I must correct my previous post, I realized I've not been exaustive enough.
In Italy the building codes contemplate 2 different situations:
-Artificial or excavated slopes, embankments, engineered works: OFS is the same or similar to that provided by the European standards
-Natural slopes: after the application of the characteristic values procedure, the FS is assigned by the professional by the evaluation of site conditions and consequences of risk.
So, actually European codes in Italy are bypassed only in the case of natural slopes.
It is interesting to notice though, that the FS intervals tend to overlap in the range of 1.35 - 1.5 in various regulations.
As to earthquake conditions, the OFS remains the same but in pseudo-static methods an horizontal component of the load is applied, which tends to lower the FS with respect to the static case. This is compensated by the fact that actions are not factored, but it also depends on the design approach (there are 3 of them and the second has a 4th one which is a subset). Yes, European regulations can be confusing...
If I were in the OP's position and barring the existance of already cited specific instances of liquefaction, rapid drawdowns and so on, I may proceed in two ways:
-After an examination of the various international laws, adopt an FS with a lower bound of 1.4 for frictional soils and an upper bound of 1.6 for analysis using undrained cohesion. These boundaries may be adjusted according to the specific site conditions.
-Adopt a recognized international code and follow it, if there are no local specifications.
Also a
tricky and fundamental choice may be the representative values of phi-c and Su. It may suffer of substantial reductions with respect to the lab values.
A friend whose case is presently in court, botched his analysis because he adopted peak values in an already failed slope. His stabilization plan was followed but another slide ensued. He should have adopted residual values even though the sliding surface could not be located exactly. In some cases, constant volume values are advised.