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BRAY, RATHJE, AUGELLO AND MERRY D Seismic Design for Lined Solid-Waste Landfills 212 GEOSYNTHETICS INTERNATIONAL S 1998, VOL. 5, NOS. 1-2 This isan important issue, because two ofthe few MSWLF units that were lined with a HDPE geomembrane as required by Subtitle D were damaged as a result of strong shaking during the 1994 Northridge earthquake (Augello et al. 1995). Tears in the geo- membrane liners at Canyons C and D of the Chiquita Canyon landfill both occurred at the top ofthe slope near the anchor trench, where the largest static, pre-seismic stresses in the liner would be expected due to side slope downdrag as the waste fill settled over time. Moreover, both 1.5 mm (60 mil) thick, smooth HDPE geomembrane liners were directly overlain with protective soil and waste, without the inclusion of a protective geotextile orsliplayer, andthe tear at Canyon Cinitiated atthe location ofanextrusion welded patch along a longitudinal seam where a sample was removed for destructive testing (EMCON Associates 1994). Fromthisexperience, theincorporation offlexibility andductility inthedesignofthe containment system isjudgedtobecrucial, eventhoughaquantitative assessment ofits merit is difficult with the available analytical tools. “Defects” and “inflexible anchor trenches” which may induce stress concentrations during static and seismic loadings shouldbeavoided,andasliplayershouldbeplacedabovethebase/side slopelinertoac- commodateslipinthesystemwithoutunderminingtheintegrityofthecritical baseliner. The 1988 static failure of the Kettleman Hills Waste Landfill Unit B-19 (Mitchell et al. 1990) emphasized the importance of evaluating the interface strengths of the many different typesofgeosynthetics incorporated inwaste containment designs. Aswiththe response of geomembranes, most studies have investigated the static response ofcom- mon interfaces (e.g. Negussey et al. 1989; O’Rourke et al. 1990; Byrne et al. 1992; Or- man 1994; Gilbert et al. 1995). Pseudo-static stability analyses of geosynthetic-lined landfills during the Northridge earthquake resulted in back-calculated interface strengths for smooth and textured HDPE geomembranes that are consistent with those from laboratory test results for similar interfaces reported in the literature (Augello et al. 1995). Shaking table tests have provided important insights regarding the dynamic response of common geosynthetic interfaces. Yegian and Lahlaf (1992) found that the dynamic friction angles measured during instrumented shaking table tests are close to those measured in static friction tests. Hence, conventional, static interface friction tests are believed to provide reasonable estimates of the dynamic interface strengths. Theclay-smooth HDPEgeomembrane interface hasbeenfoundtobe oneofthemost variable, whichemphasizes theneedforhigh-quality,project-specific interface testing. The clay-geomembrane interface strength is dependent on the compaction water con- tent, compacted density,normal stress, soiltype, andpost-compaction changesinwater content (Seed and Boulanger 1991; Stark and Poeppel 1994). Residual clay-geomem- braneinterface strengthsaslowas4_havebeenmeasured.Peakstrengthsarehigher,but peak strengths are often mobilized at shear displacements of only 2 to 4 mm, with the shear stress falling to close to residual strength at displacements of only 20 mm (Stark and Poeppel 1994). However, the clay-geomembrane interface is not necessarily the weakest interface. Stark and Poeppel (1994)showthat underlow normal stresses (upto approximately 150 to 300 kPa), geosynthetic-geosynthetic interfaces, such as geotex- tile-geomembrane, may be weaker. Forhigher normal stresses, the clay-geomembrane interface strength is likely lower. This suggests that in the case of a base liner system wherenormalstressesmaybehigh,theuseofageotextile cushionasasacrificial slipsur- face doesnotnecessarily prevent stressesfrombecoming induced inthegeomembrane.