الفهرس 
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Abstract
The effect of preshaking on the liquefaction behavior of sandy soils is systematically studied in this dissertation. The research incorporated data from both the field, and centrifuge and full-scale experiments. State of the art tools and sensors were used in the tests in order to capture as precisely as possible the response of the soil and connect it to the field. The work included three centrifuge tests performed at RPI (Experiment 1-3) and a full scale test performed at University at Buffalo, UB (Experiment 4). The tests simulated the effect of several decades to several centuries of earthquake events on a 5-6 m uniform clean or silty sand horizontal deposit, including both events that liquefied the deposit and others that did not liquefy it. The experimental program involved testing different shaking sequences, in order to explore the complex relation in the field between preshaking seismic events that increase the liquefaction resistance of young deposits, and liquefying events that may decrease it totally or partially. Different shaking sequences were applied in the four experiments. However, all deposits were subjected to three or four basic event types: Events A, B and C (or D). An Event A was defined as 5 sinusoidal cycles of a peak base acceleration, apb ≈ 0.035-0.045 g; an Event B was defined as 15 sinusoidal cycles of a peak base acceleration, apb ≈ 0.04-0.05 g; and an Event C (or D) was defined as 15 sinusoidal cycles of a peak base acceleration, apb ≈ 0.1-0.25 g, all in prototype units. The prototype frequency in all cases was 2 Hz. The 15-cycle duration of Events B, C and D corresponds approximately to an earthquake of moment magnitude, Mw ≈ 7.5; while the 5-cycle duration of the Events A correspond to Mw ≈ 6. An Event A represents a mild to moderate earthquake shaking; an Event B represents a mildly strong earthquake shaking; and an Event C (or D) represents a strong to very strong extensive liquefaction shaking in the field. The results of the experiments showed that the combination of mild/moderate (Events A) to mildly strong (Events B) shakings resulted in a significant increase in liquefaction resistance of the deposits over time. However, the occurrence of extensive liquefaction, caused by an Event C or D, resulted in a dramatic immediate reduction in liquefaction resistance to a level comparable to, or even lower than, that of the young recent deposit before preshaking had started. This suggests a complex phenomenon, with most of the shaking events strengthening the liquefiable layer but some of them weakening it, sometimes dramatically when extensive liquefaction was involved.