2021-09-07_REVISION - M1973021 - Laserfiche WebLink

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Aggregate Industries, Morrison Quarry Amendment No.7 to Permit M-1973-021 FN Morrison,CO West Quarry Development Once the West Quarry Haul Road is completed, operations will begin Phase III development of the West Quarry, starting with an upper elevation at approximately El. 7,400 and proceeding down to approximately El. 7,000. For benches above El. 7,000, the maximum highwall height will be 40 feet, and the minimum bench width will be 60 feet (60H:40V). Overburden materials from these upper benches will support remaining upper bench reclamation on the South Quarry western face from El. 7,400 to El. 7,000. Mining in the West Quarry will proceed from west to east (uphill to downhill), terminating at approximately El. 7,000. Mining below this elevation will be considered part of the South Quarry operations and will proceed to approximate El. 6,500. Reclamation of the West Quarry benches will be conducted concurrently as mining proceeds on the lower benches to reduce the duration of visual exposure. Schedule for Phase III Buildout Phase III development is anticipated to have a duration of approximately 12 years. The West Quarry will be mined concurrently with the South Quarry, and approximately 11 acres of the West Quarry and 86 acres of the South Quarry will be impacted during Phase III. Phase IV — South Quarry Deepening Phase IV finalizes expansion of the South Quarry by lowering the bottom of the South Quarry to a maximum base elevation of El. 5,840. The final base elevation will be informed by final bench configurations and the potential to re-mine areas of Phase III to steeper bench configurations. Mining operations in Phase IV are anticipated to occur within the water table, requiring dewatering and water management throughout the mining process. Additional geotechnical and hydrogeologic investigations are needed to incorporate saturated conditions into the geotechnical stability analysis and to better define a dewatering strategy. In addition, end uses of the South Quarry are still under consideration. Until such time that the hydrogeologic and geotechnical investigations are complete and end use decisions are made, Phase IV is considered conceptual. Work pursuant to Phase IV will not begin until the investigation results and final end use decisions are documented and approved in a future Permit Amendment. To satisfy bonding requirements, the end use of Phase IV is considered to be a temporary reservoir with no pit sealing required. Additional hydrogeologic studies, discussed in Exhibit G— Water Information, are planned to determine feasibility of potential reservoir end uses, evaluate reclamation requirements, and better inform plans to dewater the pit during mining activities. Similarly, additional geotechnical investigations will be conducted to inform how or if mine plans will require modification to work in saturated areas. At this time, the Phase IV plan will continue mining on western, southern, and northern portions of the quarry (east-, north-, and south-facing slopes, respectively), with a maximum highwall height of 80 feet and a minimum highwall width of 40 feet (40H:80V). As is noted above, the final end use of the South Quarry is under consideration. The end use determination may impact the slope configuration of mining in these areas (e.g., 3:1 maximum slopes above and below the potential shoreline if a reservoir is designated as the South Quarry end use). September 2021 1 19