Critical slip surface in PLAXIS

Thank you very much.

The strength reduction method in PLAXIS Finite Elements automatically gives the most critical slip surface. Where the Finite Element analysis finds the most critical slip surface against which the Limit Equilibrium results can be validated for the same failure mechanisms, the Limit Equilibrium analysis can find the factors of safety for non‑critical areas more easily than Finite Element analysis. More on comparison of critical slip surface between FEM and LEM can be found in the following article:
Slope stability for road construction project: using FEM and LEM [PLAXIS and GeoStudio]

The curve plot you create is "step vs. ΣMsf". That refers to the most critical failure mechanism found by the calculation. If you want data from other areas of the model, you can select as many curve points as you want and plot the relevant data. In order to determine the safety factor as calculated by a phi/c reduction, please check if you have a stable value for ΣMsf: prior to the calculation select a control point that is likely to be in the failure zone. Afterwards, check if the control point is indeed in the failure zone and then plot the displacements of this point against ΣMsf and check if a stable value for ΣMsf is reached.

PLAXIS computes a global safety factor. That means that the calculation will find the most critical failure mechanism in the model you have and determine a safety factor based on that. Therefore, the value you get would be the lowest safety factor based on what PLAXIS has determined to be the most critical failure zone.

Note that sometimes PLAXIS calculation focuses on a local failure that from an engineering point of view is not relevant or is to be disregarded. PLAXIS offers an option to exclude part of the model using the Apply strength reduction option. Therefore, you can partition your geometry in the area of the irrelevant local failure and uncheck the option.