2016-12-19_REVISION - M1980244 (6)

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Cripple Creek & Victor /Mr. Erik Munroe, Ms. Poppy Staub 31 October 2016 Responses to October 2016 DRMS Adequacy Review of Cripple Creek & Victor Amendment 11 Page 2 CALL & NICHOLAS, INC. • (i) Attachment 8 (September 2016 submittal). The CNI Figure 6-18 Section GH-4 Global Analysis and Geology (Looking NW) depicts a failure surface with a “FOS = 1.52”. The failure surface daylights on the opposite side of Teller County Road 82 from the high wall in question, indicating the road itself has a FOS less than 1.52. The response to Comment 31.b (second bullet) states “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… ” where “…CNI advocates for a minimum FOS of 1.3…” This response does not explicitly offer any “historical mining” that would support the acceptance of a FOS less than 1.5 for this area. Please provide analysis results for a failure surface daylighting on the slope side of County Road 82 and if it is less than 1.5, provide support that historical mining has provided appropriate testing of similar rock for characteristics and strengths to achieve a FOS of at least 1.3. • Attachment 8 (September 2016 submittal). The CNI Figure 6-32 Section GH-6 Global Analysis and Geology (Looking NE) depicts a failure surface which daylights a considerable distance on the opposite side of Teller County Road 82 from the high wall in question, indicating the road itself may have a FOS considerably less than 2.219. Again referring to the response to Comment 31.b (second bullet), please provide analysis results for a failure surface daylighting on the slope side of County Road 82 and if it is less than 1.5, provide support that historical mining has provided appropriate testing of similar rock for characteristics and strengths to achieve a FOS of at least 1.3. RESPONSE: The original reported slip surfaces and factors of safety in Figures 6-18 and 6-32 were the lowest factors of safety identified during the study. When the cross sections were analyzed for stability, the factors of safety for hundreds of different trial slip surfaces were analyzed. These slip surfaces all had different entry points, exit points, and radius of curvature (depth). Hundreds of analyzed trial slip surfaces were located both in front of and behind County Road 82. For clarity and simplicity, only the critical (lowest) factor of safety values and the corresponding slip surfaces were plotted on Figures 6-18 and 6-32. These are the slip surfaces shown in red. To demonstrate factor of safety values for different slip surface entry points, the entry point was fixed at various locations along the crest of sections GH-4 and GH-6. Hundreds of potential slip surfaces were analyzed with these fixed entry points, and the resulting slip surfaces with the lowest factors of safety were then optimized. The resulting shear surfaces from this exercise are plotted in purple on the attached revised Figures 6-18 and 6-32. As can be seen in the figures, all resulting trial optimized slip surfaces are above a factor of safety of 1.50 both in front of and behind the county road. The only exception to this is the fully saturated case for cross section GH-6. However, CNI considers this case to be conservative and unlikely under actual field conditions. Experience at CCV has shown that water from precipitation is transient and is transmitted downward below the proposed mining levels to the extensive network of historic underground workings.