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1132 GEOENVIRONMENT 2000 from the f~ilure of a New Jersey landfill where the slip surface passed through a very soft tidal marsh deposit. Failure of the marsh foundation soil occurred at relat~~ely small_ displacemen~, resulting in only a portion of the waste strength being mobilized at failure. Thus, If a factor of safety of 1.0 is assumed, back-analyses of this failure will underestimate the waste strength. In addition, the results the back- analysis for this case is highly dependent on the assumed shear strength of the f~~dation s_oil. Uncertainty associated with the foundation shear strength sigmficantly unpacts the degree of confidence with which the waste strengths back calculated from this study can be relied upon. Therefore, data from the failure of the landfill founded on a soft tidal marsh deposit was not considered in the MSW shear strength assessment. . The lower-bound parameters from Howland and Landva are also strongly influenc~ by the landfill load test performed at the Operating Industries, Inc. (Oil) landfill m Monterey Park, California. This is perhaps the most widely cited source of field data on MSW shear strength. The load test at 011 was terminated due to excessive deformation. As the landfill slope did not actually fail during the load test: ~e. actual MS,W strength is larger than the strength back-calculated using limit eqmhbnum analysts and an assumed factor of safety of LO (i.e., incipient failure). The shear strengths from Tables 1 and 2 are plotted versus normal stress in Figure 2. The data plotted on Figure 2, combined with the observation from landfill operations that excavated in trenches in waste will remain stable with vertical faces in excess of 6 m in height, appear to support a bi-linear representation of MSW shear strength. Based upon this observation and the data plotted on Figure 2, a Mohr-Coulomb strength envelope consisting of¢ = 0 with c = 24 kPa at normal stresses below 30 k:Pa and ¢ = 33 ° with c = 0 at higher normal stresses was developed for use in stability analyses of MSW landfills. 250~-------------------------- gJ 150 "" ~ [/] 100 p:: .., "" :I1 50 [/] + Richardson &: Reynolds ( 1991) 0 Lopez Canyon ¢ Operating Industries (Oil) 6 Tol!"ll of Bab.:ylon l!'l ~ Private Facility in Ohio ..._ Pagotto & Rimoldi (1987) l!'l Landva &: Clark (1990) 0 0 33" 50 100 150 200 250 300 NORMAL STRESS (kPa) FIG. 2 Shear Strength of MSW 350 MSW PROPERTIES 1133 It should be noted that the two data points that fall below the bi-linear strength envelope on Figure 2, from the Lopez Canyon landfill, are back calculated using a static factor of safety of 1.2 for a slope recently subjected to an estimated peak ground acceleration of 0.44 gin the magnitude 6.7 Northridge earthquake of 17 January 1994 without any apparent slope instability. No attempt was made in the strength assessment to distinguish between the static and dynamic shear strength of MSW. A number of other MSW landfills besides the Lopez Canyon landfill were subjected to strong ground motions from the Northridge earthquake. Several landfills were in the epicentral region where peak ground accelerations were estimated to be in excess of 0.4 g. Slopes at these landfills were typically on the order of 3H:1V. There were no reported waste slope stability problems at these landfills. Back analysis of the perfonnance of these landfills may demonstrate that the dynamic shear strength of MSW is even higher than the MSW shear strength envelope plotted in Figure 2. Such back analysis is beyond the scope of this paper. SHEAR WAVE VEWCITY OF MSW A number of investigators have reported in situ measurements of MSW shear wave velocity from geophysical surveys, including seismic refraction, down-hole, cross-hole, and surface wave velocity surveys. Field measurements of ambient vibrations and of ground motions from small earthquakes also provide a basis for evaluation of MSW shear wave velocity. Cross-hole shear wave velocity surveys have been reported at the Puente Hills landfill in Los Angeles (Earth Technology, 1988) and the Brookhaven landfill on Long Island, New York (Carey et al., 1993). Shear wave velocities varying from 240 m/s at a depth of 6 m to 270 mls at a depth of 14m were reported at Puente Hills for waste buried below 6 m of soil fill. At the Brookhaven landfill, the measured shear wave velocity is reported to have ranged from 185 to 478 mls, though the depth at which these measurements were made is not reported. The maximum depth of the landfill was reported along with a plot of Young's modulus versus depth derived from the shear wave velocity . Based upon the maximum waste depth and the maximum depth for which modulus data were reported, the maximum depth at which the shear wave velocity was measured is inferred to be between 24 and 37 m. The higher shear wave velocity is assumed to correspond to the deeper depths. Kavazanjian et al. (1994) report on shear wave velocity profiles derived from Rayleigh wave (surface wave) measurements made at eight MSW landfills in southern California using both Spectral Analysis of Surface Waves and Controlled Source Surface Wave profiling teclmiques. Shear wave velocities measured in these surveys varied from as low as 80 rnls near the surface to over 300 rnls at a depth of 30m. Sharma et al. (1990) reported on down-hole shear wave velocity measurements at a MSW landfill in Richmond, California. These investigators reported an average shear wave velocity of 198 m/s over the top 15 m of the landfill. In a down-hole survey performed as part of the Puente Hills investigation by Earth Technology (1988), an average velocity of 287 mls was reported for MSW