Experimental study on liquefaction mechanism by means of micro-mechanical method

我们的研究目标是首先开发一个基于离散单元法（DEM）的程序，这使得它可以数值模拟液化现象;其次，从微观力学的角度来弄清楚液化机制;最后，实验验证数值模拟结果的真实性。主要结果如下：1）对离散元法进行了编程，使其能够考虑颗粒与周围水体的相互作用。该方法可以成功地应用于与液化相关的向上渗流问题。2）采用离散单元法（DEM）对饱和散粒体液化过程中的微观变形机理进行了数值模拟试验。在循环加载过程中，在最大主应力方向上产生柱状结构。随着循环载荷的增加，微结构变得更加各向异性。不仅有效应力的降低，而且组织本身的各向异性也加速了组织的各向异性。通过不排水循环三轴试验，研究了微结构对液化的影响。以下事实特别令人感兴趣。起初随机分布的空隙在柱状结构之间串联。在不排水循环荷载作用下，特别是当连通孔隙垂直于其伸长方向受力时，超静孔隙水压力显著增大。这就是为什么一旦液化的砂土在随后的再液化试验中会急剧失去抗液化性的原因。

Our research objectives are first to develop a program based on Distinct Element Method (DEM), which makes it possible to numerically simulate the liquefaction phenomenon ; second to figure out liquefaction mechanism from micro-mechanical point of view ; and finally, to verify experimentally the reality of numerical simulation results. We obtained the following results :1) DEM is programmed such that the interaction between particles and surrounding water can be taken into account. It can successfully be applied to liquefaction associated upward seepage flow.2) Numerical simulation tests were carried out using the distinct element method (DEM) by paying much attention to the micro-deformation mechanism during liquefaction of saturated granular materials. During the cyclic loading, column-like structures are generated in the corresponding direction of major principal stress. The microstructure becomes more anisotropic with cyclic loading. Not only decrease of effective stress but also anisotropy of the microstructure itself accelerate the anisotropy of microstructure. Such anisotropic structure becomes unstable and finally collapses when the major principal stress rotates.3) Undrained cyclic triaxial tests are carried out to clarify the effect of microstructure on liquefaction. Following facts are of particular interest. Voids, randomly distributed at first, are connected in series between the column-like structures. The excess pore water pressure increases markedly under undrained cyclic loading, in particular, when the connected voids are stressed perpendicular to their elongation direction. This is the reason why once liquefied sand loses sharply the liquefaction resistance in a subsequent re-liquefaction test.