| There are many research about physical and mechanical properties of shanghai 4th clay, however there are no scholars conduct a comprehensive study on the structure of shanghai 4th clay. The presence of structure will leads to undisturbed soil and remolded soil have completely different mechanical characteristics under different stress path. Therefore, study the mechanical characteristics and propose unified constitutive modeling method can provide a theoretical basis for the actual project.In this study, we use blocking and thin-walling methods to get the 4th clay at different place. Making conventional consolidation experiment, triaxial and true triaxial shear experiment by the undisturbed soil, and get the mechanical properties of shanghai 4th clay under different stress path. Based on the experimental results, TS-Zhang model was corrected to some extent, making it better suited to the mechanical characteristics of 4th soft clay. Propose unified constitutive modeling method. Observing the change of pore and soil particles under consolidation process by SEM experiment. The main results are as follows:(1) One-dimensional consolidation test results of shanghai 4th undisturbed soil indicates that soil compression index at different directions are different. Soil compression index at 0°direction is greater than 90°direction, this indicates that shanghai 4th undisturbed soil has an initial anisotropy. SEM images shows that shanghai 4th clay particles are flaky. The contract between two particles mainly are surface to surface contract at 0°direction, the contract between two particles are edge to edge contract at 90°direction, diffenent kind of contract lead to the initial anisotropy. From the compress index we can make a conclusion that edge to edge contact is less stable than surface to surface contact.(2) Triaxial shear text results indicate that shanghai 4th undisturbed soil will affected by the dissipating of the overconsolidation raito and structure during shearing process. Unified constitutive modeling results indicate that the dissipation rate of overconsolidation raito is greater than structure. When at low confining pressure shearing process, the soil will mainly affected by the dissipation of overconsolidation raito, then the soil will affected by the dissipation of structure. When at high confining pressure, the soil will only affected by the dissipation of structure. This explains why the effective stress path of soil under low confining pressure is different with high confining pressure. Due to the disspation of the structure, volume change of undisturbed shanghai 4th clay at shearing process is two times of the remolded soil.(3) In three-dimension stress state,the change of midum principle stress will affect the stress-strain curve of soil apparently.True triaxial shear experiment results are more familiar with SMP criterion when project the undisturbed soil data to πplane.(4) Experiment results shows that the plastic volume strain will also lead to the dissipation of structure. Modified the development fomulas of the structure and overconsolidation ration. Propose unified constitutive model methods. The method get structure and overconsolidation development parameters by simulating the consolidation process, while get structure and overconsoildation ratio parameters by simulating isotropic consolidation process. Using unified model method and modified TS-Zhang constitutive simulate the experiment data of shanghai 4th clay under differernt stress path, the result indicates the correctness of unified model method and modified TS-Zhang model.(5) Observe the change of pore and soil particles within consolidation process by using SEM experiment. Grain size curve results demonstrate the feasibility of extracting data from SEM pictures. Soil anisotropy ratio will decrease along with the increase of consolidation pressure, but it never decrease to zero. Consolidation does not eliminate the anisotropy ratio of the soil particals. |
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