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1138 GEOENVIRONMENT 2000 potential. In matching computed results to the observed spectra from the Landers event, where most of the motions were in the small strain range, the computed results were relatively insensitive to the choice of modulus reduction and damping curves and effective strain factor. However, significant differences developed in analyses of the landfill response in the Northridge earthquake when the effective strain factor was varied. In general, the best results for each particular set of damping and modulus reduction curves were obtained using an effective strain factor greater than 0. 7. Figure 7 presents a comparison of response spectra for the longitudinal motion at the crest of the landfill calculated using the four sets modulus reduction and damping curves and an effective strain factor of 0.8. The closest fit for the four sets of curves as a group was observed for an effective strain factor of 0.8. Of all the combinations of equivalent linear parameters evaluated, the MSW curves developed by Kavazanjian and MatasoviC combined with an effective strain factor of 0.8 gave the best agreement between the observed and predicted response of the OII landfill in the Northridge earthquake. ::§1.2 T-.,-,--------------, -RECORDED (Oil-Northridge; Lona:itud.l ..... _.,., MSW (Kava.o:unjiuo &: Malo~ovic', 11'19-l , ·---CLAY ~Pl=15) (Vucetic &: Dobry, 1991) , ---CLAY Seed &: ldrias, 1970bl - -PEAT Seed &: ldriss. 1970a. Damping FIG. 7 Observed at Predicted Response at 011, Northridge Earthquake The Kavazanjian and MatasoviC MSW modulus reduction and damping curves are plotted in Figure 6 along with the Seed and Idriss (1970b) clay curves, the Seed and Idriss (1970a) peat curves, and the data used by Seed and ldriss to develop their peat modulus curve. The Kavazanjian and MatasoviC MSW curves provide as close a fit to the peat modulus data as the curves proposed by Seed and Idriss. Seed and Idriss recommended use of an effective strain factor of 0.8 for equivalent-linear analysis of peat, the same value recommended from this study for the Kavazanjian and MatasoviC MSW curves. This comparison supports intuitive suggestions from earlier investigators that the dynamic behavior of MSW be represented using modulus reduction curves for peat and damping curves for clay. MSW PROPERTIES 1139 CONCLUSION MSW properties required for seismic analyses typically employed in engineering practice include unit weight, shear wave velocity, shear strength, and equivalent-linear modulus, damping, and effective strain factor. Field observations and in situ measurements provide valuable infonnation on these MSW properties. For unit weight and shear wave velocity, the distribution of these parameters with depth, and not just an average value, should be used in evaluation of seismic response. Figure 1 presents a profile of MSW unit weight versus depth developed by the authors based upon reports of initial in-place unit weight from landfill operators, the range of published values of unit weight and compressibility, and a field investigation at the Puente Hills landfill in southern California. The unit weight profile shown on Figure 1 is consistent with average values of MSW unit weight typically cited in practice for landfill capacity estimates. A bi-linear shear strength envelope for MSW developed by the authors for use in seismic stability and deformation analyses is presented on Figure 2. This strength envelope is based upon back-analysis of a load test at an MSW landfill, observations of steep but stable slopes at four landfills, and direct shear tests performed in the laboratory and in the field. Observations of landfill performance in the Northridge earthquake suggest that the dynamic shear strength of MSW may be even larger than the shear strength corresponding to the bi-linear envelope shown on Figure 2. Figure 3 presents a profile of MSW shear wave velocity versus depth developed by the authors. This profile is based on several different types of geophysical velocity surveys at MSW landfills, including down-hole, cross-hole, and surface wave surveys. The shear wave velocity profile shown in Figure 3 is consistent with average shear wave velocities for the Oil landfill derived from seismic refraction surveys, ambient vibrations, and ground motions from small magnitude earthquakes. Figure 6 presents modulus reduction and damping curves for MSW developed from back-analysis of strong ground motions recorded at the Oil landfill in the Northridge and Landers earthquakes. These curves were developed from non-linear time-domain response analyses of the Oil landfill. The recorded motions at on were compared with results of equivalent-linear analysis using these MSW curves and using the peat and clay soil curves recommended by previous investigators. The analyses using the Kavazanjian and MatasoviC curves combined with an effective strain factor of 0.8 gave the best fit with the observed response at on in the Northridge earthquake over a broad range of periods. The MSW properties described in this paper provide a consistent set of properties for conventional equivalent-linear seismic response and pseudo-static limit equilibrium stability analysis of solid waste landfills. These properties are calibrated based upon available field observations of the behavior of solid waste. In the authors' opinion, such equivalent-linear analyses are appropriate for ground motions of intensity less than or equal to 0.4 g. In the authors experience, when ground motion intensity exceeds 0.4 g non-linear cyclic stress-strain effects start to become important in site response analyses. In such cases, equivalent-linear site response