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

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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.