Mechanism of cyclic softening of non-plastic silt and its countermeasures

侧向流动是地震期间和地震后由于液化而水平发生的地面的大的横向移动。推断出了侧向流动。在地震灾害中对建筑物和生命线的基础造成最大的破坏。为了弄清粉土对液化的易感性，我们需要一种根据地基液化来预测变形的方法。本文提出了一种预测粉土液化变形的方法，并提出了减小液化后变形破坏的对策。研究结果如下：1)粉土的后循环三轴试验结果表明，随着超孔隙水压力的增加，粉土的刚度逐渐减小。然而，当超过超孔隙水压力的0.8时，该刚度迅速下降。如果保持…的超孔隙水压力，地震后的地面变形可以大大减少采用超孔隙水压力模型预测液化后变形模数的变化。并将其纳入了模型试验中预测粉土地基变形的有限元程序中。3)在1G模型试验中，由于地基坡度不同，粉土液化引起的变形有所不同：严重坡度时，侧向流动异常；坡度平缓时，竖向变形占主导地位。液化前后粉土与砂土的作用对比表明，粉土在液化过程中变形明显延缓，但液化后即刻砂土的位移量较大。此外，最终的排水量与沙子的排水量几乎相同。这些结果强调，在地震中，既要注意砂土的液化后变形，也要注意粉土的液化后变形。4)为了减少粉土地基的液化后变形，采用了轮胎排水板和常规的碎石排水板。模型试验结果表明，轮胎排屑法不仅能有效地减小侧向位移，而且能有效地减小竖向位移。这些效果与在相同条件下安装碎石排水沟的情况几乎相同。这表明，轮胎碎石排水法是防止液化引起的粉土变形的一种经济的对策。较少

Lateral flow is a large lateral movement of the ground that occurs horizontally as a result of liquefaction during and after earthquakes. Lateral flow is inferred. to cause most damage to foundations of structures and lifelines during earthquake-induced disasters. For clarifying vulnerabilities to liquefaction of silts, we require a method of forecasting the deformation according to ground liquefaction. This study proposes a method for predicting the liquefaction-induced deformation of silty soil and proposing the countermeasure to reduce damage due to post-liquefaction deformation.Results of the present study are summarized as follows :1)Results from post-cyclic triaxial tests on silt indicate that stiffness decreases gradually as excess pore water pressure rises. However, that rigidity decreases rapidly when a ratio of 0.8 of the excess pore water pressure is exceeded. Ground deformation after earthquakes can be decreased considerably if the excess pore water pressure is maintained b … More elow 0.8 using gravel drains as a countermeasure.2)The excess pore water pressure model was adopted to predict changes in the deformation modulus after liquefaction. It was incorporated into the FEM code for predicting silt ground deformation in model tests. Using the proposed FEM code, we obtained good agreement between observed and predicted deformation of sloping silt ground.3)In 1G model tests on sloping silt ground, deformation attributable to the liquefaction of silty soil differs depending on the ground slope : lateral flow is exceptional when the ground is severely sloping ; vertical deformation is dominant when the slope is gentle. Moreover, comparison of the actions of silt with that in sand before and after liquefaction showed remarkable delay of silt deformation in the transformation, but the amount of sand displacement is greater immediately after liquefaction. Moreover, the final amount of displacement is nearly identical to that of sand. These results emphasize that, during earthquakes, attention should be given to post-liquefaction deformation of silt as well as that of sand.4)The tire-chip drain was adopted for reducing post-liquefaction deformation of silt ground along with the conventionally used gravel drain. Results of model tests demonstrated that the tire-chip drain method is available for reducing not only lateral displacement but also vertical displacement. These effects are almost identical to those of the cases in which gravel drains are installed in the same condition. This suggests that the tire-chip drain method can be an economical countermeasure for preventing liquefaction-induced silt deformation. Less