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Geomechanical classifications of rock masses are of great interest and importance for the design of civil and mining works, as slopes, open pits, and underground excavations stability are directly related to the mechanical properties of the rock mass classes. Some of the most common geomechanical rock mass classifications currently used are the RMR (Rock Mass Rating), the Q system, and the GSI (Geological Strength Index). Recently (2011), it was presented a new approach for RMR in which the author had pointed an erroneous understanding by some of the RMR users related to the weights adopted for some parameters. Due to the great difficulty found by some professionals to classify highly weathered rock masses by using RMR89, various authors have proposed a correction of for RMR89 values by considering an adjustment index. On this context, the present work has the aim of verifying, for several rock masses from Quadrilátero Ferrífero, Minas Gerais, Brazil, the effectiveness of these three proposed adjustments on RMR89. The results has shown that the corrections proposed better represent the weathered rock masses usually found on open pit on Quadrilátero Ferrífero. [ABSTRACT FROM AUTHOR]