Need a book? Engineering books recommendations...

Re: Sliding factor of safety for retaining walls

• To: seaoc(--nospam--at)seaoc.org
• Subject: Re: Sliding factor of safety for retaining walls
• From: Thomas Chiu <Tomchiu(--nospam--at)worldnet.att.net>
• Date: Wed, 03 Sep 1997 20:32:06 -0700

```Bill:

I would use method 2 based on reasoning that your resisting forces are
the combined passive pressure and the footing's frictional forces.
Sometime I even neglect the passive pressure at the footing toe because
of backfill condition.  For example, if you have a line of 3 frames in
the building resisting say 100 kips. wouldn't your F.S. = sum of the
capacities of the 3 frames / 100kips.  I don't think you can subtract 1
frames from 100 kips and use the other 2 frames to calculate the F.S.

Tom Chiu, SE
Thomas Engineering

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.)
>
>

```