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New Coal Mine Roof Rating (CMRR) Improves Safety in U. S. Underground Mines.
Proceedings of the 27th International Conference of Safety in Mines Research Institutes, 20-22 February 1997, New Delhi, India 1997:1001-1011
A new coal mine roof rating (CMRR) system for evaluating the condition of the roof strata of coal mines for ground control purposes was discussed. The system was developed by NIOSH with the goal of bridging the gap between descriptions of the geological characteristics of a coal mine roof and the needs of engineers responsible for designing a roof support or reinforcement plan. The CMRR system assumes that the structural stability of mine roof rock depends primarily on discontinuities that weaken the rock bedding. The system concentrates on identifying the individual units in the bolted sequence which define the number of unit borders and then evaluates their strength. The data needed for the system can be obtained by observation and simple contact tests that can be performed with a ball peen hammer, a 9 centimeter mason channel, a tape measure, and sampling bags. The CMRR system was based specifically on dividing the mine roof into a series of structural units which are rated individually. Each unit rating is based on evaluating any discontinuities and their characteristics, the compressive strength of the rock, and the moisture sensitivity of the rock. An adjustment is applied to account for a dead load from weak rocks above the bolted section. The ratings of all units within the bolted section are combined into a thickness weighted average, after which a series of roof adjustment factors is applied to obtain the final CMRR score. CMRR scores can range from 0 to 100 and specifically represent the inherent strength of the bolted section. The CMRR system has been applied to pillar and entry design, roof hazard assessments, and selecting appropriate roof supports. Examples of these applications were discussed.
Coal-mining; Risk-analysis; Rock-falls; Geophysics; Underground-mining; Industrial-safety; Failure-analysis;
Proceedings of the 27th International Conference of Safety in Mines Research Institutes, 20-22 February 1997, New Delhi, India
Page last reviewed: May 5, 2020
Content source: National Institute for Occupational Safety and Health Education and Information Division