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Top1. Introduction
Liquefaction has become one of the most interesting, complex and controversial topic of research for geotechnical engineers, especially after the Alaska and Nigata earthquakes in 1964. Most of the earlier studies on liquefaction have focused on clean sands containing little or no fines. However, a number of case histories have revealed that silty sands are also prone to liquefaction (Kuribayashi & Tatsuoka, 1975; Seed et al., 1983; Chang, 1990; Yamamuro & Lade, 1998). Recent laboratory test results have demonstrated that silty sand/sandy silt is more liquefiable than sand (Vaid et al., 1990; Baziar & Dobry, 1995; Lade & Yamamuro, 1997; Zlatovic & Ishihara, 1997; Yamamuro & Lade, 1998; Amini & Qi, 2000; Xenaki & Athanasopoulos, 2003).
Despite these numerous reported results, the effect of non-plastic fines on liquefaction resistance is not very well understood as the problem lies in the fact that the reported results are conflicting and full of ambiguous. While several researchers (Dezfulian, 1982; Tokimatsu & Yoshimi, 1983; Seed et al., 1983; Kuerbis et al., 1988; Chang, 1990; Pitman et al., 1994; Amini & Qi, 2000) reported that increasing the fines content increases the liquefaction resistance, other researchers also reported that increasing the fines content decreases the liquefaction resistance (Shen et al., 1977; Troncoso & Verdugo, 1985; Troncoso, 1990; Lade & Yamamuro, 1997; Yamamuro & Lade, 1997; Zlatovic & Ishihara, 1997).
Effect of non-plastic silt on liquefaction resistance of sand is better understood in terms of sand skeleton/ intergranular void ratio rather than silt content (Shen et al., 1977; Troncoso & Verdugo, 1985; Kuerbis et al., 1988; Vaid, 1994; Polito & Martin, 2001; Xenaki & Athanasopolous, 2003). When clean sand is mixed with non-plastic silt, the maximum and minimum void ratios as well as the range of void ratios change and highly unstable and compressible particle structures may formed in loose deposits (Lade & Yamamuro, 1997; Yamamuro & Lade, 1997). Some of the specimens tested by Shen et al. (1977) had skeleton void ratios lower than the minimum void ratio of the specimen. Shen et al. (1977) also reported that, the trend of cyclic strength with respect to density changed markedly as the silt content exceeded 20%, an observation that is consistent with the postulated change in fabric at this value of fines content.