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= 1.3 (assuming strength measurements resulting from multiple tests have been performed; otherwise a minimum FOS of 1.5 is required for critical structures, such as the Globe Hill Pipe zone where Section 1.2.3,p. 1-3 states "shear strength was estimated based on previous experience in similar type materials"). Based on Figure 1- 1, it appears the higher FOS is required for Sectors 2, 10, 12, 14, and possibly 4. Please confirm the minimum FOS in the vicinity of critical structures is consistent with DRMS Table 1 and include critical structures on Appendix 5 figures. RESPONSE: CNI agrees with the criteria of 1.5 minimum FOS(factor of safety)for failure mechanisms that may impact critical structures (such as Teller Co. Road 82), given that the strengths were estimated based on previous experience in similar rock types in the Cripple Creek and Victor mining district. We also agree that if multiple tests have been conducted to determine the strengths that the minimum FOS of 1.3 required by the Division is satisfactory. However, we advocate a minimum FOS of 1.2 for slope instability mechanisms that will be fully contained within the surface mining operation and not impact off-site critical structures. • The last paragraph on p. 4-9 cites Wiles(2000) suggesting the"coefficient of variation for rockmass strength estimation may be 25 to 35 percent". Given this variability in an anisotropic fractured system, the Division is concerned that despite the numerous tests performed to estimate rock strength parameters,this expected variability, an FOS of 1.5 may be appropriate. Please provide appropriate rationale if you disagree with the Division requiring a FOS of 1.5 in the vicinity of critical structures (e.g., Co Rd 82, SGVLF). RESPONSE: CNI agrees that a FOS of 1.5 for failure mechanisms that may impact critical structures is appropriate,provided no prior experience has been gained in historical mining of slopes in rocks with the same rock mass characteristics and strengths. If historical mining in the same rock mass has occurred and appropriate strength testing has been conducted, then CNI advocates for a minimum FOS of 1.3 for slopes that could impact critical structures (e.g., Co Rd 82, SGVLF). 32. Appendix 5 Geotechnical Slope Recommendations, p. 5-8, Section 5.4.1. The report states "Current design interramp angles do not meet the required 80%reliability in Sectors 1, 2, 6 and 7". • Please provide a source and rationale for the 80%reliability requirement. RESPONSE. The rationale for the catch bench design methodology is presented in Ryan and Pryor(2000). The referenced paper is attached as Attachment 9. Catch- bench reliability refers to the percentage of time a catch-bench width will satisfy the minimum acceptable catch-bench width for a given bench height. The purpose of maintaining catch benches of adequate width is to provide rockfall protection to the working level at the bottom of the slope. At CCV, CNI recommends using an 80 percent reliability of maintaining a catch-bench width of 19 feet for single benches (35 foot- high) or maintaining a catch-bench width of 26 feet for double benches (70 foot-high). This specified width is the achieved catch-bench width after mining and scaling has finished. Industrywide, catch-bench reliabilities ranging from 60 to 90 percent have been used successfully. Note that the term "catch bench reliability" used refers to the reliability of achieving the criteria of 19 or 26 feet(for single and double benching, respectively). A catch bench reliability of 80 percent does not indicate that 20 percent of the mine benches will be totally lost, instead, it indicates that 20 percent of the benches are expected to be less wide than the specified criteria. • Is this 80%reliability for slope stability or catch bench functionality? Page 14 of 30