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元法（DEM）的程序，使液化现象的数值模拟成为可能；二是从微观力学角度分析液化机理；最后通过实验验证了数值模拟结果的真实性。我们得到了以下结果：1)对DEM进行了编程，可以考虑颗粒与周围水的相互作用。它可以成功地应用于液化相关的向上渗流。2)采用离散元法（DEM）进行了数值模拟试验，重点研究了饱和颗粒材料液化过程中的微观变形机理。在循环荷载作用下，柱状结构沿主应力方向形成。随着循环加载的增加，微观结构的各向异性增强。有效应力的减小和组织本身的各向异性都加速了组织的各向异性。这种各向异性结构在主应力旋转时变得不稳定并最终崩溃。3)进行不排水循环三轴试验，明确微观结构对液化的影响。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起来。在不排水循环加载下，超孔隙水压力显著增加，特别是当连接的孔洞受到垂直于其延伸方向的应力时。这就是液化后的砂土在随后的再液化试验中液化阻力急剧下降的原因。

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.