ratios between the two clean-sand base curves, plotted in Figs.4 and 2, respectively, range from 5 to 8—values that areslightly higher than those expected for clean sands. Shiftingthe CPT base curve to the right by 10 to 15% would increasethose ratios to unusually high values ranging from 6 to 9.Second, base curves, such as those plotted in Figs. 2 and 4,were intended to be conservative, but not necessarily to encompassevery data point on the plot. Thus the presence of afew points beyond the base curve should be allowable. Finally,several studies have confirmed that the CPT criteria in Fig. 4are generally conservative. Robertson and Wride (1998) verifiedthese criteria against SPT and other data from sites theyinvestigated. Gilstrap and Youd (1998) compared calculatedliquefaction resistances against field performance at 19 sitesand concluded that the CPT criteria correctly predicted theoccurrence or nonoccurrence of liquefaction with >85% reliability.
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