Study on Time-Dependent Behavior of Saturated Soils due to Loss of Structure based on Elasto-Plastic Mechanics

处于高结构、超固结和各向异性状态的天然沉积粘土，由于采样扰动和“二次固结/延迟压缩”、“等痛”等随时间变化的行为，比重塑粘土表现出更显著的不排水抗剪强度降低。本文基于弹塑性力学和土-水耦合分析，结合试验工作，对土体的这种特性进行了研究。主要结论如下：1)室内自重固结重塑的胶体粘土具有较高的含水率，与解体/完全重塑粘土相比，具有较高的结构性，即体积较大。即使在一维应力作用下，随着塑性变形的进行，粘土也接近变形状态。2)在数值模拟中，考虑了土体结构性、超固结和各向异性三个概念及其演化规律，提出了天然粘土的弹塑性本构模型。此外，在分析中，用Inv-Lnp‘代替了v-Lnp’来描述自重固结过程，即在天然沉积土中发生的大量压缩行为，而v和p‘分别表示比体积和平均有效应力。3)从数值角度研究固结仪试验中观察到的天然土的二次固结。这种现象不仅与天然粘土本身的高度结构性有关，而且与“取样作业”伴随的超固结度的增加/产生有关。此外，在取样方向与各向异性轴方向不同的情况下，更容易发生次固结。

Natural deposit clay in highly structured, overconsolidated and anisotropy states is said to exhibit decrease of undrained shear strength due to sampling disturbance and time-dependent behavior such as "secondary consolidation/delayed compression" and "isotache" more remarkably than its remolded clay. In the present study, such behavior of the soil is investigated based on elasto-plastic mechanics and soil-water coupled analysis with experimental works. The main concluding remarks are as follows : 1) Colloidal clay reconstituted through self-weight consolidation in laboratory, which has significantly high water content, is highly structured, I.e. bulky compared with destructured/fully remolded clay. The clay approaches to destructured state as plastic deformations proceeds even in one-dimensional stress application. 2) In the numerical works, an elasto-plastic constitutive model for natural clays is newly proposed considering three concepts of soil structure, overconsolidation and anisotropy and their evolution rules. Furthermore, partially in the analysis, the v-lnp ' relationship is replaced by Inv-lnp ' to describe the self-weight consolidation process I.e. the large amount of compression behavior which occurs more remarkable in natural deposit soil, while v and p 'show specific volume and mean effective stress respectively. 3) Secondary consolidation of natural soil observed in oedometer tests if investigated from the numerical point of view. The behavior is attributed to not only high degree of structure of natural clay in itself but also increase/generation of overconsolidation accompanied with "sampling operation". Furthermore, in case that direction of sampling is different from that of axes of anisotropy, the secondary consolidation happens more easily.