The Hoek-Brown constant mi for the intact rock matrix (miMax in the IUCM) is an important parameter that can largely control how damage initiates and propagates in the numerical model. More importantly, this constant can control the confinement dependency of the modelled rock under high-stress conditions.
Hoek and Brown (1997), Brown (2008), and Hoek et al. (2002) recommended the use of a series of well-conducted triaxial compression tests to provide the most reliable means of establishing the value of mi.
Without triaxial test data, practitioners often use the recommended mi values for various rock types proposed by Hoek and Brown (1980, 1997). However, as recently demonstrated by Richards (2011), there is often a poor correlation between mi values obtained from well-controlled triaxial tests and those provided in the guideline charts.
Carter et al. (2007), Diederichs et al. (2007), Brown (2008), and Richards (2011) suggested that the ratio of uniaxial compressive to true uniaxial tensile strength provides a more reliable estimate of mi than published guidelines. As a result, in the absence of triaxial test results, this ratio should be used to establish the mi value, and the published guidelines should only be used as a last resort when none of the above methods is available.
