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<br />-4- <br />Significant portions of the area of projected vertical subsidence (14 feet <br />maximum vertical subsidence) are comprised of steep sloped ridge topped and <br />ridge slope areas, which may be subject to significant secondary mass movement <br />phenomena. It will be necessary for the applicant to prepare feasible methods <br />of detection and mitigation, in order to assure prevention of material damage <br />to the affected and adjoining permit areas. The application observes, on page <br />39a of appendix 4.5-1; "...there is no present indication of significant slope <br />stability on existing natural slopes." Review of existing stereographic <br />aerial photography obtained by the Division in 1983 determined that <br />significant areas of potential slope instability appear to exist on the <br />northeast facing slopes of those portions of the Grand Hogback, in close <br />proximity to the mined seam's outcrop. Portions of these areas are projected <br />to undergo subsidence as a result of the proposed hydraulic extraction plan. <br />The applicant should amend the application to include a more comprehensive <br />discussion of slope stability within the permit area. As an example, the <br />applicant's attention is called to the area in the immediately proximity of <br />the proposed airway raise/escapeway, immediately east of the peak of Horse <br />Mountain, in which modern landslide activity appears to have occurred as a <br />result of recent exploration road construction. <br />The application presents a proposed subsidence monitoring program, which the <br />Division considers to contain significant deficiencies. The applicant <br />proposes the installation of one linear set of widely spaced subsidence <br />monitoring monuments, oriented perpendicular to the outcrop strike. The <br />application should be amended to provide a detailed description of the <br />rationale of the proposed monitor monument installation configuration. <br />The Division anticipates, depending upon the final approved version of the <br />amended subsidence survey's projection of slope stability consequences due to <br />subsidence, that monument at ion will be needed to specifically verify the slope <br />stability projections. <br />The application observes that survey of all actively subsiding monument points <br />will cease when a point has failed to demonstrate subsidence movement during <br />two consecutive surveys. Succession of monument survey monitoring may occur <br />only with the written consent of the Division. <br />Rule 2.06.4 Special Categories of Mining - Steepslope Mining <br />Portions of the permit appear to qualify as Steepslope mining areas. <br />Projected areas to be subsided during the first five year permit period appear <br />to constitute Steepslope mining areas. Rule 2.06.4, however, does not require <br />a specific response, but rather requires compliance with the performance <br />standards within Rule 4.27.3. Final determination of whether or not the <br />application is in compliance requirements of Rule 2.06.4 will have to occur <br />following resolution of the concerns regarding potential slope instability due <br />to subsidence. <br />