Seismic deformation of composite breakwaters and embankments on saturated sandy ground with reference to torsional cyclic behavior of sand after phase transformation

1995年神户大地震中，神户港某复合型防波堤因地面强烈震动，出现了超过2.5m的沉降量。本研究通过对30g和50g离心条件下考虑粘性结垢的防波堤-路堤-地基饱和砂土体系进行了一系列振动台试验，以阐明这种大沉降量的机理。结合各向异性固结松散砂土的不排水循环扭转剪切试验结果，讨论了地基的变形特征。这些试验的结果总结如下：(1)在离心机模型试验中，复合防波堤主体下方的地基在地面震动过程中可能会经历较大的轴向变形(垂直压缩与水平延伸相关联)，这是由于不排水累积剪切变形机制，一旦有效应力…该机制就开始运行。更多的达到相变状态。中层和/或下层(深层)的垂直压应变大于浅层。(2)在循环扭剪试验中，应力路径到达转换线后，垂直和扭转剪切应变都有明显的发展。垂直应变累积，但应变增量随着循环剪切次数的增加而减小。初始固结应力比K=σ‘&lt；Z0&gt；/σ’&lt；I0*&gt；(σ‘_Z0&gt；σ’&lt；R0&gt；分别为初始垂直有效应力和水平有效应力)越大，相变时的有效围压越大，砂样的累积竖向应变越小。通过对防波堤-地基复合体系自重应力的线弹性有限元分析，发现地基浅层在自重作用下产生较大的剪应力。在离心机试验中观察到的垂直压缩剖面可以用单元特性来解释，上述结果表明了相变后土的特性的重要性，这可能在目前的分析方案中被较少地考虑。例如，在这种分析中，本构参数通常可以根据由预相变过程控制的循环剪应力比-剪切重复关系的次数来确定。较少

In the 1995 Hyogoken-Nambu earthquake, some composite breakwaters in Kobe port experienced over 2.5m settlement due to strong ground shaking. In this study, a series of shaking table tests were conducted on breakwater- and embankment-foundation saturated sandy ground system is centrifugal conditions of 30-g and 50-g with viscous scaling incorporated, in order to clarify the mechanism of such large settlements. The deformation characteristics of the foundation ground were then discussed referring to the results of undrained cyclic torsional shear tests on anisotropically consolidated loose sand. Results obtained from these experiments are summarized as follows:(1) In centrifuge model tests, foundation ground just beneath the main part of a composite breakwater may undergo large axial deformation (vertical compression associated with horizontal extension) during ground shaking, due to the undrained cumulative shear deformation mechanism, which becomes operational once effective stresses … More reach phase transformation states. The vertical compressive strains in the middle and/or lower (deep) layers were larger than those in the shallow layers.(2) Vertical and torsional shear strains developed significantly after the stress paths reached the transformation lines in cyclic torsional shear tests. The vertical strains cumulated, however the strain increments decreased with the number of cyclic shearing. Larger the initial consolidation stress ratio K=σ'_<z0>/σ'<I0*> (σ'_z0> and σ'_<r0> are respectively the initial vertical and horizontal effective stresses), the effective confining stress at phase transformation is larger, leading smaller cumulative vertical strain of sand specimen. According to linear elastic finite element analysis on stresses induced by the self-weight of composite breakwater-foundation ground system, large shear stresses were introduced prior to shaking at the shallow part of the foundation ground. The profiles of vertical compression observed in centrifuge experiments can be explained with the elemental performance.The above-mentioned results indicate the significance of soil behavior after phase transformation, which may have been incorporated to less extent in current analytical schemes. In such analyses, for example, the constitutive parameters may often be determined based on the cyclic shear stress ratio - number of shear repetition relationship governed by pre-phase transformation process. Less