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c~ <br />,~ Lf~ <br />SMITH WILLIAMS CONSULTANTS, INC. <br />Project: OSA St <br />bilit <br />A <br />a <br />y <br />nalysis Job No. 1125C <br />Calculation Title: Squaw Gulch and East Cresson Stability Calculations <br />Pre ared B :Justin Hall <br />Checked B :John Lu o Date: 3/13/08 <br />Date: 3/13/08 <br />OBJECTIVE: <br />To assess the global stability of the overburden storage areas within Squaw Gulch and East Cresson <br />under static and earthquake loading conditions based on the available site information and <br />assumptions stated herein. <br />The plan map showing the location of the stability sections is presented in Figure 1. <br />METHOD: <br />For all failure mechanisms considered in the analyses, slope stability was evaluated using limit <br />equilibrium methods based on Spencer's method of analysis (Spencer's method) (Spencer, 1967). <br />Spencer's method is a method of slices (consideration of potential failure masses as rigid bodies <br />divided into adjacent regions or "slices," separated by vertical boundary planes). It is based on the <br />principle of limiting equilibrium, i.e., the method calculates the shear strengths that would be <br />required to just maintain equilibrium along the selected failure plane, and then determines a "safety <br />factor" by dividing the available shear strength by the required shear strength. Consequently, safety <br />factors calculated by Spencer's, or by any other limiting equilibrium method, indicate the <br />percentage by which the available shear strength exceeds, or falls short of, that required to maintain <br />equilibrium. Therefore, safety factors in excess of 1.0 indicate stability and those less than 1.0 <br />indicate instability, while the greater the mathematical difference between a safety factor and 1.0, <br />the larger the "margin of safety" (for safety factors in excess of 1.0), or the more extreme the <br />likelihood of failure (for safety factors less than 1.0). The minimum required safety factor in this <br />assessment is 1.3. <br />The stability analyses were conducted using SLIDE 5.0, a commercially available computer <br />program (Rocscience, 2007), and the input parameters presented in the following section. <br />ASSUMPTIONS/INPUT PARAMETERS: <br />• Groundwater was not encountered in geotechnical borings or test pits within the project <br />areas and was not included in the stability models. <br />• Earthquake (seismic) loading conditions were simulated using a pseudo-static approach. <br />~~ Pseudo-static-based analyses are commonly used to apply equivalent seismic loading on earthfill <br />structures. In an actual seismic event, the peak acceleration would be sustained for only a fraction of <br />a second. Actual seismic time histories are characterized by multiple-frequency attenuating motions. <br />The accelerations produced by seismic events rapidly reverse motion and generally tend to build to a <br />peak acceleration that quickly decays to lesser accelerations. Consequently, the duration that a mass <br />is actually subjected to a unidirectional, peak seismic acceleration is finite, rather than infinite. The <br />