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

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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