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1130 GEOENV!RONMENT 2000 using good waste placement practice. Furthermore, the constant unit weight used at depths in the interpreted Puente Hills profile is not consistent with the assumption that waste will continue to compress with additional confining pressure, even at very large confining pressures. Therefore, based upon reported initial in~place waste densities for modern landfills and the waste compressibility values reported by Fassett et al. (1994), the interpreted Puente Hills profile was modified to develop the authors' profile shown on Figure 1. In the absence of site specific information ,the authors have used this profile in engineering analysis of the seismic response of modern MSW landfills. MSW SHEAR STRENGTH MSW shear strength values reported in the technical literature vary widely, with friction angles as low as 10 degrees and as high as 53 degrees and cohesion values varying from 0 to 67 kPa. Many of the lower values are directly contradicted by observations of stable landfill slopes in the field. Therefore, a critical re- evaluation of the available shear strength data on MSW was performed by the authors. The authors used selected data from this re-evaluation, supplemented with shear strength back-calculated from observations of stable waste slopes in the field, to develop the assessment of MSW shear strength presented herein. In performing the assessment of MSW shear strength, the authors recognized that factors such as waste compressibility and strain compatibility should be considered when developing strength parameters for limit equilibrium analysis. The authors assessment of MSW shear strength was based primarily on shear strengths back-calculated from case histories and the results of in-situ testing. With the exception of one set of data from large scale tests, laboratory test data on MSW shear strength was not used. The excluded laboratory data was not considered reliable due to either the use of processed waste in the testing program or the small size of the MSW samples relative to the inhomogeneity of MSW landfills. Field and laboratory test data considered reliable and used in the re-evaluation of MSW shear strength are summarized in Table 1. TABLE 1 DATA USED IN TilE MSW SHEAR STRENGTH REEVALUATION - REFERENCE DATA TYPE RESULTS COMMENTS Pagono and Back calculation ¢ = 22°, c = 29kPa No data on waste types or test Rimoldi from plate procedures are provided. (1987) bearing tests. Landva and Clark Laboratory direct ¢ = 240, c = 22 kPa Normal stresses up to 480 kPa. (1990) sh= tests on to Lower strength not used in Fig. MSW. ¢ = 390, c = 19 kPa 2, corresponds to shredded waste. Richardson and Large direct tP = 18° to 43° Normal stresses range from 14 to Reynolds (1991) shear tests and 38 kPa. Unit weight of waste performed in situ c = 10 kPa and cover estimated as 15 kN/m3 • MSW PROPERTIES 1131 Pagotto and Rimoldi (1987) reported the results of plate-bearing tests performed at an "urban waste" landfill in Italy. No details are given on waste properties, test methods, or test interpretation. Considering the limitations of plate- bearing tests, these results must correspond to very low normal stresses in the waste. Landva and Clark (1990) presented the results of direct shear tests on MSW materials conducted in a 434 mm by 287 nun direct shear box at normal stresses up to 480 kPa. Results reported for fresh shredded refuse containing a large amount of plastic were not used in the re-evaluation as they are not considered representative of MSW in situ. Richardson and Reynolds (1991) reported the results of tests performed on MSW using a 1.5 m by 1.5 m direct shear box. The box was loaded with concrete blocks to normal stresses between 14 and 38 kPa. The data in Table 1 was supplemented by the authors with data from back- analyses of existing MSW landfill slopes known to be stable. Table 2 presents back calculated MSW shear strengths from four existing landfills (GeoSyntec Consultants, 1993). TABLE 2 BACK ANALYSIS OF EXISTING LANDFILL SLOPES - AVERAGE MAXIMUM SLOPE WASTE STRENGTH, ¢ SLOPE LANDF1LL Height Slope Height Slope FS = FS= FS= (m) (H'V) (m) (II'V) 1.0 1.1 1.2 Lopez Canyon, CA 120 2.5:1 35 1.7:1 25° 27° 29° OII,CA 75 2:1 20 1.6'1 28° 30° w Babylon, NY 30 1.9: l 10 1.25: l 3QO 34° 38° Private Landfill, OH 40 2:1 10 1.2' 1 30° w 37° Note: FS = Factor of safety for back analysis assuming c = 5 kPa. The back-calculated friction angles for MSW presented in Table 2 were obtained assuming a cohesion of 5 kPa using the modified Bishop method of slices. As the slopes at these landfills have been standing for up to 15 years without excessive deformation or other signs of impending instability, the factors of safety against slope failure within the waste are certainly larger than 1.0 and probably greater than or equal to 1.3. To be conservative, the authors used results for a factor of safety of 1. 2 in the MSW shear strength assessment. The authors assessment of the available shear strength data was performed in a manner similar to Howland and Landva (1992). These investigators presented available data from the technical literature on MSW strength on a plot of shear stress at failure versus normal stress. From the plot, they derived strength parameters that represented the "lower bound" for the entire body of data. These lower-bound strength parameters are strongly influenced by the back-calculated waste strengths