Due to a bug in the 3D 2011 program it is not possible to run a Safety phase for models containing a Hoek-Brown material model.
There are no direct workarounds available. However a possible way of getting insight into the safety factor could be to replace the Hoek-Brown material with a Mohr Coulomb material with equivalent parameters followed by a safety phase. Also see the chapter on the Hoek-Brown model from the Material Models manual for more information on the conversion from Hoek-Brown to Mohr Coulomb.
This is fixed in PLAXIS 3D2012