Slope/W Probability of failure
When i do a probabalistic calculation i am among other things interested in the likelihood of failure for the specific Slope. That would be P(failure).
When i do the calculation i only get P(failure) for the most critical slope, why is that? Should P(failure) not be presented for all valid slope calculations?
Answers
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Hi @DavidJersenius,
P(Failure) is calculated for every slip surface, it's just that your critical slip surface is the only slip surface that has any probability of failure. You can see that the minimum factor of safety for the critical slip surface is below 1 and the other slip surfaces all have a minimum factor of safety above 1.
I hope this helps. Please let me know if you have any other questions.
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Hi @TrevorKent
Thank you for your answer!
I am not quite sure i follow. The selected slip surface, as depicted in the picture above, has a factor of safety of 1,1, and a P(failure) of 0,005. All calculated slip surfaces has a probability of failure even if they are very low. So i should be able to see the P(failure) of all slip surfaces regardless of their Fos?.
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Hi @DavidJersenius,
The probability of failure is determined by counting the number of safety factors below 1.0 and then taking this number as a percentage of the total number of converged Monte Carlo trials. For example, if there are 1000 Monte Carlo trials with 980 converged safety factors and 98 of them are below 1.0, then the probability of failure is 10%. Your slip surfaces show P(Failure)=0 because none of converged trials returned a factor of safety below 1.0.
The reliability index describes the stability by the number of standard deviations separating the mean factor of safety from its defined failure value of 1.0. That means Slip # 4851 is almost 20 standard deviations from failure. For context, 6 standard deviations would be a P(Failure) of 1 in 1 billion. So you could calculate P(Failure) from the reliability index, but it would be exceedingly small in your case.
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