Hi all,
Can anyone give some advice about unstable situations due to soil collapse. I mean, can anyone recommend a way to calculate safety factors (<1.0) if you got a collapse in plastic phase using only plaxis results (without a limit equilibrium model)?
Thanks in advance!
Francesc
I did not get the question. Can you elaborate? You want to get a safety factor less than 1 or you are getting a safety factor less than 1?
Sorry, I will explain it. Imagine that you get a soil collapse during your plastic calculation, then you check and confirm that is global failure (not a local collapse). This obviously means that you have a safety factor less than 1 but you do not know how far from that value you are. I am suggesting that in some situations could be interesting to know if safety factor is 0.9, 0.6 or whatever. For sure one option is to build and limit equilibrium model and find it but I am sure that some of you get this value from Plaxis models perfoming additional calculations with higher soil parameters or something equivalent. I hope this helps to understand better my question.
I get it. Look at this document I prepared for you. Of course, this is just a rough sketch. If you can elaborate your problem with model outputs then the forum will help you better.
PDF
Hi Jamal! Thanks a lot! I checked with some examples and results look good. I don't know if you are a member of plaxis staff. If not, let's see if someone of them is proposing the same approach.
I'm not a member of the Plaxis staff. But I have a question about Jamal's suggestion. If you limit the MPhase to something like 0.5, then at the end of that phase you would only have about 1/2 of the condition that you're trying to model. In the example in the pdf this would be 1/2 of the mobilization of shear stresses to account for the non-horizontal ground. Now if you follow this condition with a FOS phase which starts reducing the shear strength of the soil,and trying to compute equilibrium (for the partially complete prior phase), I'm not sure of how to interpret the meaning of the computed FOS.
I my opinion (and I could be wrong about this) you can't calculate a valid FOS less than 1.0 using methods based on equilibrium, and Plaxis is great at calculating equilibrium. I'd recommend starting from equilibrium, say by removing the surcharge and then increase the surcharge to get failure.
Martin
Answer Verified By: Dennis Waterman
Dear francesc an Martin,
Of course I am not part of the plaxis support system. I do agree with Martin here. What I gave is a rough estimation. Normally, in a situation like this, I use limit equilibrium, but to get a rough idea, I use the above approach. Most of the time in my line of work, I encounter some local failures, and hence to overcome that I use plaxis 2D with Plaxis LE. I hope this helps. Thanks Martin for your contribution.