A simplified modelling of a pos-triggering flow liquefaction problem

[RJAguiar]

Hi everyone!
How are you?
I am trying to model a flow liquefaction problem of a tailing stockpile. I am not interested in evaluating whether the liquefaction will be triggered since I am already assuming that it will be triggered for some reason. I just want to have a preliminary idea about the mobilized volume and the resulting run-out. By the way, the whole stockpile is not liquefiable, but only the saturated bottom layer. In the first step, I generated the in situ (initial) stresses by running the elastic gravity loading condition. Then, I tried to reduce the undrained shear strength of the bottom layer step by step (little by little) until starting to observe a continuous shear zone inside the mass, but it did not seem to work properly.
Do you agree with this procedure?
Have you ever run a similar analysis?
Does anyone have any idea where I might be failing?
Does anyone have a better idea to model this problem, provided it is not so complicated?
Thank you very much indeed in advance for your help!