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May 11, 2011 Page 4 <br />occurring at cover depths of greater than 400 ft is negligible. 3 4 Given the anticipated cover <br />depths range from 450 to 1,200 ft, it is highly unlikely that any perceptible surface deformation <br />will result from room and pillar development operations. <br />4.0 SUBSIDENCE CONTROL, MONITORING, AND MITIGATION PLANS BY <br />SURFACE STRUCTURE CLASS <br />Subsidence prevention and control will be achieved in the current mining plan by the use <br />of development -only mining with adequate pillar stability factors. Monitoring and mitigation <br />plans were developed by defining broad classes of structures, and classifying each structure <br />within the permit area that could be affected by unplanned mining- induced subsidence. The <br />classification was achieved by taking into consideration the specific function and construction <br />attributes of each structure. The classification of all the surveyed surface structures is presented <br />in Table 2, along with the allowable ground deformation should unplanned subsidence occur. <br />Deformation below these values is not likely to cause damage. Allowable damage criteria were <br />not found for structures on concrete skids /footings, structures on metal skids /footings, and <br />structures on metal frames. Therefore, AAI selected a horizontal strain criterion of 5 millistrain <br />for these three classes, a criterion thought to be conservative. <br />Generally, the structures which are of the greatest concern for unplanned subsidence <br />within the permit boundary are the gas wells and associated pipelines constructed after mining <br />was suspended in the 1980s. No mining will take place within 150 ft of an active gas well, per <br />Mine Safety and Health Administration (MSHA) requirements for mine safety. Subsidence <br />control, monitoring, and mitigation plans for the aforementioned structures are presented below. <br />4.1 Gas Wells <br />A significant number of gas wells are located directly above or within the influence of the <br />Allen or Apache seam room - and - pillar panels. AAI reviewed state and federal statutes related to <br />mining in the vicinity of gas wells. Mine Safety and Health Administration (MSHA) rules <br />(30 CFR 75.1700) state that "when located, such operator shall establish and maintain barriers <br />around such oil and gas wells in accordance with State laws and regulations, except that such <br />barriers shall not be less than 300 feet in diameter, unless otherwise permitted." Colorado coal <br />mining regulations [Colorado Revised Statutes 34- 33- 111(1)(k)] require that "damage, <br />destruction, or disruption of services provided by oil and gas wells shall be minimized." AAI's <br />communications with the Colorado Division of Reclamation, Mining and Safety indicated that <br />protective pillar sizing for gas wells typically defers to the MSHA rule. <br />2 Gray, R. E. and R. W. Bruhn (1984), "Coal Mine Subsidence — Eastern United States," Geological Society of <br />America, Reviews in Engineering Geology, Volume VI, pp. 123 —150. <br />s Karfakis, M. G. (1987), "Mechanisms of Chimney Subsidence over Abandoned Coal Mines," Proc. 6` Intl. <br />Conference of Ground Control in Mining, Morgantown, WV. <br />a Matheson, G. M. (1990), "A Probabilistic Function for Prediction of Chimney Subsidence Sinkhole Development," <br />Proc. Mine Subsidence — Prediction and Control, AEG 33` Annual Meeting, Pittsburg, PA, pp. 233 — 246. <br />Agapito Associates, Inc. <br />