The Coal Mine Roof Rating (CMRR)—a decade of experience

Texture, mineralogy, and rock strength in horizontal stress-related coal mine roof falls

“…Much work has been performed on determining the strengths of various sedimentary rocks, and several engineering classification systems such as the widely used Rock Mass Rating system (Bieniawski, 1973(Bieniawski, , 1979 and newly developed Coal Mine Roof Rating (Molinda and Mark, 1999;Mark, 1999;Mark and Molinda, 2005) have attempted to relate rock strength to mine roof stability. For purposes of rock classification schemes, rock strength has been regarded as a mechanical property that can be quantified by such laboratory tests as the triaxial or uniaxial compression tests, or the splitting tensile (Brazilian) strength test.…”

“…Those falls exhibited characteristics of horizontal stress-induced failure, involving flexure or buckling of the roof beam without the presence of pre-existing structural geologic discontinuities to act as planes of preferential weakness. Horizontal stress is attributed to the transmission of forces through the crust by the action of plate tectonics (Mark, 2001). Although the presence of geologic structural discontinuities can provide planes of preferential weakness along which a roof span may fail, roof failure in the absence of such structures may be influenced by some textural or mineralogical property inherent to the rock.…”

Influence of fracture-induced weakening on coal mine gateroad stability

“…Empirical methods [3][4][5] are often utilized based on the statistical analyses of adequate data collected in field practices. Many studies employ analytical methods [6][7][8] for better understanding of the failure mechanism and rock support performance.…”

Multi-seam mining of the deep Waterberg resources

“…Further research using the NIOSH database of US coal mines confirmed that longwall mining is possible with a low CMRR of about 30 (Mark and Molinda, 2005). A number of the mines with low CMRR were mining high tonnages using longwall mining methods but using additional roof support and narrower bord Multi-seam mining of the deep Waterberg resources VOLUME 116 widths when the chain roads are developed.…”

“…Stability of the immediate roof of the seam mined. This is dependent on aspects such as the strength, discontinuities, water content, and moisture sensitivity of the roof above the seam mined, which (among other factors) give rise to a coal mass rock rating (CMRR) value (Mark and Molinda, 2005). CMRR values of greater than 65 are considered high, and those less than 40 as weak 4.…”