ABSTRACT
The Hoek-Brown failure criterion, which is used worldwide in rock engineering design, relies on accurate assessment of two key intact rock material parameters – namely UCSi and mi , in addition to, for rock masses, definition of rockmass quality, through GSI, directly, or via a correlation relationship from Q or from RMR. Although much has been written regarding appropriate procedures for GSI characterization of rockmasses for Hoek-Brown evaluation, there still appears much uncertainty in reliably defining appropriate values of intact mi . This paper thus looks at a couple of suggestions that may help practitioners improve current procedures for more accurately defining mi than existing approaches. The two aspects specifically discussed are – (1) inferences to aid mi estimation utilizing tensile test data, and (2) a methodology for visually assessing mi based on simple textural characterization of fabric, grain size and interlock, conducted as part of a field core logging or lab. testing program. It is hoped that together, these suggestions will be helpful for improving estimation of mi values, particularly for rocks with differing competence or age, but of similar character and mineralogy, hence identical names.
