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• the Magnitude 8'synthetic accelerogram generated by Seed and Idriss [1969] to simulate distant large-magnitude event on the San Andreas fault, scaled to 0.10 g to represent both the MCE and MPE. 3e2 Analytical Method The slope stability analyses were performed using the computer program XSTABL. The program calculates slope stability using a limit equilibrium analysis based on the method of slices.The method of slices estimates slope stability by assuming a shear surface and calculating the forces that would cause slope movement, and the forces resisting slope movement for the selected shear surface. The ratio of available shear strength (resisting forces) to mobilized shear strength (driving forces) is known as the factor of safety. The computer programs employ a searching routine to determine the critical shear surface with the minimum factor of safety. A factor of safety equal to 1.0 under static loading conditions represents a condition of imminent failure. For temporary slopes, a minimum factor of safety of 1.3 under static loading conditions is generally considered adequate. Permanent slopes are typically designed to achieve a minimum static factor of safety of 1.5. During a seismic event, the propagation of bedrock motions induces a sequence of cyclic shear stresses on the soil and refuse. These cyclic shear stresses result in cyclic strains. When the stresses are above yield, a certain amount of strain remains, which produces permanent seismic deformations in the soil or refuse. To estimate these seismic deformations, a procedure based on the concept proposed by Newmark(1965)for calculating seismic permanent deformations and refined by Makdisi and Seed(1978)was used. The method assumes that failure occurs on a well-defined slip surface and that the material behaves elastically at stress levels below failure, but develops a plastic behavior above yield. When the maximum, average acceleration in the potential sliding mass (kn.) exceeds the calculated yield acceleration for each sliding surface (ky), movements are assumed to occur along the direction of the failure plane. The ky is the seismic coefficient that results in a factor of safety of 1.0. The overall deformation is obtained by summing the strains over the failure surface. The strains are estimated based on a time-step finite-element analysis using the equivalent linear method(Seed et al., 1973). The theory of one-dimensional wave propagation through layered media can be used to model the response of the landfill mass to the rock motions associated with the design earthquake at the site. The SHAKE computer program (Schnabel et al., 1972; Idriss and Sun, 1992)was used to predict the response of the landfill mass to the input base rock motions. SHAKE models the one-dimensional response of individual vertical columns of material(s) that are representative of actual field conditions. The vertical column can consist of different layers of materials or one material. Accordingly,the modeling for a landfill consists of columns with layers of waste and soil. The shear-wave velocities are used to compute the maximum dynamic shear modulus of each layer of the vertical column. The strain-dependent modulus and damping relationships are entered into the SHAKE program for each layer in the modeled column. Because the modulus and damping values are strain-dependent and are not known at the outset, an iterative procedure is required to estimate these values for each layer. The strain in each layer of the vertical column is estimated, and the strain-dependent values of modulus and damping are calculated for that layer. Based on the computed strains in each iteration, new values of modulus and damping are obtained. The iterative analysis is continued until the values of modulus and damping are compatible with the strain developed in each layer. After the strain-compatible shear modulus and damping for each layer are determined, horizontal displacements at each layer are calculated using the closed-form wave equation. The SHAKE computer program uses these displacements to calculate the accelerations at the top of each layer. The ground accelerations at the top of the refuse-soil columns were then WMU FU-10 REPORT 3-3