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