Exclude some zones in Strength reduction calculation

In some cases, the zones in slope is out of our interest during strength reduction calculation, such as bedrocks. So if it possible to pick them out from our SR calculation? (but not assign a null cm for these zones!) it definetely will save the calculation time and without change the result.
Acturally, this method is widely used by other FEM software, such as Phase2.

I don’t think we can specify a range of zones in FOS command. However, you can customize the FOS scheme with a fishcall which is straightforward.
For you case, why don’t you simply assign the elastic model to the bedrocks?

Yes, we can apply the fish or python to manually exclude the bedrock, but this is not so straightforward.
The fos calculation is not support the elastic cm in the model.

As a workaround, consider assigning the Mohr-Coulomb (MC) model to the bedrock instead of the elastic model. The key is to set a very high cohesion value for the bedrock. This approach essentially mimics the effect of the elastic model while enabling its use in the Factor of Safety (FOS) calculations.

Coming from Phase2/RS2 I also think exclusions zones would be useful. Although for bedrock materials, generally the strain levels in my models are so low that the solver takes only a few cycles to reach equilibrium and therefore the speed of the factor-of-safety calcs are unaffected.

Since factor-of-safety doesn’t work when zones are assigned the elastic cmodel, I follow two approaches:
a) for inconsequential shallow slip surfaces on the surface of a slope (generally mohr-coulomb), I select zones within a polygon range and multiply the cohesion and friction angle for those zones by the expected FoS using a short fish routine. (Although I haven’t gotten around to make it use the current FoS stage yet so an idea of the expected FoS is required (e.g. 1.5).
b) for bedrock materials outside of the critical failure surfaces, I consider reducing the stiffness of the the zones which reduces the number of characteristic steps and speeds up the FoS calcs.
b) I find the FoS algorithm in FLAC2D/3D frustrating when it keeps searching near FoSs that are clearly failing. Therefore I generally check a single FoS around where I expect the critical FoS to be and then bracket quite tightly around that value for the final result.

Hope it helps!