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BRAY, RATHJE, AUGELLO AND MERRY D Seismic Design for Lined Solid-Waste Landfills 224 GEOSYNTHETICS INTERNATIONAL S 1998, VOL. 5, NOS. 1-2 1. Estimate MHARock from the applicable groundmotion maps (e.g. Frankel et al. 1996 USGS maps for the continental USA) based on an appropriate probability level (generally 2% exceedance in 50 years for Subtitle D). 2. Use k =0.75(MHARock /g) as the seismic coefficient for base and cover sliding. 3. Calculate ky for the base and cover using conservative strengths (e.g. residual) with Equations 2 and 1, respectively. 4. If ky t k,thecalculated seismically induced displacements will generally belessthan 150and 300mm forbase and cover sliding, respectively. The engineer must decide if these commonly accepted displacement levels are tolerable. 3.3 Simplified Procedure Unstable geologic conditions, such as liquefaction, soft clay instability, native slope instability, and surface faulting should be assessed first. This simplified procedure is not appropriate for landfills with potentially unstable foundations. 3.3.1 Characterize Ground Motions Estimate MHA, Tm , and D5-95 for each design earthquake event . (a) Review the regional and site geology and seismicity, and identify potential earth- quake sources. (b) Assign the earthquake magnitude and distance to each earthquake source. Select representative earthquakes: likely, a high-intensity, short-duration, near-field event (higher MHA,lowerTm ,andlowerD5-95 ),andalower-intensity,long-duration, inter- mediate-field event (lower MHA , higher Tm , and higher D5-95 ). (c) Use rock relationships (Figure 2) to estimate MHARock ,Tm ,andD5-95 at the site. Check the design MHA values against the USGS maps for the 475 year and 2,375 year earthquake events (i.e. 10 and 2% exceedance in 50 years, respectively.) 3.3.2 Develop Seismic Loading Estimate MHEA of waste column for base sliding and MHA at top of landfill for cover stability assessment . (a) Estimate MHEABase of the waste column for base liner sliding using the plot of MHEABase /[(MHARock )(NRF )] versus Ts /Tm (Figure 6). Record the median and 16% probability of exceedance values. From Section 3.3.1,MHARock and Tm have been estimated. Estimate NRF from Figure 6. Estimate Ts =4H/Vs ,whereH =waste height, and Vs = weighted average initial shear wave velocity of waste. Estimates of Vs forsolid-waste are available in Figure 3. Figure 6may also beused toestimate MHEA atintermediate depthswithin thewaste fill if Ts iscalculated using theheight and the average initial shear wave velocity of the potential sliding surface. (b) Estimate MHATop using the plot of MHATop /[(MHARock )(NRF )] versus Ts /Tm (Figure 8). Use the median and 16% probability of exceedance values. For localized cover sliding near the crest and along the slope of the landfill, use MHEACover =1.25 2.Use k =0.75 (MHARock /g) as the seismic coefficient for base and cover sliding. 3.3 Simplified Procedure Unstable geologic conditions, such as 3.3.2 Develop Seismic Loading Estimate MHEA of waste column for base sliding