Examined are effect of an isotropic stress condition of sand on liquefaction and post-liquefaction behaviors. Both loose sand and dense sand are tested by means of cyclic torsional shear test in which both axial strain and radial stress are kept constant in order to reproduce an in-situ stress condition. The K_0-value at the time of consolidation before the cyclic loading (initial K_0-value), a ratio of lateral normal stress to the overburden stress, ranges from 0.67 to 2.5, while keeping the initial effective mean principal stress constant. Both liquefaction strength and post-liquefaction volumetric change caused by excess porewater pressure generation are examined. Liquefaction strength is the smallest when K_0 = 1.0, i.e., an isotropic stress states and increases as K_0-value increases or decreases. The increase is not so large for loose sand, but it increases up to 15% for dense sand. On the other hand, effect of K_0-value on the post-liquefaction volume change for dense sand is almost negligible. In case of loose sand, however, post-liquefaction volume change tends to increase with K_0-value.
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