2021-11-03_REVISION - M1973021 - Laserfiche WebLink

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Aggregate Industries, Morrison Quarry Amendment No.7 to Permit M-1973-021 FN Morrison,CO concluded that 40H:70V bench configurations at El. 7,000 and El. 6,650 had a corresponding global stability factor of safety (FOS) greater than 1.25, considering both competent and poor rock conditions. Local stability FOS evaluations provided similar results at El. 7,000. Less favorable results were documented at El. 6,650; however, the geotechnical consultant recommended a 40-foot-wide bench at this elevation to meet stability criteria. Adopting a lower 60-foot highwall and wider 40-foot bench in this amendment for all mining at these elevations results in a FOS that meets or exceeds the minimum requirements. As has been successful with South Quarry operations to date, monitoring the mine highwalls will continue in accordance with the South Quarry monitoring plan, which utilizes control points fixed to concrete-filled 55-gallon drums and a visual monitoring program. This plan will be supplemented to include quarry face monitoring using Light Detection and Ranging (LiDAR) scan of the rock face, a recommendation from the 2020 Geotechnical Stability Exhibit. Monitoring will be increased to a minimum of biannual inspections and will be conducted by a qualified geotechnical professional. These inspections will be documented in technical reports and summarized in the annual Geotechnical Addendum. Biannual monitoring will support prompt identification of rock instabilities in completed highwalls, ensuring these issues are promptly addressed—via catchment berms, rock scaling, buttressing, or other techniques—to support safe mining and reclamation working conditions. Below El. 6,600: 40H:40V Benches and application of the Observational Method The 2020 Geotechnical Addendum (Figure 7) identified the potential for large deposits of poor quality rock (e.g., sillimanitic gneiss) in the South Quarry below El. 6,600. Large deposits of poor quality rock may require more conservative slope configurations to safely mine these areas. As mining has not advanced below El. 6,600, there is insufficient information to predetermine absolute final slope configurations below this elevation. As a result, consistent with Amendment Nos. 3, 4, and 5, the "Observational Method" will be used to support mining in these areas. The "Observational Method" has been modified to reflect proposed operations (see Exhibit D.3). Below El. 6,600, operations will mine to an initial 40-foot bench and 40-foot highwall configuration. This conservative configuration was recommended in Appendix 1 for areas of sillimanitic gneiss and will allow for mining to safely proceed while a qualified geotechnical professional observes and evaluates exposed rock. It is anticipated that these observations will be conducted biannually, and initial observations will focus on geologic mapping and visual observations. The results of these observations will be documented in technical reports and summarized in the annual Geotechnical Addendum. If observations identify favorable conditions for potentially steeper configurations (e.g., competent rock, fault orientation), a qualified geotechnical professional will conduct additional mapping and laboratory testing necessary to conduct a stability evaluation to support a steeper (i.e., 40H:60V) bench configuration. If evaluations result in a FOS greater than 1.25, the results of this analysis will be presented in a technical revision that will be submitted to the Division of Reclamation, Mining and Safety for review and approval. Mining at configurations steeper than 40H:40V will not proceed without DRMS approval. Conversely, if large quantities of poor quality rock are present or instabilities are identified, a steeper bench will not be pursued. October 2021 1 17