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# Sliding factor of safety for retaining walls

• To: seaoc(--nospam--at)seaoc.org
• Subject: Sliding factor of safety for retaining walls
• From: "Bill Sherman" <SHERMANWC(--nospam--at)cdm.com>
• Date: Wed, 3 Sep 1997 16:01:16 +0500

```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.)

```