Microstructural Behavior Of Saturated Soft Clay And An Elasto-Plastic Constitutive Model Considering Microstructure

With the high-speed developing of coastal region, there are more and more large-scale geotechnical structures, all of which are almost located in marine deposited clay. Then the geomechanical problems over marine deposited clay have caught more attention. In the recent decades, some constitutive models of soft clay, based on traditional macro-mechanics, have been developed but there are often a lot of differences between theory analysis and practical situation in calculating the deformation and strength. As a consequence, making a constitutive model of marine deposited clay considering microstructure has become an important task. In this study, we have a constitutive model based on the typical Shanghai clay. The research includes four parts.In the first part, the mineral component and microstructure were investigated by using X-spectrum analysis test and scanning electron microscopy (SEM). It is summed up the microstructure characteristics of Shanghai clay. Then SEM tests were dealt with on Shanghai in different stresses during one dimensional consolidation test, moreover the evolution of particle (including particle shape, mean area and orientation) was quantitatively analysed based on the horizontal cut plane and vertical cut plane of the specimen. We can point out that the clay has the characters of damage gradually and material anisotropy during loading.In addition, the remolded Shanghai clay was also examined by means of oedometer test and scanning electron microscopy test. Then the quantitative results (the shape, the mean area and orientation) were compared to the corresponding results of intact clay, the microstructure character and mechanism behavior of remolded Shanghai clay can be pointed out.In the second part, mercury intrusion tests were carried out on Shanghai clay to investigate the relationship between pore distribution and permeability under different vertical stress levels. The results show that the pore size distribution is dominant for small pore size, with the main size distribution varying from 0.2μm to 1.2μm. Moreover, the pore diameters become smaller and the whole pore volume decreases with the increased vertical stress level. The pore size distribution is dominant for micro-pore size. Then several permeability models were proposed to correlate the permeability with the pore size of Shanghai clay, the hydraulic-radius model is suitable for Shanghai clay, then the quantitative equation is derived.The third part concerns the development of an elasto-plastic constitutive model considering microstructure. The elasto-plastic model for granular materials with microstructural consideration (Chang & Hicher, 2005) has been extended to model the behavior of clays. The clay microstructure is assumed to be made of platelet aggregates which cannot be destroyed during loading. The deformation is computed by integrating displacement at the particle contacts in all orientations. The behavior of contact plane is assumed to follow an elastic-plastic law based on a Mohr-Coulomb criterion. A Hertz-Mindlin's elastic law is used for the elastic part and a double-yield surface is used to reflect the plastic behavior. The predictions of the proposed model were compared with experimental results of drained and undrained triaxial tests on normally and overconsolidated clays. The results demonstrate that the model is able to reproduce the main features of clay behavior.In addition, the stress-strain behavior of Shanghai clay was studied with laboratory tests. Drained triaxial tests were employed using a GDS triaxial test machine. At the same time, undrained triaxial test data of mucky clay and both drained and undrained triaxial tests data of mucky-silty clay were collected and analysed. They showed that the Shanghai clay has a characteristic of strain hardening, and the relationship between stress and strain is hyperbolic. The proposed model can described the stress-strain behavior of Shanghai clay and the effective stress path behavior. Another aspect, the model enables us to describe the evolution of stresses between aggregates on the contact plane. The results show that the shear stresses are different along different typical orientations, only few orientations are fully mobilized to reach the failure state at the strain levels examined. Therefore, the model can explain the failure of clays based on the microstructure.The fourth part concerns the description of the anisotropic behavior of clay. A second-order symmetric fabric tensor is introduced to the model, such as the stiffness of the contact planes, the hardening parameter, the dilatant/ contractant coefficient are made dependent of the orientation of the contact plane in order to capture the inherent and stress induced anisotropic behavior of clay. Then the proposed model was validated using results on Shanghai clay, San Francisco clay respectively, based on drained and undrained triaxial tests with loading in vertical and horizontal directions. It is shown that the model can describe the inherent anisotropic behavior of clay. Moreover, the model was also used to simulate undrained triaxial tests on Kaolin clay under different K₀-conditions. Is shown that the model can describe stress induced anisotropic behavior of clay, but there are differences in high overconsolidated experimental data and predictions by the model.