2008-04-15_REVISION - M1980244 (377)

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D WX SMITH WILLIAMS CONSULTANTS, INC. Project: Phase 5 VLF Job No. 1125 Calculation Title: Arequa Phase 5 VLF Stability Calculations Prepared B : Justin Hail Date: 3119/08 Checked B : John Lupo Date: 3119/08 OBJECTIVE: To assess the global stability of the Valley Leach Fill (VLF) within Arequa Gulch under static and earthquake loading conditions based on the available site information and assumptions stated herein. Nine cross sections, A through I, were evaluated for phases I-IV and phase 5. The plan map showing the location of the stability sections is presented on Drawings A700, A710, and A720. METHOD: For all failure mechanisms considered in the analyses, slope stability was evaluated using limit equilibrium methods based on Spencer's method of analysis (Spencer, 1967). Spencer's method is a method of slices (consideration of potential failure masses as rigid bodies divided into adjacent regions or "slices," separated by vertical boundary planes). It is based on the principle of limiting equilibrium, i.e., the method calculates the shear strengths that would be required to just maintain equilibrium along the selected failure plane, and then determines a "safety factor" by dividing the available shear strength by the required shear strength. Consequently, safety factors calculated by Spencer's, or by any other limiting equilibrium method, indicate the percentage by which the available shear strength exceeds, or falls short of, that required to maintain equilibrium. Therefore, safety factors in excess of 1.0 indicate stability and those less than 1.0 indicate instability, while the greater the mathematical difference between a safety factor and 1.0, the larger the "margin of safety" (for safety factors in excess of 1.0), or the more extreme the likelihood of failure (for safety factors less than 1.0). The minimum required safety factors in this assessment are 1.3 and 1.1 for static and pseudostatic conditions, respectively. The stability analyses were conducted using SLIDE 5.0, a commercially available computer program (Rocscience, 2007), and the input parameters presented in the following section. ASSUMPTIONS/INPUT PARAMETERS: • Groundwater was not encountered in geotechnical borings or test pits within the project areas and was not included in the stability models. • Earthquake (seismic) loading conditions were simulated using a pseudo-static approach. Pseudo-static-based analyses are commonly used to apply equivalent seismic loading on earthfill structures. In an actual seismic event, the peak acceleration would be sustained for only a fraction of a second. Actual seismic time histories are characterized by multiple-frequency attenuating motions. The accelerations produced by seismic events rapidly reverse motion and generally tend to build to a