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• SMITH WILLIAMS CONSULTANTS, INC. <br />3.0 Stability Analyses <br />3.1 Methodology <br />For all the potential failure modes considered in this study, slope stability was evaluated <br />according to the Spencer's Method of Analysis (Spencer's Method). Spencer's Method <br />considers potential failure masses as rigid bodies divided into adjacent regions or "slices" <br />separated by vertical boundary planes and is based on limit equilibrium, i.e., the method <br />calculates the shear strengths that would be required to just maintain equilibrium, and then <br />calculates a Factor of Safety (FOS) by dividing the available shear strength by the required shear <br />strength. Consequently, the FOS calculated by Spencer's Method indicates the percentage by <br />which the available shear strength exceeds, or falls short of,-that required to maintain <br />equilibrium. Therefore, an FOS equal to or in excess of 1.0 indicates stability and those less than <br />1.0 indicate instability. The greater the mathematical difference between the FOS and 1.0, the <br />larger the "margin of safety" (for an FOS in excess of 1.0), or the more extreme the likelihood of <br />failure (for an FOS less than 1.0). <br />• The stability analyses were conducted using SLIDE V5.0 (RocScience, 2007), a commercially <br />available computer program, with the input parameters presented in this section. For both the <br />wedge and the circular failure modes, the SLIDE critical surface search routine was initially used <br />to determine the least stable failure surface. The program automatically iterates through a <br />variety of potential failure surfaces, calculates the safety factor for static and pseudo-static <br />conditions for each surface according to Spencer's Method, and selects the surface with the <br />minimum FOS, commonly referred to as the critical surface: Static analyses were conducted <br />with no applied horizontal forces, while pseudo-static analyses modeled design seismic <br />conditions by incorporating a constant horizontal force. For the pseudo-static analyses, a <br />conservative design coefficient of 0.08g (which is equal to the currently approved PGA for the <br />Cresson Project) was used in the slope stability models, which is consistent with that used for <br />Amendment Nos. 6, 7, and 8 (CC&V 1993, 1998, 2000). <br />3.1.1 Input Parameters <br />3.1.1.1 Conceptual Model <br />A representative conceptual model, incorporating the areal distribution and engineering <br />properties of the foundation materials that underlie the SGOSAE and ECOSA, was developed <br />from the results of field investigations. The critical stability cross sections analyzed are shown <br />• on Drawings A100 and A110. It is important to note that the OSA configurations shown have <br />S:\PROJECTS\1125C CC&V CRESSON OSA\H2 -DESIGN\OSA STABILITY FINAL REPORT V3.DOC <br />