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October 11, 2011 Page 12 <br />4.0 SUBSIDENCE PLANS BY SURFACE STRUCTURE CLASS <br />In order to prepare subsidence prevention and control measures for the all structures that <br />could possibly be affected by mining induced subsidence, broad classes of structures were <br />defined and each structure was classified. The classification of structures was achieved by <br />taking into consideration the specific function and construction attributes of each structure. The <br />classification of all the surveyed surface structures is presented in Table 3, along with the <br />allowable ground deformation. Deformation below these values is not likely to cause damage. <br />Allowable damage criteria were not found for structures on concrete skids /footings, structures on <br />metal skids /footings, and structures on metal frames. Therefore, AAI selected a horizontal strain <br />criterion of 5 millistrain for these three classes, a criterion thought to be conservative. <br />The values of strain, slope, and curvature in Table 2 may be compared to protection <br />criteria for surface structures and features above the mining area to assess damage potential. It <br />may be noted that the estimated maximum strains and slopes in Table 2 exceed the allowable <br />values for the structures under consideration; therefore, subsidence prevention and control <br />measures are likely necessary. Subsidence control, monitoring, and mitigation plans for affected <br />structures are presented below. <br />Generally, the structures which are of the greatest concern for subsidence within the <br />permit boundary are the gas wells and associated pipelines constructed after earlier mining on the <br />property was suspended. Protective pillars 300 ft- square will be left around active gas wells, per <br />Mine Safety and Health Administration (MSHA) requirements for mine safety. In order to <br />prevent damage to the gas pipelines, a buffer zone will be created on either side of the gas <br />pipelines that will not have any retreat mining extractions. This should limit the maximum <br />horizontal strains experienced by the pipelines to less than 2 millistrain, i.e., the damage criterion <br />for such structures. <br />A separate subsidence prevention plan was developed for the alluvial valley floor of the <br />Purgatoire River. That plan is presented at the end of this report. <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 <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 />A literature review was performed regarding design guidelines for protection of gas wells <br />that intercept underground mine workings. A guideline developed by the Commonwealth of <br />Agapito Associates, Inc. <br />