Static and Dynamic Instability of Liquefiable Soils

0201317，罗纳尔多·博尔哈，斯坦福大学“可液化土壤的静态和动态不稳定性”这个项目的目标是开发一个有限元模型来分析土壤的动态不稳定性和流动液化，包括地震期间和地震后伴随的侧向流动和大的地面运动。该模型基于两相混合物理论，具有以下基本组成部分：(A)复制液化前超孔压积累的本构模型；(B)流动液化不稳定的开始判据；以及(C)流动液化后残馀状态的本构响应。边界面塑性模型用于模拟流动液化开始前土体的各向异性循环应力应变行为，并预测随之而来的孔压累积。通过使用基于变形梯度乘法分解的有限变形理论来使用大变形。该项目的主要部分致力于将单调不排水三轴试验和循环不排水单剪试验的结果作为边值问题来分析。解释这些试验结果的一个普遍假设是，试件是均匀变形的，这意味着任何测量到的试件响应都可以解释为材料响应。然而，有强烈的迹象表明，这远远不是正确的。通过执行边值问题模拟解决的问题之一是，在中等密度的饱和砂样中通常观察到的准稳态条件是否是非局部的。此外，还研究了缺陷和其他扰动对作为结构系统的土样响应的影响。最后，设计了一种系统的方法来重新标定有限变形区域的本构模型参数，并将有限元模型用于1971年圣费尔南多地震后的下圣费尔南多大坝坍塌的重新分析。

Abstract0201317, Ronaldo Borja, Stanford University"Static and Dynamic Instability of Liquefiable Soils"The objective of this project is to develop a finite element model for analyzing the dynamic instability and flow liquefaction of soils, including the accompanying lateral flow and large ground movement during and following an earthquake. The model is based on a two-phase mixture theory with the following essential components: (a) a constitutive model that replicates the buildup of excess pore pressure prior to liquefaction; (b) a criterion for the onset of flow liquefaction instability; and (c) a constitutive response at residual state following flow liquefaction. A bounding surface plasticity model is used to model the anisotropic cyclic stress-strain behavior of soils before the onset of flow liquefaction, as well as to predict the accompanying pore pressure buildup. Large deformations are employed through the use of a finite deformation theory based on a multiplicative decomposition of the deformation gradient.A major component of the project is devoted to analyzing the results of monotonic undrained triaxial tests and cyclic undrained simple shear tests as a boundary-value problem. A universal assumption employed in interpreting the results of these tests is that the specimen is deforming homogeneously, suggesting that any measured specimen response may be interpreted as a material response. However, there are strong indications that this is far from being correct. Among the issues addressed by performing boundary-value problem simulations is whether or not the quasi-steady state condition commonly observed in saturated sand specimens of intermediate density is non-local. Also being investigated are the effects of imperfections and other perturbations on the response of soil specimens taken as a structural system. Finally, a systematic methodology is being devised for re-calibrating the constitutive model parameters in the finite deformation regime, and the finite element model is used to reanalyze the collapse of the Lower San Fernando Dam following the San Fernando earthquake of 1971.