Influence of silt size and content on static liquefaction potential of sand Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/hx11xj067

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  • Different specimen preparation methods such as moist tamping, dry funnel deposition, slurry deposition, dry air pluviation have been reported in the literature to investigate the undrained behavior of silty sands. Similarly, different means have been used to densify the soils prepared with such methods. However, the influence of the densification technique, utilized within a particular deposition method, on undrained behavior (e.g. change in initial peak deviator stress and instability angle) was not known. Therefore, a new densification technique is developed for the dry funnel deposition method, which avoids tamping, vibrating or mold tapping. This new method of densification is thought to create a much consistent soil fabric for different amounts of densification than other specimen densification techniques. The experimental results show that the change in undrained behavior with increasing density produced by densification is much less pronounced when compared to the other densification methods reported in the literature. Prior research efforts regarding the effect of non plastic silts on the liquefaction behavior of sands mainly focused on the influence of fines content, confining stress, and depositional techniques. However, there is no consensus in the literature regarding the influence of fines content on the undrained behavior of silty sands. Strain-controlled monotonic undrained triaxial compression tests were performed on a single base sand mixed with three different essentially nonplastic silts. First, silt size effects are investigated while other factors like fines content (20%), confining stress (30kPa) and deposition method (dry funnel deposition) were kept the same. The results show that silt size is indeed an important factor which influences the liquefaction potential of silty sands. Different comparison bases for undrained behavior such as the loosest possible density after deposition, intergranular void ratio, void ratio and relative density were also evaluated. It was observed that as the mean grain diameter ratio (D₅₀/d₅₀) of the sand grains (D₅₀) to silt grains (d₅₀) decreases, liquefaction potential for a silty sand increases. This tendency is attributed to more metastable contacts with increasing silt size. Finally, the influence of fines content on the static liquefaction potential of silty sand is investigated for different silt types. It was found that if the mean grain diameter ratio (D[subscript 50-sand]/d[subscript 50-silt]) is sufficiently small, the liquefaction potential of the sand increases steadily with increasing fines content for the studied range (0-20%). As mean grain diameter ratio (D[subscript 50-sand]/d[subscript 50-silt]) increases, the liquefaction potential of the sand first decreases then increases with fines content. For such cases, liquefaction potential of the silty sand might be less than the liquefaction potential of the clean sand. Differences in undrained behavior are explained based on the influence of mean grain diameter ratio (D[subscript 50-sand]/d[subscript 50-silt]) on the initial soil fabric.
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