A Mamdani fuzzy inference system for the geological strength index (GSI) and its use in slope stability assessments

The strength and deformation modulus of closely jointed rock masses can be estimated by using the Hoek and Brown empirical criterion in conjunction with the Geological Strength Index (GSI) classification system. To minimize the uncertainties of the rock masses, experienced engineers commonly use the engineering judgment. Intervals of the GSI, rather than point-valued observations, are also likely to be obtained when using the original GSI chart. This study is an attempt to provide an approach on the fuzzy set theory for determining GSI ratings to be used by the empirical failure criterion. For the purpose, the original GSI system and its quantitative modified form were defined by fuzzy sets, and Mamdani fuzzy algorithm was constructed employing 23 "if-then" rules. In addition, the validity of the approach and methodology of parameter estimation were checked by back analysis of the well-documented slope instabilities occurred in the heavily jointed rock masses.