2020-11-05_REVISION - M1973021 (12)

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1. Introduction 1.1 General GEI Consultants, Inc. (GEI) has prepared this report to evaluate revised highwall configurations in Aggregate Industries (AI) Morrison Quarry mine near Morrison, Colorado. The evaluations are focused on revised configurations at the existing (active) South Quarry and proposed highwall configurations in the planned West Quarry. The Morrison Quarry mine is operating under permit M-1973-021 from the Colorado Division of Reclamation Mining and Safety (DRMS). This Geotechnical Stability Exhibit is included as an appendix to DRMS permit Amendment No. 7 (AM-07) and summarizes results of stability analyses performed to evaluate proposed quarry geometric configurations (e.g., cut slopes and bench widths) relative to the minimum required factor of safety (FOS) for rock slope stability (FOS greater than [>] 1.25) as indicated in MLRB Policy 30. Morrison Quarry currently consists of three mining cells; the North Quarry,the Central Quarry, and the South Quarry. Active mining no longer occurs within the North Quarry or Central Quarry and Al is actively mining within the South Quarry, utilizing highwalls and benches primarily on the western (east-facing)mountain slopes of the South Quarry. Three below-grade benches have been excavated down to about elevation(El.) 6,600 feet on the eastern side of the quarry. Al has begun reclamation on some areas within the South Quarry, primarily at the high elevation benches (El. 7,450 up to El. 7,550 feet) on the west side of the quarry. Al is proposing to modify mining operations in the South Quarry to permit a steeper highwall configuration on the main, east-facing highwall. The proposed modifications include increasing the highwall slope to a maximum configuration of 30-ft wide benches, 70-ft high wall benches with vertical cut faces. This will involve re-mining existing benches below El. 7,000 feet in addition to increasing the steepness of the slope for future excavations. AI is also planning to create a fourth mining cell on a ridge located north of the South Quarry and west of the Central Quarry,referred to as the West Quarry. The planned top and bottom benches are approximately located at El. 7,360 and El. 7,040 feet, respectively. Below El. 7040 feet is considered the lower South Quarry, which has an anticipated bottom at El. 6195. The West Quarry is currently inactive and in the final stages of planning for future development. Mining within the West Quarry is anticipated to begin in 2021. 1.2 Site Description The Morrison Quarry is a hard rock quarry, owned and operated by Al, located west of Colorado Highway 8 and approximately 1/2 mile north of U.S. Highway 285 in Morrison, Colorado. The mine site is located west of the Dakota hogback along the Front Range of the Rocky Mountains west of Denver, Colorado, in Section 10 of Township 5 South, Range 70 West of the 6th Principle Meridian. The site location is shown in Figure 1 in the figures Aggregate Industries—Morrison Quarry GEI Consultants, Inc. AM-07 Geotechnical Stability Exhibit October 2020