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，使液化现象的数值模拟成为可能；其次从细观力学的角度揭示液化机理；最后从实验上验证数值模拟结果的真实性。1)编制了DEM程序，考虑了颗粒与周围水的相互作用。利用离散单元法(DEM)对饱和散体材料液化过程中的微观变形机理进行了数值模拟试验。在循环荷载作用下，相应的主应力方向上会产生柱状结构。随着循环加载的进行，微观组织变得更加各向异性。不仅有效应力的降低，而且微观组织本身的各向异性也加速了微观组织的各向异性。这种各向异性结构在主应力旋转时变得不稳定并最终坍塌。3)进行了不排水循环三轴试验，以阐明微结构对液化的影响。以下事实特别有意义。最初随机分布的空隙串联连接在柱状结构之间。在不排水循环荷载作用下，超孔压显著增加，特别是当连接的孔洞受到垂直于其伸长方向的应力时。这就是为什么曾经液化过的砂土在随后的再液化试验中急剧丧失抗液化能力的原因。

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.