Reliability of the linear correlation of Rock Mass Rating (RMR) and Tunnelling Quality Index (Q)

With the advent of the RMR and Q classification methods for underground excavation support design, a linear correlation between the two methods was suggested by linear regression analysis of the data obtained from several case studies. The data used in deriving the relationship was widely scattered and the range of values covered by the 90% confidence limits demonstrated that the relationship had very little practical value. In subsequent publications, the 90% confidence limits were omitted when referring to the relationship. Consequently, some practitioners in the field of rock engineering assumed that this relationship, expressed as a semi logarithmic equation, is universally applicable for transforming the ratings assigned by one system to the ratings of the other. This assumption is erroneous and deserves scrutiny. This paper reviews some of the relevant published information and illustrates that there is no sound scientific basis to assume a universally applicable linear relationship between the two.