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January 2, 2020 Page 3-2 <br />Agapito Associates, Inc. <br />The CMRR was developed for coal mine roof quality assessment. AAI calculated CMRR <br />values for specific core holes using the lithology of the immediate roof (first 6 ft), Rock Quality <br />Designation (RQD) values from core logs and inspection of core photographs, judgements <br />regarding weathering resistance based on the slake test data, and the appropriate physical property <br />datasets for each HWM area. CMRR ratings were converted to RMR values based on an AAI- <br />developed relationship derived from Western US experience: <br /> <br /> RMR = 0.9 CMRR + 23 (Eqn. 3-1) <br /> <br />The conversion to RMR is useful in that stand-times (the time between mining and eventual <br />roof collapse) have been correlated to RMR and opening width (Bieniawski 1989) and provide <br />insight to what the various CMRR and RMR values may mean from a practical roof stability <br />standpoint. <br /> <br />Representative roof stability assessment results for each target seam are presented in <br />Table 3-2. <br /> <br /> <br />Table 3-2. Roof Stability Summary <br />Area Seam CMRR RMR Stand Time <br />(days) <br />I Pit East F 34.0 53.6 32 <br />I Pit Middle and West F 28.8 48.9 10 <br />J Pit East G2 30.6 50.5 15 <br />J Pit West G2 32.2 52.0 21 <br />N Pit <br />L 29.5 49.6 12 <br />M 31.8 51.6 19 <br />Q 30.6 50.5 15 <br />L Pit <br />H 27.0 47.3 6 <br />K 31.8 51.6 19 <br />Q 38.5 57.6 89 <br /> <br /> <br />Typical required stand-times are on the order of 14 hours or less, based on an assumed <br />production rate of 350 tons per hour (tph), full penetration depth of 1,200 ft, and the average seam <br />height for each of the seams. Thus, even for the weakest roof, the projected stand-times are <br />significantly greater than the time required to mine each opening. Should localized roof failures <br />occur, leaving approximately 6 inches of roof coal may improve conditions. <br /> <br />