Studies of deformation of composite breakwaters under earthquake shaking

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This research focused on the deformation mechanism of composite breakwaters resting on sandy deposits under conditions of earthquake shaking. Specifically, two interrelated approaches were made. First, a comprehensive set of centrifugal earthquake shaking tests were performed on composite breakwaters that rested on deposits of sand. The relative density of the foundation soil was varied over a wide range, while the intensity of earthquake shaking was held constant. All of the centrifuge tests were carried out under a steady-state centrifugal acceleration of 30 gravities. Viscous scaling was introduced so as to match the time scaling laws of vibration and consolidation. In fact, each centrifuge model was subjected to 20 cycles of sinusoidal excitation, with a peak acceleration of 3 gravities. The experimental results indicated that the caisson structure resting on a loosely packed sand layr underwent an S/D ratio (ratio of settlement S to sand thickness D) as high as 30%. With increased relative density of the supporting sand layr, the S/D ratio of the caisson decreased almost linearly with increasing relative density, realizing an S/D ratio of 3% when the supporting sand layr densified to a relative density of 81%. It is noteworthy that the seismic instability of the foundation soil was a form of repeated bearing-capacity failure, although the soil in the free field or well away from either toe of the rubble mound underwent liquefaction.The second approach was concerned with the development of a finite-element code that permitted a systematic dynamic analysis of the nonlinear seismic behavior of saturated granular soil under earthquake shaking. The theoretical formulations, the related finite-element formulations and the developed source program are described in detail in the final report of this research.