Abstract: On the basis of the YAI(yield approach index) concept and its phase complementary parameter ω called danger coefficient, which were proposed in the previous paper by the authors, the new concept, FAI, is introduced. It is the sum of danger coefficient ω and FD(failure degree) which describes the damage degree of rockmass in the plastic zone. Two cases are presented to analyze the validity and utility of FAI. The results proved that this parameter can line out the position and extent of failure zones, damage zones and disturbed zones, and characterize the stability rules of rockmass. In the stability assessment of slope engineering, the sliding surface denoted by this parameter perfectly agrees with that measured in situ. So, FAI is of very importance in the stability assessment, the election of excavating and supporting schemes.

Key words: tunnel, slope, stability evaluation, failure approach index, excavating damage zone, strength reduction