Miners Toolbox Index Rock Mechanics  Rock Mass Classification Coal Mine Roof Rating

Coal Mine Roof Rating

To help quantify the engineering properties of mine roofs, the Coal Mine Roof Rating (CMRR) was proposed (Molinda and Mark, 1994). The CMRR weighs the geotechnical factors that determine roof competence, and combines them into a single rating on a scale from 0 to 100. The rating system is based on over 20 years of research on geologic hazards in underground coal mining with worldwide experience with rock mass classification systems. 

The Coal Mine Roof Rating (CMRR) estimates the structural competence of bolted mine roof. The important geotechnical features of the rock are assigned a relative strength value and their effect is combined into a final single value (CMRR). The CMRR ranges from 0-100. The CMRR is intended to sum up all of the geologic parameters which determine the strength of the rocks. Numerous other factors are known to affect the stability of a roof sequence. Among them are horizontal stress, multiple seam loadings, pillar strength, depth of cover, and roof support. The CMRR is not intended to characterize unusual or large-scale disruptions of the roof beam such as faults.  Where such features exist, a specific stability analysis for that area would be needed.

The Coal Mine Roof Rating consists of the Unit Ratings which assess the strength of individual members of the bolted interval, and adjustments which consider the strength of the association of all the Units together. The bolted interval is partitioned into Units (usually 1-3) based on rock members of comparable properties. The thickness-weighted average of the Unit Ratings is then adjusted for the effects of the Strong Bed, Ground Water, Surcharge and Unit Contacts to determine the final CMRR.

The CMRR assumes that it is primarily the discontinuities within the rock which determine its structural competence or weakness. Rocks are generally weaker in tension than compression and this is due to laminations or bedding. Because of the sedimentary deposition of coal measure rocks, internal bedding is almost always an important discontinuity. Other internal defects in rocks include fossil partings, mud bands, slickensides, faults, or mica beds. The CMRR evaluates the cohesion on bedding and other discontinuities by a chisel splitting test (hand sample) or point load testing (core). The roughness on the surface of the discontinuity can act to resist or facilitate shearing or delamination. Small asperities (sand and silt grains) can resist by interlock or cementing. Bedding that is composed of stacked clay minerals can easily delaminate and shear when flat or planar. The high frequency or intensity of discontinuities also causes weakness. The spacing and persistence of each discontinuity set is measured directly or estimated remotely by observation.

The material strength of the rock matrix itself ( unconfined uniaxial compressive strength (UCS)) contributes to its structural competence. A field test, the ballpeen hammer test, is a good index test of rock U.C.S. Point Load Testing, or UCS tests can also be used.

Many clay-rich rocks degrade on contact with water. Water movement by capillary action on bedding surfaces can cause slaking or delamination. Expansion and shrinkage of unconfined skin clays with seasonal moisture cycles can also cause slaking and delamination. Large pressures developed from the expansion of clay minerals confined up in the roof can damage roof beams. For these reasons moisture sensitivity of roof rocks is tested by water immersion or observation.

References:

Molinda GM, Mark C [1994]. Coal Mine Roof Rating (CMRR): A practical rock mass classification for coal mines. Pittsburgh, PA: U.S. Department of the Interior, Bureau of Mines, RI 9387, 83 p.

Mark C, Molinda GM, Schissler AP, Wuest WJ [1994]. Evaluating roof control in underground coal mines using the coal mine roof rating. Paper in the Proceedings of the 13th Conference on Ground Control in Mining, Morgantown, WV, pp. 252-260.