What is the critical SRF?

The Critical SRF is identical to the Factor of Safety in limit equilibrium slope stability analyses (Slide).  A value of 1.0 means the slope is in a state of limit equilibrium and that any reduction in shear strength will result in failure of the slope.  You do not design for a factor of safety of 1.0 unless you are using limit states and factoring input parameters (Eurocode).  Design values for slope stability analyses are up to the user and depend on the type of slope and consequences of failure.

If you are looking for guidelines on what factors of safety to use, a suggested reference is John Read and Peter Stacey's book, "Guidelines for Open Pit Design".  This book contains typical safety factors for use in design.