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 proceed on the northwestern, northern, and eastern faces of the South Quarry until all benches have been advanced to El. 6,500. On the northwestern and northern faces, the maximum highwall height will be 80 feet and the minimum bench width will be 40 feet (40H:80V). Based on recommendations from the 2020 Geotechnical Report, pending further studies, the eastern portion of the quarry (west-facing) will be mined using benches with a maximum highwall height of 40 feet and a minimum bench width of 40 feet (40H:40V). As mining continues in Phase III, additional geotechnical evaluations will be conducted to 1) determine if any southwestern corner benches below El. 7,000 and eastern face benches can be re-mined to steeper slopes (up to a maximum 40H:80V) before reclamation; and 2) collect additional information to better understand the global and local rock stability in the West Quarry area. The results of these studies, the ongoing application of the observational approach, and the bench configurations identified in the section General Mining Approach will inform decisions regarding the ability to re-mine areas of the South Quarry and the need to modify bench configurations (i.e., ones with less-steep slope angles) to meet factor of safety minimum requirements. Any decision to re-mine or increase bench configurations will be done in a mining plan modification that will be submitted to the Colorado Division of Reclamation, Mining and Safety (DRMS) for approval. Groundwater monitoring wells located south and east of the South Quarry (MW2019-02 and MW2019-01) show average water level elevations of 6,581 and 6,459 feet, respectively. Based on these static measurements, there is the potential that groundwater will be encountered and require dewatering at the base of Phase III. A Colorado Division of Water Resources (DWR) well permit application is underway, and a Substitute Supply Plan is in development. If groundwater is encountered before these documents are approved, the excavation will be backfilled or sealed to prevent groundwater exposure, and mining will cease in that area. After approvals are obtained, any necessary dewatering will be conducted by installation of temporary sumps to pump and discharge groundwater from the site through Outfall 003. As mining progresses, stormwater will collect in the lower elevations of the South Quarry. Per the Storm Water Management Plan (Rev. 6, June 2021), visual observation will be conducted during and after storm events, and stormwater that has not infiltrated into the subsurface within 72 hours will be pumped to Outfall 003 for discharge. If Phase IV is delayed or postponed following Phase III completion, more permanent stormwater and groundwater management structures may be implemented. These are discussed in Exhibit E— Reclamation Plan, Dewatering. Temporary spur roads from the West Quarry Haul Road will re-establish access to the upper benches of the South Quarry western slope up to El. 7,400. Benches above El. 7,000 will continue to be reclaimed in accordance with Exhibit E— Reclamation Plan. Reclamation activities are planned to occur above actively mined benches as mining progresses downhill. South and West Quarry reclamation activities will be completed on upper benches above El. 7,000 to address visual impact concerns. Once completed, reclamation activities will continue in a manner that minimizes open benches in accordance with Exhibit E—Reclamation Plan. September 2021 1 18