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Aggregate Industries, Morrison Quarry <br /> Amendment No.7 to Permit M-1973-021 FN <br /> Morrison,CO <br /> (40H:80V) configuration approved in AM05. On the West Wall, mining has advanced to the <br /> current bench, approximately El. 6,635, using a planned 60-foot highwall and 40-foot bench <br /> (40H:60V) configuration with actual mined highwalls ranging from 40 to 60 feet. Bench highwall <br /> modifications were operational decisions, not the result of geotechnical issues. On the East <br /> Wall, mining has advanced to the current bench, El. 6,600, using conservative 40-foot highwall <br /> and 40-foot bench (40H:40V) configurations, due to the presence of unstable rock. Mined <br /> highwalls ranged from 20 to 40 feet. <br /> Annual geotechnical observations and performance monitoring of exposed highwalls is <br /> conducted to support the development of an annual Geotechnical Addendum and assess <br /> highwall stability. As stated in Appendix 1, Rule 6.5 Geotechnical Stability Exhibit, Section 7.1.1, <br /> annual reporting and field inspections indicate that the South Quarry highwall is generally <br /> stable. Although rockfall debris is commonly observed, the rockfalls are generally minor and <br /> likely a result of production blast-induced rock fracturing and freeze-thaw in the rock face above <br /> the benches. The bench widths are adequate to collect highwall rockfalls. The observed rock <br /> conditions are predominately "competent" rock quality (i.e., granitic and biotite gneiss), with <br /> localized poor zones resulting from highwall proximity to existing (pre-mining) grade or zones of <br /> sillimanitic gneiss. <br /> Mining activities and configurations to date are effective in working through unstable rock <br /> conditions and resulted in stable configurations. Completed South Quarry reclamation and <br /> mining activities can be found in Exhibit C-2. <br /> General Phase III Mining Approach <br /> Under Phase III, mining will not occur within the groundwater table. Instead, mining will continue <br /> in the South and West Quarries to maximum depths before groundwater is encountered. <br /> The South and West Quarry maximum highwall and minimum bench design configurations are <br /> based on elevations described in the following subsections. <br /> Above El. 7,000: 60H:40V Benches <br /> This bench configuration is unchanged from AM05 and will be employed on the remaining West <br /> Quarry western face to reduce visual impacts. <br /> El. 7,000 to El. 6,600: 40H:60V Benches <br /> The proposed bench configuration at these elevations are based on geotechnical information <br /> gathered during South Quarry annual inspections, observations of completed 40H:60V <br /> highwalls at similar elevations (discussed above), and findings from the 2020 Geotechnical <br /> Stability Exhibit. <br /> Appendix 1, Rule 6.5 Geotechnical Stability Exhibit, was completed with the goal of determining <br /> if steeper (e.g., 30H:70V) slopes were feasible. Although this approach is not currently under <br /> consideration, selected sections of the report support the revised 40H:60V bench configuration. <br /> Specifically, slope stability analysis included in this report (Appendix 1, Tables 7.3 and 7.4) <br /> October 2021 1 16 <br />