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SHANNON F~WILSON, INC. <br />• <br />6.4 Stability Analysis Results <br />The four sections shown on Figures 2 and 8 were analyzed using Spencer's Method for limit <br />equilibrium analysis using GeoStudio's Slope/W software (Referencel9). A circular type failure <br />and a block type failure refined with optimization procedures (as provided in Slope/VV) were <br />used in each of these trial runs. The optimization procedure involves randomly altering the <br />shape of the critical block failure surface until the process converges on the failure surface with <br />the minimum factor of safety (FS). <br />Using the soil/rock parameters discussed above, a baseline case was analyzed for each of the four <br />sections. The circular analysis resulted in near surface failure arcs, approaching the infinite slope <br />condition. The calculated FS at the 3.8H:1 V slope is 2.2 for this condition. We forced deeper <br />block type fai]ures down to the soil layer in the base of the valley, such that more global stability <br />conditions were assessed. The FS for this condition were similaz or slightly higher. <br />The 3H:1 V slope in Section A-C is also controlled by infinite slope conditions. The calculated <br />factor of safety for this case is 1.7. <br />The baseline cases (Table C-1) indicated that when lazger block type failures are considered, <br />The results our parametric study indicate that the static FS against slope instability is greater than <br />1.5 as required by CDMG Regulations. The parametric study indicated that the stability of the <br />slope is sensitive to the strength parameters of the spoil and overburden soil (cohesion and <br />friction angle), with the friction angle of both the spoil and the overburden soil having the most <br />influence on global stability. Changes in unit weight/density of the spoil and overburden soil had <br />negligible effect on the calculated FS. <br />Section F-G in the East Valley Fill was the most critical section with a calculated Factor of <br />Safety (FS) of 2.2. Our parametric study was performed on this section. The results of the <br />parametric study are summarized on Figure 9. Appendix C contains a detailed summary of <br />information regazding the analyses, including output plots for each Slope/W run. <br />Our parametric analysis also indicated that a rise in groundwater elevation of up to 40 feet into <br />the spoil did not significantly affect the FS. Modeling groundwater at 40 feet above the <br />spoil soil interface resulted in only a slight reduction in FS. It is our opinion that a rise in <br />groundwater greater than 40 feet is unlikely, provided the drains aze properly constructed and <br />maintained. <br />• <br />23-I-01105-200-R].doc 23-1-01105-200 <br />