The Coal Mine Roof Rating (CMRR) system was developed ten years ago to fill the gap between geologic characterization and engineering design. It combines many years of geologic studies in underground coal mines and worldwide experience with rock mass classification systems. Like other classification systems, the CMRR begins with the premise that the structural competence of mine roof rock is determined primarily by the discontinuities that weaken the rock fabric. Since its introduction, CMRR has been incorporated into many aspects of mine planning, including longwall pillar design, roof support selection, feasibility studies, and extended cut evaluation. It has also become truly international, with involvement in mine designs and funded research projects in South Africa, Canada, and Australia. Most recently, a new streamlined process to determine the CMRR from exploratory drill cores has been developed. Just three types of information are now required:
• Fracture spacing Rock Quality Designation (RQD) from a standard geotechnical drill log
• Uniaxial compressive strength from standard lab tests, geophysical downhole logging, or axial point load tests, and;
• Diametral point load testing.
The CMRR has been implemented in a computer program, which can be obtained from NIOSH free of charge. The program facilitates calculation of the CMRR from either underground or drillcore data. Values from many locations can be saved in a single file, and an interface with autocad allows CMRR contour plots to be integrated into mine planning.