Studies of deformation of composite breakwaters under earthquake shaking

研究了地震作用下砂质复合防波堤的变形机理。具体地说，提出了两种相互关联的方法。首先，对基于砂土沉积物的复合防波堤进行了一套全面的离心地震振动试验。在地震强度保持不变的情况下，地基土的相对密度在较大范围内变化。所有离心机试验均在30重力的稳态离心加速度下进行。为了匹配振动和固结的时间标度规律，引入了粘性标度。实际上，每个离心机模型都受到了20个周期的正弦激励，加速度峰值为3个重力。试验结果表明，松散堆积砂层上沉箱结构的S/D（沉降量S与砂厚D之比）高达30%。随着支撑砂层相对密度的增加，沉箱的S/D比几乎随相对密度的增加而线性降低，当支撑砂层相对密度达到81%时，沉箱的S/D比为3%。值得注意的是，地基土的地震失稳是一种重复的承载力破坏形式，尽管自由场或远离碎石堆趾的土壤发生了液化。第二种方法是开发一种有限元程序，允许对地震震动下饱和颗粒土的非线性地震行为进行系统的动力分析。在本研究的最终报告中详细描述了理论公式、相关的有限元公式和开发的源程序。

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.