Simple yet tricky question on reporting failure

[soil1999]

If you are performing a stability analysis using for example limit equilibrium method (or limit state analysis) , and you failure surface (mode) giving FOS of 0.7 and another failure surface (mode) giving FOS of 0.9. Which failure will occur first (which mode is more critical) in this case.?

I am guessing that the answer to this question is much more complicated than one expects..?
Pls provide your comments..Thank you for participating in this threat.

 

[dgillette]

That's a typo, right? If not, slope instability is a threat I try not to participate in.

Seriously, you are right when you say it's more complicated. The limit-equilibrium analysis is only that - finding the ratio of resisting force to driving force assuming equilibrium at the limit state on an assumed failure surface. It doesn't account for strain to peak strength, strain softening/strain compatibility, different pre-yield stiffness of different materials, etc., except as you tweak your strength assumptions by judgment, based on lab shear tests. LE also assumes a distinct rupture surface, which isn't necessarily what happens in the field, and (generally) some specific location and vector direction for the forces between slices. Right off, I'm not even sure whether the results mean much when FS is less than one, in part because of the interslice assumptions.

I would be hesitant to use it to try to predict what movement would happen when. I think that would be going beyond what it can legitimately be expected to do. If you really need to know that, might be time to find someone to run FLAC or PLAXIS for you.

Bon chance!
DRG