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RULE 4 PERFORMANCE STANDARDS <br />a <br />4.23 Auger and Highwall Mining <br />4.23.1 Scope <br />This Section establishes environmental protection performance standards in addition to those <br />applicable performance standards in Rule 4, to prevent any unnecessary loss of coal reserves and <br />to prevent adverse environmental effects from auger mining incident to surface mining activities. <br />4.23.3 Performance Standards <br />4.23.4 Maximize Recoverability of Mineral Reserves <br />Colowyo has currently identified a number of areas that may be suitable for highwall mining that <br />will be accessed from within the West Pit (refer to Map 23, Mine Plan). Highwall mining should <br />allow for the recovery of additional coal resources beyond the currently permitted pit final <br />highwalls and endwalls. These resources were not classified as reserves until the concept of <br />highwall mining was conceived — all coal was previously classified as unmineable due to being <br />uneconomic to mine either by modern surface or underground mining methods. <br />From a strip mining perspective, the previously mined and future permitted limits of the East, <br />West, and Section 16 pits clearly delineate the maximum recoverable coal resources attainable <br />today with modern surface technology and coal market demand and pricing. The proposed <br />highwall mining of many of the same seams around the perimeter and underneath the mined out <br />pits, represents recovery of reserves that would not have been recovered by any other means <br />utilizing either surface or underground mining techniques. <br />The inability to recover these coal reserves by any other mining technique is primarily based on <br />insurmountable geologic factors. The majority of all coal seams experience splitting and <br />thinning to the point of not being economical to mine due to lack of adequate coal quality <br />(unacceptable ash from coal seam splits that cannot be selectively mined from the seam) and/or <br />coal seam thicknesses getting too thin to be mined from either the surface or underground. <br />Additionally, all coal seams proposed to be mined by highwall mining, have experienced natural <br />historic in place burning of the seams which severely limits the extent of mineable coal and also <br />precludes finding intact outcrop locations from which to access these same seams by <br />underground outcrop mining. To the north and east, changing geologic structure gives rise to <br />severe dips and unstable fractured strata that also precludes mining by either surface or <br />underground mining methods. Areas proposed for potential highwall mining activities are <br />shown on Map 23. <br />Colowyo was acquired by Tri-State Generation & Transmission Company in December of 2011. <br />However, when Kennecott Energy Corporation acquired Colowyo in 1994, numerous <br />optimization, exploratory drilling, geologic evaluation, and pre -feasibility studies and programs <br />were undertaken to identify all future mining options. By year-end 2003, Colowyo completed <br />pre -feasibility studies on three separate surface mining options and on underground mining <br />options that evaluated all coal seams and all areas within Colowyo's Logical Mining Unit (LMU) <br />and beyond. This comprehensive body of work definitively and unequivocally shows that all <br />Rule 4 Performance Standards 4-87 Revision Date: 4/2/18 <br />Revision No.: RN -07 <br />