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-~- This U.S. Bureau of Mines Contract Report, dated tlarcn, 1983, presents the results of research performed oy Geo-Hydro Consulting, Inc., under USBi4 contract J0215032. The abstract of the report summarizes its content. "The report presents the evaluation of the risk of slope failure for a slope that contains a coal seam outcrop. It deals with potential slope stability problems prior to mining, during mining when an outcrop barrier pillar is created, and after mining when water may be impounded behind the pillar. Stability conditions of the slope have been analyzed for various topographic and geologic configurations. Potential modes of failure have been determined and guidelines are presented for the estimation of the safe outcrop barrier pillar width." (U.S. Bureau of Mines, Mining Research Contract Report, Contract J0215032, "Stability of Mountain Slopes Undercut by Coal i~1ines", by Geo-Hydro Consulting, Inc., ,March, 1983) This report presents recommendations for barrier pillar design which appear to be particularly germane to the Mt. Gunnison No. 1 coal mine. The Division finds that such an investigation is necessary to ensure continued environmental protection and compliance with the subsidence-related requirements of Rules 4.20.3(1), Surface Owner Protection, and Rule 2.05.6(b)(f)(ii)(B)(II) and (III), measures to prevent material subsidence damage. Therefore, in light of the modification of the mine plan since the original approval, West Elk Coal Company must prepare an analysis of its amended mine plan configuration in order to determine whether the outcrop barrier pillar design is adequate to comply with the Rules cited above. The study presented within the USBM Mining Research Contract Report entitled "Stability of Mountain Slopes Undercut by Coal Mines" (March, 1983) will serve as the basis for determining adequacy. Further, if this analysis determines that a redesign of their presently approved mine plan is necessary to ensure compliance with the regulations, then the operator will be required to expeditiously complete and submit an appropriate redesign of its existing underground mine plan. I6. Advisory Comments: The fo]lowing items were identified during the review that pertain to issues outside the 5 year permit area. These items are not of immediate concern but may need to be considered in future permit terms. Mt. Gunnison is not obligated to respond to these concerns under this permit term. 1. Future impacts in the Minnesota Creek watershed will oe analyzed using intervention analysis. Page 17 of Exhibit 2.8.5 (Protection of Minnesota Creek 'Rater Supply) cites references Box and Tiao (1975) and Hipet, et al. (1978). These citations should be included in the references cited list on page 19. Also, a brief explanation of the methods, assumptions, etc. of intervention analysis would be appropriate to be able to assess your proposed modeling technique.