Micromechanics Study on Liquefaction-Induced Lateral Flow and Soil-Pile Interaction

地震液化引起的横向扩展使地面产生较大的位移和剪切应变，从而使桩基受到阻力的破坏。模拟这个问题有两个困难；对液化土的本构模型进行评价，并选择合适的模拟工具来模拟这种大变形问题。本研究的目的是从微观力学的角度寻找解决这些困难的方法。首先，我们开发了一些实验技术，使用高速相机和PIV（粒子图像测速）。PIV是一种从序列数字图像中检测各种实验速度场的强大工具。我们进行了两种类型的实验：一种是Toyoura砂的振动台试验，观察小尺寸地面模型的表面；另一种是渗透性试验，通过调整试样底部的流体注入速度来保持任意液化状态。在这两个实验中，都安装了一根圆柱形管道，以测量由于试件与管道之间的相对位移而作用在管道上的阻力。研究发现，液化土具有固体特性（通过土与桩之间的粘滑运动来检测）和粘性流体特性（以速率相关响应为特征）。采用离散元法（DEM）进行进一步研究。DEM是一种基于牛顿第二运动定律计算各个固体颗粒的模拟方法，颗粒之间的相互作用简单地用弹簧-阻尼器-滑块系统来模拟。一系列的等体积简单剪切模拟为我们提供了观察液化前后基本微观机制的方法。特别地，还成功地模拟了液化土的再凝固过程，这是预测液化引起的侧向流动位移的重要因素。基于以上信息，我们提出了一种基于细观力学的颗粒组合本构模型，该模型可以很好地应用于组合体的剪切破坏。最后，我们研究了SPH (Smoothed Particle Hydrodynamics)，这是一种基于颗粒的连续介质模拟方法，可以应用于固体和流体。结果表明，SPH可以很好地模拟桩-液化土的相互作用，包括再固化过程。少

Lateral spreading induced by seismic liquefaction causes the ground to experience large displacement and shear strains, which in turn damage pile foundations due to the drag force. There are two difficulties in simulating this problem ; the evaluation of constitutive model for liquefied soil and the choice of adequate simulation tool suitable for such large deformation problem. The objective of this study is to find a way for these difficulties from a micromechanical standpoint.Firstly, we developed some experimental techniques using high-speed camera and PIV (Particle Image Velocimetry). PIV is a powerful tool to detect a velocity field of various experiments from sequential digital images. We performed two types of experiments : one is a shaking table test of Toyoura sand to observe a surface of small-sized ground model, and the other is a kind of permeability test to keep arbitrary liquefaction state by tuning a fluid injection speed from the bottom of the specimen. In both experime … More nts, a cylindrical pipe is installed to measure the drag force acting on it due to the relative displacement between the specimen and the pipe. It was found that the liquefied soil has both properties as a solid (detected by a stick-slip motion between the soil and the pile) and as a viscous fluid (characterized by the rate dependent response).Further investigation was made by DEM (Discrete Element Method). DEM is a simulation method to compute each solid grains based on Newton's second law of motion, and the interaction between grains is simply modeled by a spring-dashpot-slider system. A series of constant-volume simple shear simulations provided us to observe essential micro-mechanisms up to and after liquefaction. In particular, the re-solidification of liquefied soil was also successfully simulated, which is quite important factor to predict the lateral flow displacement due to liquefaction.Based on the above information, we proposed a constitutive model of granular assembly based on micromechanics, which can be well applied up to shear failure of the assembly.Finally, we studied SPH (Smoothed Particle Hydrodynamics), a particle-based simulation method for continuum, which can be applied both in solid and fluid. It was found that the pile-liquefied soil interaction including the re-solidification regime was successfully simulated by SPH. Less