FLAC can perform a factor-of-safety calculation by using the strength reduction method as described
in note 12 in Section 3.8 in the User’s Guide. The method is implemented by invoking the SOLVE
fos command. In this approach, the strength of the material is reduced until a failure of the slope
occurs. Both friction angle and cohesion are reduced simultaneously by a constant factor, and
FLAC runs are automatically made with each new pair of strength parameters using a bracketing
approach until a safety factor is found.