Date of Award
Master of Science
Severe damages to physical infrastructures as well as lifeline facilities have been observed during past earthquakes. Saturated sands as well as sands containing fines liquefied. The effect of fines on liquefaction resistance of sand is not fully understood till today. The basis of comparison and types of fines are reported as important factors affecting effects of fines on liquefaction. The fly ash is non-plastic and finer. Hence, study of liquefaction behavior of sand-fly ash mixtures may be helpful for the understanding of effects of fines on liquefaction resistance. The objective of this study were (1) to investigate effects of addition of fly ash on pore water pressure generation and deformation characteristics of sand, (2) to study effects of confining pressure on liquefaction resistance of sand-fly ash mixtures, and (3) to study Youngs modulus and damping ratio of sand-fly ash mixtures. Stress controlled cyclic triaxial tests were performed on clean sand, fly ash and sand-fly ash mixtures containing 10, 20 % of fly ash. For the evaluation of effects of confining pressure in liquefaction resistance, three series of tests were conducted at 5, 10 and 15 psi initial effective confining pressures. The reversible shear stress was applied systematically by varying CSR (Cyclic Stress Ratio) from 0.1 to 0.5. But, the tests were conducted only in 5 psi effective confining pressure in case of pure fly ash. The results obtained from the tests were used to compare the effects on liquefaction resistance in terms of pore water pressure build up, deformation behavior and effective confining pressure. Based on the results, it was observed that, the liquefaction resistance decreases with an increase in cyclic shear stresses at a given initial confining pressure. Liquefaction resistance also decreases with an increase in confining pressure for any CSR values. Further, the effects of fly ash content on liquefaction resistance was found to depend upon the confining pressure. For all effective confining pressure, liquefaction resistance decreased with an addition of 10 % fly ash. However, when the fly ash content was increased to 20 %, the liquefaction resistance increased. Moreover, the liquefaction resistance of sand containing 20 % fly ash was higher than clean sand at 5 psi effective confining pressure. On the other hand, it was lower than clean sand for the effective confining pressure 10 and 15 psi. The brittleness of the sample was found to increase with an addition of fly ash. Youngs modulus and damping ratio were also determined. The Youngs modulus was found to decrease with an increase in axial strain for clean sand, and for sand containing 10 and 20 % fly ash. It was also noted that, Youngs modulus increases with an increase in confining pressure. The damping ratio increases with an increase in axial strain. No distinct variation of damping ratio with confining pressure was observed.
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