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SubTerra, Inc. Sage Creek Mine 11/18/2009 <br />Initial Evaluation of Subsidence Impacts for Continuous Miner Development <br />• Pillar Failure (Worst Conditions) <br />Coal mine entry pillars are designed to yield under stresses that are redistributed during longwall <br />extraction and therefore have very high safety factors during entry development. Niosh's ALPS <br />(Analysis of Longwall Pillar Stability) program has been used to illustrate this condition. <br />Pillar stress will increase during development mining to accommodate the weight of the overlying <br />overburden which will be transferred to the pillars and adjacent coal ribs as entries are advanced. <br />In situ vertical stresses will be directly proportional to the maximum overburden height and <br />increase at 1 -psi per foot of depth. <br />The ALPS program computes stability factors for conditions progressing from development through <br />longwall extraction. ALPS stability factors less than 1.2 indicate unsatisfactory conditions (e.g., <br />pillar instability) for coal mine roof ratings (CMRR) of less than 35 (i.e., worst case). Lower stability <br />factors are considered adequate for higher coal mine roof ratings (CMRRs). <br />We ran the forward model in the ALPS program using the worst case coal mine roof rating of 35 <br />and the pillar and mine geometry data previously discussed with the following results: <br />Stability Factor <br />Development Loading 2.4 to 2.8 <br />Headgate Loading 1.5 to 1.8 <br />Tailgate Loading 1.1 to 1.3 <br />These model results illustrate a high confidence in pillar stability during development mining even <br />• when worst case conditions are considered. <br />Pillar Punching (Worst Conditions) <br />This phenomenon occurs when redistributed stresses, reacted through the coal pillars exceed the <br />capacity of the mine floor causing the pillar to "punch" downwards into the mine floor. This <br />phenomenon is typically only associated with high extraction ratios and mine workings at great <br />depth. <br />Pillar punching is analogous to bearing capacity failure of a foundation and is typically analyzed <br />using traditional foundation design methods. We elected to use the method suggested by Vesic <br />(1969) as reported in CMLRD (1986) to calculate the bearing capacity of the mine floor. <br />During development the vertical stress carried by the entry pillars at the Sage Creek Mine is <br />expected to increase from 1,200 psi (worst case) to 1,700 psi assuming no load transferred to the <br />solid coal ribs (exceeds worst case). The factor of safety for pillar punching is the ratio of bearing <br />capacity to pillar stress; stability is indicated by a safety factor greater than 1. <br />The bearing capacity was computed using the Vesic equation, a shear strength for the mine floor <br />of 400 psi and an immediate floor thickness of 2 -ft. This results in a bearing capacity of 3,200 psi <br />for the narrow, 40 -ft wide pillar and a safety factor greater than 1. <br />2 Mark C (1992). Analysis of Longwall Pillar Stability (ALPS) - -An Update. Paper in the Proceedings of the <br />Workshop on Coal Pillar Mechanics and Design, USBM IC 9315, pp. 238 -249. <br />3 CMLRD, 1986. Colorado Mined Land Reclamation Division, 1986. Boulder County Abandoned Mine <br />• subsidence Investigation Report. 1986 <br />in <br />