The assessment of rock mass strength is a key element for the analysis of many rock excavations, both open pit and underground. There have been attempts to assess this strength using a suitable rock mass strength criterion having input derived using a Rock Mass Classification system. The most well developed criterion is the Hoek-Brown Failure Criteria using the Geotechnical Strength Index (Hoek, 2002) as input. Despite its popularity the GSI has some inconsistencies. Rock masses with differing fracture frequencies and joint conditions can have the same GSI value and therefore the same strength characteristics would be applied. To address this issue, this paper describes a method to determine the input to the criterion on the basis of the joint condition and fracturing of the rock mass.