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1128 GEOENVIRONMENT 2000 (1994) have demonstrated that use of average values for unit weight and initial shear wave velocity in seismic response analyses can lead to discrepancies of over 100 percent for the peak acceleration at the top of the landfill and 40 percent for the peak average acceleration of the landfill compared to analyses in which these properties vary with depth. Typically, no attempt is made to differentiate between dynamic and static shear strength for MSW. However, available data (Siegel et al., 1990) and the authors observations indicate there may be some difference between the shear strength that is mobilized for these different loading conditions. The "dynamic" properties of shear modulus and equivalent viscous damping are generally provided as a function of a representative cyclic shear strain. The representative cyclic shear strain is generally set equal to some fraction of the maximum shear strain. Therefore, the effective strain factor used to relate the maximum cyclic shear strain from the analysis to the representative cyclic shear strain is also a required parameter for equivalentwlinear site response analyses. For a Newmark permanent seismic deformation analysis, no additional material properties are needed. The required input parameters are not material properties, but are derived from the results of the seismic response and limit equilibrium analyses. MSW UNIT WEIGHT Values of MSW unit weight immediately after placement in a landfill are typically reported by landfill operators based upon daily estimates of waste toiUlage, cover soil quantities, and waste lift thicknesses. Average inMplace unit weights for MSW landfills derived from waste and cover soil quantities and landfill surface elevations over the life of MSW Iandftlls cited by landfill owners and operators for the purposes of evaluating landfill capacity are often used for engineering analyses of stability and seismic response. However, few studies have attempted to evaluate the distribution of unit weight with depth within the landfill or changes in unit weight with overburden pressure and/or time. Kavazanjian and MatasoviC (1994) have demonstrated that the assumed distribution of unit weight within the landfill can have a significant influence on the results of seismic response analyses. Average in-place unit weights used by owners and operators for landfill capacity estimates are typically in the range of 8.6 to 10.2 kN/m3 • Values in this range have also been used for seismic analysis by Singh and Murphy (1990), Sharma and Goyal (1991), and Repetto et al. (1993). Fassett et al. (1994) provide a summary of reported values of in-place unit weight for MSW from a variety of sources. Unit weight values summarized by these investigators vary from 2.9 kN/m3 to 14.4 kN/m3• with a value of 15.6 k:N/m 3 reported for one special case. While few details are provided as to the conditions under which these values apply, the lower values may be assumed to correspond to uncompacted or poorly compacted waste immediately after placement and the higher values may be assumed to correspond to older waste under relatively high overburden pressures. Initial in-place unit weights of MSW reported by Fassett et al. vary from 2.9 tO 7.5 kN/m3 , or 500 to 1300 pounds per cubic yard in the units typically used in practice. The higher end of this range of initial in-place unit weights may be MSW PROPERTIES 1129 assumed to correspond to modern landfills using good standards of practice for waste compaction and daily cover application. In fact, some southern California landfills have permit conditions requiring minimum initial waste unit weights of over 7.0 kN/m3 (1200 pounds per cubic yard). Primary factors influencing the initial in-place unit weight of MSW include waste composition, volume of daily cover soil, and compactive effort employed during placement of the waste. The initial in~place unit weight will increase with compression immediately following application of overburden pressure due to waste placement. The in-place unit weight may also increase with the additional compression that occurs over time. Earth Technology (1988), in reporting on a field and laboratory studies performed at the Puente Hills landfill near Los Angeles, developed an interpreted profile of unit weight versus depth for that landfill. This interpreted profile was developed from measurements of unit weight on drive samples recovered for laboratory testing and down-hole geophysical gamma-gamma logging. The resulting interpreted unit weight profile, shown in Figure 1, varied from 3.3 kN/m3 at the surface to 12.8 kN/m3 at depths greater than 60 m. 0 2 ~ s ~ :0:: 50 E-< A.. "" Q ····· ............. .. .. . ............. -... ·- -This Study ---Earth. Technology: (1988) --Extreme Values (Fassett et al., 1994) ::-:.: Range of Typical Average Values c-...cw,cw..: Derived from Fassett et al. ( UNIT WEIGHT (kN/m3 ) FIG. 1 Unit Weight Profile for MSW l\ Ctyp Maximum and minimum MSW unit weights and derived values of maximum, typical, and minimum MSW compressibility, from Fassett et al. (1994), and the range of typical unit weights used for capacity estimates are plotted in Figure 1 for comparison with the profile developed by Earth Technology for the Puente Hills landfill. The Puente Hills profile agrees well with the range of unit weight and waste compressibility values reported by Fassett et al. and with the typical average values used for capacity estimates. However, the initial in-place unit weight used at the surface in the interpreted Puente Hills profile is too low for modem landfills