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2.05.6(6) Subsidence Survey, Subsidence Monitoring, and Subsidence Control Plan <br />The undeveloped land is primarily related to those portions of the poorly <br />vegetated steep slopes that offer little opportunity for any managed <br />endeavor. The postmining land use of the permit area will be classified <br />the same as the premining land use which is rangeland and fish and <br />wildlife habitat. See Figure 4.1-1. <br />Some subsidence is predicted to occur over all mining panels. The magnitude of <br />subsidence will be a function of overburden depth and coal extraction ratio. The <br />Red Cliff Mine EIS prepared by the BLM (January 2009) contains a prediction of <br />subsidence and strains for five selected panels at the McClave Canyon Mine <br />(EIS Volume II, Appendix D, pages D-6 and D-7). Maximum subsidence <br />predictions range from 1.52 to 2.56 feet. Overburdenjor the five selected panes <br />range from 355 to 900 feet. Pages D-6, D-7 and Figure 7 from the referenced <br />EIS follow this section. <br />Subsidence cracks are likely to develop at the surface when large tensile strains <br />are induced by mining. These will generally be oriented parallel to retreat lines <br />except where they may be influenced by geologic structures such as faults. The <br />retreat line is orientated perpendicular to the mining panel. Literature studies <br />suggest a reasonable limit to overburden height for the formation of surface <br />cracks. The Applicant believes there is a slight risk of subsidence cracks <br />developing at the surface where retreat pillar mining is practiced under 500 to <br />800 feet of overburden. Areas where less than 500 feet of overburden overlies <br />the coal seam should be considered a high risk for subsidence cracks. <br />The light use roads located above the mining area could be damaged by <br />subsidence. The worst possible consequence would be a subsidence crack that <br />develops on the road. Cracks could be a few inches wide and be long enough to <br />go across the entire roadway. However, the subsidence cracks would not cause <br />material damage or diminution of use of the road. The roads located above the <br />mining area are not maintained so there are frequently eroded areas along the <br />road which make travel difficult. Subsidence cracks on the road would be less of <br />a hazard than naturally occurring heavily eroded areas. <br />Since the range fences are designed so they can be disconnected from the fence <br />posts and lay flat on the ground between grazing seasons they will not be <br />damaged by subsidence. <br />PR -02 2.05-70 10/12 <br />