2008-04-15_REVISION - M1980244 (256)

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5 -~ SMITH WILLIAMS CONSULTANTS, INC. • 5.0 Geotechnical Analysis 5.1 VLF Stability Analysis 5.1.1 Methodology For all the potential failure modes considered in this study, slope stability was evaluated according to the Spencer's Method of Analysis (Spencer's Method). Spencer's Method considers potential failure masses as rigid bodies divided into adjacent regions or "slices" separated by vertical boundary planes and is based on limit equilibrium, i.e., the method calculates the shear strengths that would be required to just maintain equilibrium, and then calculates an FOS by dividing the available shear strength by the required shear strength. Consequently, the FOS calculated by Spencer's Method indicates the percentage by which the available shear strength exceeds, or falls short of, that required to maintain equilibrium. Therefore, an FOS equal to or in excess of 1.0 indicates stability and those less than 1.0 indicate instability. The greater the mathematical difference between the FOS and 1.0, the larger the "margin of safety" (for an FOS in excess of 1.0), or the more extreme the likelihood of failure (for an FOS less than 1.0). • For the Phase 5 VLF, all stability analyses were conducted using SLIDE V5.0 (RocScience, • 2007), a commercially. available computer program, with the input parameters.presented in this section. For both the wedge and the circular failure modes, the SLIDE critical surface search routine was initially used to determine the least stable failure surface. The program automatically iterates through a variety of potential failure surfaces, calculates the safety factor for static-and pseudo-static conditions for each surface according to Spencer's Method, and selects the surface-with the minimum FOS, commonly referred to as the critical surface. Static analyses were conducted with no applied horizontal forces, while pseudo-static analyses modeled design seismic conditions by incorporating a constant horizontal force. For the pseudo-static analyses, a conservative design coefficient of 0.14g (which is equal to the currently approved PGA for the Cresson Project). was used in the slope stability models, which is consistent with -that used for Amendment Nos. 6, 7, and 8 (CC&V 1993, 1998, 2000). For the post closure configuration, Smith Williams used the PGA of 0.08g, which is also consistent with Amendment Nos. 6, 7, and 8 (CC&V 1993, 1998, 2000). S:\PROJECTS\1125 CC&V PHASE 5 VLF\H2 - DESIGNWREQUA VLF\VLF PHASE 5 FINAL REPORT V2.DOC 29