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# Re: Sliding factor of safety for retaining walls and passive resistance

• To: seaoc(--nospam--at)seaoc.org
• Subject: Re: Sliding factor of safety for retaining walls and passive resistance
• Date: Wed, 03 Sep 1997 20:36:58 -0700

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Bill Sherman wrote:

> 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
> deigns.)
>
>

Hi Bill          Thanks for replay  that was very good response, i'm still waiting
for more
engineer to give thier opinion
I'm from old school , and  I would go with method #2

Ok ,regard the passive resistance pressure
I see that way
The bearing pressure is consider the reaction due to active pressure against the
wall
and as structural engineer I do not use the reaction to resist the action ,
because
that rection will not fully devolped unless I have  100% moment connection and
that
we assume but actualy it does not happened because the wall at the top deflect and

move , so what we really calculate the soil pressur not 100% accurate ,plus if you

draw as you said free body diagram you find that like I said before that soil
bearing
pressure is the reaction of soil active pressure and the function of the
stem,heel,and toe
is to transfere that horizontal force to vertical force supported by the soil.
Plus the passive force will not be devolped unless the wall move ,so which one
occures
first the wall movement or the wall rotation which will devlope the soil pressure?

or both will occures in the same time?

Dave Anderson P.E.

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