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BRAY, RATHJE, AUGELLO AND MERRY D Seismic Design for Lined Solid-Waste Landfills <br />216 GEOSYNTHETICS INTERNATIONAL S 1998, VOL. 5, NOS. 1-2 <br />forthe stability evaluation ofthe entire cover slope. Asshownin Figure7b, theequiva- <br />lent acceleration acting on a potential cover sliding mass decreases as the normalized <br />lengthoftheslide massincreases, duetoincoherence oftheseismic responseofthelong <br />cover slope. <br />One-dimensional nonlinear dynamic analyses were performed to identify factors af- <br />fecting theseismic loading forcoversystems. Atlow inputrock accelerations (MHARock <br /><0.2g),theD-MODresultsareconsistent withaccelerations recorded attheOIIlandfill <br />andthescatter issmall; however,athigheraccelerations, thescatter increases andcover <br />acceleration estimates are less reliable (Bray and Rathje 1998). This scatter is a result <br />ofdifferentlandfillheightsandstiffnesses,anddifferentinputgroundmotions. Thenor- <br />malization usedfor MHEABase wasapplied tothe MHATop results(Figure8),and thisnor- <br />malization allows for a better estimate of the seismic loading at the top of a landfill. <br />2.7 Seismic Stability Evaluation <br />2.7.1 Seismically Induced Permanent Deformations <br />The seismic stability of a MSWLF unit can be best evaluated in terms ofseismically <br />induced deformations. Deformations should be expressed in terms of deviatoric and <br />volumetric components. The overall response of a cover system will be a function of <br />seismically inducedfillsettlement duetocontractive volumetric strainsandseismically <br />induced fill bulging/lurching toward a slope-face due to deviatoric strains. <br />Simplified techniques, such asthe Tokimatsu and Seed (1987)method forclean sand <br />deposits, however, are not available for evaluating seismically induced settlements of <br />MSWLFs.Hence, thisanalysis isnottypically performed,although manyofthesurface <br />cracks observed in landfill coversfollowing the1994Northridge earthquake havebeen <br />attributed tothismechanism (Augello etal. 1995).Simplified techniques, such asMak- <br />Figure 8. Normalized maximum horizontal acceleration at the top versus the <br />normalized fundamental period of the waste fill (from Bray and Rathje 1998). <br />0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 <br />0.0 <br />0.2 <br />0.4 <br />0.6 <br />0.8 <br />1.0 <br />1.2 <br />1.4 <br />1.6 <br />1.8 <br />2.0 <br />2.2 <br />MHATop/[(MHARock)(NRF)]Ts-WASTE /Tm-EQ <br />Rock site median <br />16th and 84 <br />th probability of exceedance lines