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Sliding factor of safety for retaining walls

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How do you calculate the sliding factor of safety for a retaining wall?  For 
example, if you have an active sliding force due to soil of 2000 lb, a passive 
resisting soil force of 400 lb, and a frictional resistance of 2400 lb: 
Method 1:   
Net lateral force = active force - passive force  
= 2000 - 400 = 1600 lb. 
Factor of Safety (FS) = base friction / net lateral soil force   
= 2400 / 1600 = 1.50 
Method 2: 
Total resisting force = base friction + passive soil force  
= 2400 + 400 = 2800 lb. 
FS = total resisting force / active sliding force  
= 2800 / 2000 = 1.40 
Which number is correct?  I've seen designs and guidelines done both ways.  
(Note: when passive soil resistance is neglected, the question goes away.)  
I've typically used Method 2 on the basis that the intent is to have a total 
resisting force greater than the force causing the tendency to slide.  
However, this would mean that a wall with equal soil height on both sides 
under at rest pressures doesn't meet stability criteria without adequate 
frictional resistance!  Thus I am starting to lean in the direction of Method 
1.  (Note: this question also occurs when calculating safety factors for 
overturning - should a passive soil force be included in the numerator or 
denominator?  The answer affects calculated factors of safety and resulting