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C� San Andreas Fault Event Great Valley Fault Event 5.3.2.2 Gross Stability Analyses Magnitude (Mw) Peak Horizontal Ground Acceleration Duration of Significant Shaking (D5-95) Magnitude (Mw) Peak Horizontal Bedrock Acceleration Duration of Significant Shaking (D5-95) owl 7.9 0.06g 45.4 sec 6.7 0.158 15.3 sec The gross stability of the landfill and liner system was assessed using the maximum slope height and slope gradient for the proposed landfill development (Figures 1 and 2 of Appendix K). Because the actual interface strengths of landfill liner materials will vary with the manufacturers of particular products used, a parametric analysis was done to develop a range of acceptable strength parameters (i.e. combination of cohesion and angle of friction) that will provide a minimum static factor of safety of 1.5. The range of parameters used in the analysis are included in Appendix K. As determined in CH2MHill's geotechnical evaluation of the Forward Landfill (1992), the native soils in the area consist of interbedded layers of sand, silt, and lean clay. The cohesive soils can be characterized as very stiff to hard, and non -cohesive sands are dense to very dense. The stability conditions of the native subgrade beneath anticipated refuse loads are considered non -problematic. Assuming a refuse height of approximately 200 feet above the subgrade, CH2MHill calculated that the factor of safety for failure of a lean clay layer that is 10 feet below the liner grade would be 2.2 to 2.3. 5.3.2.3 Gross Stability Results As shown in Appendix K, the results of the stability analyses indicate that a static factor -of - safety of 1.5 or more can be achieved for the planned landfill geometry using a range of liner interface shear strength values. The pseudo -static analyses that were completed for the project to evaluate earthquake loading conditions indicate yield accelerations from about 0.10g to 0.11g for the range of interface shear strength parameters (see Figure 3 in Appendix K). The yield acceleration is the seismic coefficient (ky) that yields a static factor of safety of 1.0. As shown in the calculations included in Appendix K, using these yield accelerations and the Bray & Rathje method (1998), an insignificant amount of seismic -induced permanent displacement of the landfill liner system is calculated for the two design earthquake events (i.e., Great Valley and San Andreas fault MCE's). 5.3.3 Final Cover Slope Stability As discussed in Section 5.2.4.2, the proposed alternative final cover at the landfill will 0 consist of a four -foot thick monolithic alternative final cover section composed of on-site Forward Landfill SWT Engineering Joint Technical Document - April 2014 z:\projects\allied waste\forward\five year permit rvw 2013\jtd-5 yr pr 2013\text\sec 5_final.doc