2025-07-30_PERMIT FILE - C1981019 (7)

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RULE 4 PERFORMANCE STANDARDS South Taylor – Rule 4, Page 13 Revision Date: 7/21/25 Revision No.: MR-267 4.23 AUGER AND HIGHWALL MINING 4.23.1 Scope This Section establishes environmental protection performance standards in addition to those applicable performance standards in Rule 4, to prevent any unnecessary loss of coal reserves and to prevent adverse environmental effects from auger mining incident to surface mining activities. 4.23.2 Maximize Recoverability of Mineral Reserves Colowyo has identified areas suitable for highwall mining at South Taylor (Map 23). Highwall mining allows for the recovery of additional coal resources beyond the final pit highwalls and end-walls. These coal reserves are economically recoverable using highwall mining methods. From a strip mining perspective, the South Taylor Pit clearly delineates the maximum recoverable coal resources attainable today with modern surface technology and coal market demand and pricing. The highwall mining of the G-seams on the west side of the pit represents recovery of reserves that would not have been recovered by any other means utilizing either surface or underground mining techniques. Section 4.23 of Volume 1 contains additional discussion regarding the removal of coal using highwall mining methods. The document includes a summary of the geologic factors that limit removal of the coal using conventional methods, the requirements for leaving undisturbed areas of coal in un-mined sections, procedures for working around underground mines, surface and groundwater pollution prevention procedures, reduction in fire hazards, backfilling, grading, and PMT requirements for all areas previously permitted. 4.23.2(1) Undisturbed Areas of Coal Shall Be Left in Unmined Sections As for the rules requirements [Rules 4.23.2(1)(a)-(c)] for leaving undisturbed areas of coal in unmined sections, Colowyo requests a variance from the requirements of this rule for the South Taylor Pit. Colowyo’s highly successful highwall mining methods that have been used and will be in the future in the South Taylor Pit, will maximize production and ensure no subsidence occurs. Using this particular method of highwall mining by leaving pillars and barriers allows the seams to be mined below each other and still ensures geologic stability once all seams have been mined out. Please see Exhibit 27, Item 6 and Item 7 in Volume 20 for further discussion on the geotechnical design and operational considerations implemented highwall mining the South Taylor Pit. 4.23.2(2) Abandoned or Active Underground Mine Workings No abandoned or active underground mine workings have ever existed or currently exist in any of the coal seams in the areas to be highwall mined. Highwall mining will not take place within 500 feet of any abandoned or active underground mining operation. 4.23.2(3) Surface Mining Activities and Highwall Mining The highwall mining shall follow the surface coal mining activities in a contemporaneous manner consistent with the applicable requirements of Division Rule 4. Due to active pit progressions and sequencing of mining (in addition to meeting the Permit requirements for contemporaneous reclamation), it is required that highwall mining occurs timely if not immediately following conclusion of pit mining activities. Also, as described more fully in 2.06.9(2), the need to backfill, is mandatory for Colowyo in order to build the pit floor from which to work from to mine the successively higher (in the geologic column) coal seam. Hence successful highwall mining is in part dependent upon timely backfill of the pit.