Document Type

Conference Paper

Publication Date


Publication Details

This conference paper was originally published as Calleja, J, CMRR – Practical Limitations and Solutions in Aziz, N (ed), Coal 2008: Coal Operators' Conference, University of Wollongong & the Australasian Institute of Mining and Metallurgy, 2008, 92-103.


The Coal Mine Roof Rating (CMRR) is a rock mass classification which was developed empirically, from a database of coal mines in the USA. The CMRR weighs some of the geotechnical factors which may effect the competence of mine roof and combines them into a single rating on a scale from 0 to 100. The Australian underground coal industry has, in recent years, wholeheartedly embraced this system as a method of geotechnical characterisation. CMRR is a very simple system which is quick and easy for any engineer or geologist to learn and implement. It also provides a standard process and methodology and an output which can be compared between mine sites, for geotechnical characterisation and design which neatly fulfils the current requirements of the Occupational Health and Safety Act and the Coal Mine Health and Safety Regulation 2006. However, the use of CMRR on its own will potentially lead to flawed geotechnical characterisation and design. The pitfalls of rock mass classification systems have long been known to respected geotechnical experts such as Brady and Brown (1985) who caution, “Although the use of this approach is superficially attractive, it has a number of serious shortcomings and must be used only with extreme care. The classification scheme approach does not always fully evaluate important aspects of a problem, so that if blindly applied without any supporting analysis of the mechanics of the problem, it can lead to disastrous results”. The objective of this paper is to explore the risks and practical limitations associated with the use of CMRR, and to consider strategies and guidelines for the use of CMRR in characterisation and design which will minimise the risks.