The instability due to rapid generation of pore water pressure in loose to medium dense soil is one of the major causes of catastrophic failure of geotechnical structures. Most of the previous studies on instability/liquefaction behavior have focused on clean sands, although sands with a significant amount of fines (particle size ＜ 0.075 mm) arc common in field conditions. Some recent studies on artificially mixed sands with silts have shown that these soils are much more liquefiable than clean sand. Thus, this study investigated the instability behaviour of naturally occurring silty sand. The effect of in-place void ratios and the initial mean effective stresses on the mechanical behaviour of the silty sand was investigated. It was observed that for the same in-place void ratio, the liquefaction resistance increased with initial mean effective stress, which is consistent with the so called "reversed behaviour". The instability stress ratio and relative density showed a single correlation, which can be used to predict the triggering of instability. Further, the flow behaviour was able to be described with respect to phase transformation points, at which the effective stress path changes soil behaviour from compressive to dilative displacement.
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