# Question about the criterion to define instability or stabilty

**URL:** <https://forum.itascainternational.com/t/question-about-the-criterion-to-define-instability-or-stabilty/1315>\
**Category:** Uncategorized\
**Created:** [February 2, 2024, 2:49pm UTC](https://forum.itascainternational.com/t/question-about-the-criterion-to-define-instability-or-stabilty/1315 "2024-02-02T14:49:31Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![MiguelN](https://avatars.discourse-cdn.com/v4/letter/m/6f9a4e/32.png) [@MiguelN](https://forum.itascainternational.com/u/MiguelN)\
**Post date:** [February 2, 2024, 2:49pm UTC](https://forum.itascainternational.com/t/question-about-the-criterion-to-define-instability-or-stabilty/1315/1 "2024-02-02T14:49:31Z")

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Hi everyone.

My question maybe is some basic but very important. In FLAC Manual (Factor of Safety) is the following text:

“The detection of the boundary between physical stability and instability is based on an objective  
criterion in FLAC that decides whether the system is in equilibrium or a state of continuing motion.”

Can you say me please which are these criterion if I used ofr example Solve FoS?

In the practice, I put histories of displacement, velocity, unbalanced forces, etc… and run the model. Then I see if the histories with the plasticity indicators if the model have instability or not (using Strength Reduction Method “manually” in FLAC). I know too that exists some velocities to define more or least instability or stabilty (could be 1e-5 or 1e-6 in FLAC/FLAC3D, obviously verifing with some histories previously). But which is the internal criterion in FLAC to define stabilty or instabilty? Are these criterion used in FLACSLOPE for example? Thanks in advance.

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**Author:** ![jwang](https://avatars.discourse-cdn.com/v4/letter/j/b782af/32.png) [@jwang](https://forum.itascainternational.com/u/jwang)\
**Post date:** [February 2, 2024, 3:06pm UTC](https://forum.itascainternational.com/t/question-about-the-criterion-to-define-instability-or-stabilty/1315/2 "2024-02-02T15:06:58Z")

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On pages 1-7 through 8 of Factor of Safety volume(FLAC8.1 MANUAL), there is a section discussing the procedure for implementing the strength reduction technique in FLAC and what criterion is used.