineers design manual EM 1110-2-2502 (1989) on 
"Retaining and Flood Walls" sums the net forces at the base of the footing and 
only applies the safety factor to the friction/cohesion resistance at the 
bottom of footing.  Thus the passive force is used to reduce the driving force 
to get the "net horizontal force" before applying the safety factor.  In 1994, 
ASCE published a design guide entitled "Retaining and Flood Walls" which is 
based on the COE design manual and uses the same method to determine the 
factor of safety.  I am currently working on the design of a floodwall in 
accordance with these documents.   
UBC states that "Retaining walls shall be designed to resist sliding by at 
least 1.5 times the lateral force" - but it does not define whether this is 
the "net lateral force" or "total driving force".   
Thus the codes and available literature are not clear/consistent.  

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Based on my textbook "Foundation Analysis and Design" by Joseph Bowles,
Second Edition, page 380, FS sliding=sum resisting forces/sum driving forces.
This is what  I use.  Also, if I combine friction and passive resistance, I
reduce the friction force by 1/3 based on what I've read from soils report on
some of my projects.

Ernie Natividad 

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