2021-09-07_REVISION - M1973021 (4)

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Aggregate Industries, Morrison Quarry Amendment No.7 to Permit M-1973-021 FN Morrison,CO The standard Phase III and Phase IV maximum bench design configuration from AM05 remains unchanged and consists of: • 60H:40V (horizontal to vertical) benches on upper portions of western exposures to reduce visual impacts (West Quarry and South Quarry western faces, above 7,000 feet elevation); and • 40H:80V benches below 7,000 feet elevation. The mining plan includes several exceptions to these standards based on South Quarry highwall observations to date and addresses key findings and recommendations in the 2020 Geotechnical Stability Report(see Appendix 1). These exceptions require conservative benching approaches while additional site-specific geotechnical evaluations are completed, including: • 40H:40V benches in areas with thick units of sillimanitic gneiss (less than 5 feet); and • 2H:1 V maximum bench configuration in areas where foliations or fractures are steeply dipping into the excavation and the strike is parallel to the bench face. As has been stated in previous amendments and proven effective in South Quarry Phase III work to date, the "Observational Method" will continue to be employed to refine slope configurations as various portions of the quarry are developed and rock mass characteristics are modeled. Operational considerations, rock quality, localized slope stability, and visual inspections will all influence the actual buildout and final configurations. Refer to AM03 and AM04 for detailed descriptions of the Observational Method. Monitoring the mine highwalls has been critical in identifying rock stability issues to help inform the Observational Method and supporting less aggressive bench configurations to mitigate potential slope failures in the South Quarry. It is also critical to identifying rock instabilities in completed highwalls, ensuring these issues are promptly addressed, either via catchment berms, rock scaling, buttressing, or other techniques, to support safe mining and reclamation working conditions. Performance monitoring of highwalls is conducted 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 continue and 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. Detailed discussions of Phase III and IV activities, including surface water and groundwater management, and timelines are presented below. Phase III — South Quarry and West Quarry Development To date, mining operations in the South Quarry have resulted in lowering the Quarry to Elevation (El.) 6,690 in the southwestern corner and El. 6,600 on the eastern face. Mining and reclamation will proceed from the present configuration, using the same mining methods described in AM05. Under Phase III, mining will continue in the South and West Quarries to El. 6,500, where it is anticipated that groundwater will be encountered. September 2021 1 16