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AFS Response to DRMS Geotechnical Review#2 September 28,2021 Risk of rockfall resulting from blasting will be minimal because the vibration caused by blasting within the mining area can be controlled by distance and blasting techniques that minimize vibration. The relatively high frequency of blast vibrations causes the waves to attenuate over much shorter distances than earthquake signature vibrations and confine any damage to manageable distances. See Geotechnical Stability Exhibit for discussion on ground vibration during blasting,pages 32-34 of Section 6.5.6 (Pit Geometry and Mining Practices). Blasting has been safely performed within the Existing Quarry for the past 30 years. There have been no incidents of rockfall on undisturbed slopes outside of the Existing Quarry. See Attachment 1, cross- sections 1 and 2 for cross-sections of mined areas and undisturbed outslopes along Highway 6 in the Existing Quarry. As shown in cross-section 1,blasting in the Existing Quarry was conducted within approximately 425 horizontal feet of Highway 6 with an average 1.2H:1 V outslope. Cross-section 2 shows a similar case where blasting was completed within approximately 650 horizontal feet of Highway 6 with an average 1.2H:1 V outslope. In contrast, the mining area of the Spur Amendment is typically 750 feet to 1,500 feet from the closest area of concern (i.e., Highway 6 or Clear Creek, whichever is closer), with the exception of the ravine on the north side of the oxbow(near Tunnel 6)which is approximately 500 feet from Clear Creek. See Attachment 1, cross-sections 3-6 for cross-sections of the proposed Spur Amendment area along Highway 6 and Clear Creek; these cross-sections show the expansive buffer between the mining area and the Spur Amendment permit boundary. In addition to showing the horizontal and vertical distance from the mining to the area of concern, an example of mining at the 7,800-foot level is shown reflecting the "mining from behind"approach that will minimize the potential for rockfall on the outslopes. Also, locations where temporary rockfall berms may be placed(downslope from mining) are shown. The separation of the mining area from Highway 6 or Clear Creek is shown in the cross sections, as follows: • Cross-section 3 shows mining occurring a minimum of 750 horizontal feet from Highway 6 with an average 1.3H:1 V outslope. • Cross-section 4 shows mining occurring a minimum of 1,500 horizontal feet from Clear Creek with an average 1.7H:1 V outslope. The area closest to the creek has a steeper 1 H:1 V slope, but that slope is 900 feet from the nearest mining area. • Cross-section 5 shows mining occurring a minimum of 500 horizontal feet from Clear Creek with an average 2.5H:1 V outslope. • Cross-section 6 shows mining occurring a minimum of 800 horizontal feet from Clear Creek with an average 1.8H:1 V outslope. In summary, the cross-sections show that mining in the Spur Amendment will be similar to mining in the Existing Quarry. However,the slopes in the Spur Amendment tend to be longer and less steep than the examples from the Existing Quarry (generally less steep than 1.5H:1 V in the Spur vs. 1.2H:1 V in the Existing Quarry), and the mining will generally be done further from Highway 6 and Clear Creek. In order to mitigate the risk of rockfall, AFS commits to four actions: 1. Conduct only controlled blasting techniques within 50 feet of the Affected Area line (on the southern/downslope side of the mining area), with blasts designed to minimize vibration and overbreak. The rock type and quality will be monitored to optimize performance of the blasts. Furthermore, the Spur Amendment has a notably higher Rock Mass Rating (RMR)than the area previously blasted in the Existing Quarry; this will result in less overbreak and vibration. These Page 2 of 3