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

在1995年的小古根-南部地震中，由于强烈的地面震动，神户港的一些复合防波堤沉降超过2.5米。本研究对防波堤和堤防基础饱和砂土地基系统在30 g和50 g离心条件下进行了一系列振动台试验，并加入粘性结垢，以阐明这种大沉降的机理。结合各向异性固结松散砂土不排水循环扭剪试验结果，探讨了地基的变形特征。结果表明：(1)在离心模型试验中，由于不排水的累积剪切变形机制，复合防波堤主体下方的地基在地震动过程中可能发生较大的轴向变形（垂直压缩与水平拉伸相结合），一旦有效应力达到相变状态，该机制就开始运作。中下层（深层）的垂直压缩应变大于浅层。(2)在循环扭剪试验中，应力路径到达转变线后，竖向和扭转剪切应变显著发展。随着循环剪切次数的增加，垂直应变逐渐累积，应变增量逐渐减小。初始固结应力比K=σ′_<z0>/σ′<I0*>越大（σ′_<z0>和σ′_<r0>分别为初始垂直和水平有效应力），相变有效围应力越大，导致砂体的累积垂直应变较小。通过对复合防波堤-地基系统自重引起的应力进行线弹性有限元分析，在地基浅部震动前引入了较大的剪应力。离心实验中观察到的垂直压缩曲线可以用元素性能来解释。上述结果说明了相变后土壤行为的重要性，但目前的分析方案中可能没有考虑到这一点。例如，在这类分析中，本构参数通常可以根据预相变过程控制的循环剪应力比-剪切重复次数关系来确定。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