2016-12-19_REVISION - M1980244 (2)

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Technical Memorandum Newmont CC&V Gold Mine East Cresson Overburden Storage Area Stability Review/Update NewFields Job No. 475.0106.005 9 December 2016 Page 3  Exterior, eastern aspect slopes are be considered “critical” as slope failures could impact off‐site areas, and as discussed in Section 3.2 the strength characterization of the materials within ECOSA is considered robust. Based on these attributes, the minimum acceptable factors of safety are 1.3 and 1.15 for static and pseudostatic conditions, respectively.  Interior, western aspect slopes are considered “non‐critical” and the associated minimum acceptable factors of safety for these slopes are 1.25 and 1.1 for static and pseudostatic conditions, respectively. Design Ground Motions 3.1 To assess the stability of slopes during seismic loadings, a pseudostatic approach was utilized in which the potential sliding mass is subjected to an additional, destabilizing horizontal force that represents the effects of earthquake motions and is related to the peak ground acceleration (PGA). The seismic force is the weight of the sliding mass multiplied by a horizontal pseudo‐ static earthquake coefficient (kh). The kh is typically considered as a portion of the PGA because during an actual earthquake the acceleration within the potential sliding mass is cyclic and varies over the duration of the earthquake and the direction of the acceleration changes throughout the seismic event. Therefore, an average horizontal coefficient is assigned that is typically less than the PGA experienced at the base of the structure. Hynes‐Griffin and Franklin (1984) discussed that use of one‐half of the PGA for the horizontal pseudostatic earthquake coefficient will result in slope deformations that will be within tolerable limits. This reduction in the pseudostatic seismic coefficient is typically coupled to a 20 percent strength reduction for materials that are saturated or can soften during shaking. The PGA used in the current evaluation was based on a review of publically available information regarding the seismic hazard at the ECOSA site using the United States Geologic Survey (USGS) interactive deaggregations tool. The updated PGA was listed as 0.035g and 0.11g for the 475‐year and 2,475‐year‐return seismic events, respectively. NewFields considers these appropriate risk level for operational and closure conditions and the current understanding of the seismic region. The output graphic from the seismic hazard analysis identifying the PGA, mean earthquake magnitude, and mean rupture distance is presented in Attachment B. The current evaluation applied a one‐third reduction to the PGA for kh which we consider reasonable and conservative based on the recommendations of Hynes‐Griffin and Franklin (1984). The evaluation also considers a 20 percent reduction in the colluvium strength during seismic events to account for any strength loss from the development of excess pore pressure.