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Mr. Edgar Hunter 1 3 November 19, 1992 <br />XIX. Stability of Critical Slopes Under Pseudo Static Loading <br />In order to determine a factor of safety for critical slopes associated with this <br />project, a value for anticipated peak ground motions during the design earthquake <br />must be developed. The Vector Engineering report submitted with this amendment <br />application utilizes a design earthquake of magnitude 5.0 on the Oil Creek Structure <br />with a return period of 500 years. Given the available date, specifically the 1985 <br />study by Butler and Hicholl of the 1979 Divide earthquake, this design event <br />appears to be appropriate. The Vector Engineering report develops a value of .095g <br />far peak ground acceleration at the project area resulting from the design event, <br />and utilizes a value of .tg to perform slope stability analysis on critical slopes under <br />pseudo static loading. <br />In determining the peak ground motions at a given site for a given return period, the <br />attenuation of ground motion with distance from the origin of the earthquake is of <br />great imporcance. A portion of the Oil Creek Fault lies within 8 km of the project <br />area. Vector Engineering has utilized attenuation relationships developed by <br />Campbell (19811 to calculate the value for peak ground acceleration at the site at <br />.095g. The Division used attenuation relationships developed by Donovan (1973) <br />and by Esteva (1974) to calculate peak ground acceleration values of .1358 and <br />.133g respectively. These values would result in more conservative estimates of <br />the factor of safety for the critical slopes. <br />Attenuation relationships are based on the magnitude of the design event, <br />determined in this case to be 5.0, and either the epicentral distance of the focal <br />distance from the project area, in this case 8 km. A possible epicentral distance of <br />8 km places the project within the epicentral area. Attenuation equations are <br />generally inappropriate for the epicentral area, i.e., within a distance of about 5-20 <br />km from the epicenter. This area requires special consideration, for example, there <br />are great problems in relating peak ground acceleration to maximum earthquake <br />intensity within the epicentral area. Based on the foregoing discussion, it is <br />apparent that the operator must re-address the value computed for peak ground <br />acceleration that is used in the stability analysis, and submit documentation to <br />back-up the value determined to 6e appropriate. <br />2. Minimum Acceptable Factor of Safetv <br />Slope stability analysis included in the Vector Engineering report are based on obtaining a 1.3 <br />factor of safety under static loading. Appendix E of the Division's Guidelines for Cyanide <br />Leaching Operations" states that "the overall static factor of safety cannot be less than 1.5." <br />The operator should provide further design specifications which would provide for a minimum <br />factor of safety of 1.5 under static conditions. <br />3. Leach_Pad Foundation <br />Section 6.3.2 of the Vector Engineering report indicates that the geometry of the leach pad <br />results in favorable pad slopes (level, or sloping into the heap) at all locations. The operator <br />should submit details, which should include maps and/or cross-sections, to depict the geometry <br />of the prepared leach pad foundation. <br />