Time-dependent Behavior And Uniqueness Of Its Key Parameters For Soft Clays

Soft clays are widely distributed in the coastal area of China. In these regions, the economic development and urbanization is rapidly developing. The behavior of soft clay is very complicated and brings big challenge to the Engineers in the design and construction of geotechnical projects. The time-dependent properties are the most important factor of natural clays for estimating long term settlements and instability of structures. Therefore, investigations on time-dependent behavior of natural clays are significantly important for reliable design in practice. The uniqueness, a wide concept, refers to a variety of things that have the same essence. Now, studies on time-dependent behavior are seperated by creep, rate-dependency and stress relaxation behavior of soft clays. However, the different mechancal behaviors are decided by the essential viscosity properties of clays. Theoretically, there must have a uniqueness between different time-dependent behaviors of soft clay. Hence, it is necessary to investigate their internal links and build the uniqueness of constitutive models.In this thesis, based on tests on Shanghai clay and investigations on various clays, the time-dependent behavior and uniqueness of its key parameters were studied. The uniqueness of different rate-dependency parameters and the uniqueness of creep, rate-dependent and stress relaxation behaviors of soft clay were presented. The anisotropic creep behavior of three different clays from Southeast China was investigated. A nonlinear creep formulation was proposed. An enhanced elasto-viscoplastic model by incorporating time-dependent parameter was developed accounting for both the packing density and the structure. In general, some developments for soft clays are involved in this dissertation as below:1. Based on the investigations of rate-dependent experiments on various clays, the uniqueness of rate-dependency behivior of soft clay under different conditions was studied. The applicability of five rate-dependeny formulation with its key coefficients in correlating the preconsolidation pressure and the undrained shear strength with strain-rate was discussed. With various experimental results, the suitbale rate-dependeny formulation in describing rate-dependency of preconsolidation pressure and undrained strength was determined. The uniqueness of rate-dependency for soft clays under 1D and triaxial compression and extension and different OCR conditions was studied. Based on the 1D CRS tests, the triaxial compression and extension tests and multi-loading-rate triaxial compression tests on Shanghai clay with OCR=1, 2 and 4, the uniqueness of rate-dependent behavior was verified and the variation of rate-dependeny coefficient for Shanghai clay was determined.2. By deriving the analytical solution of stress relaxation, the uniqueness between different time-dependent coefficients was proposed. Based on the summary of laboratory test results, a stress relaxation coefficient was proposed. Based on the rate-dependency formulation, the analytical solution of stress relaxation with time was derived. Then, the relationship between stress relaxation coefficient and rate-dependency coefficient was obtained. Combining secondary consolidation coefficient(or creep index), the unique relationship between rate-dependency coefficient, secondary consolidation coefficient and stress relaxation coefficient was established and verified. 1D relaxation and creep tests were conducted on reconstituted Shanghai clay, together with the rate-dependent tests conducted above, the uniqueness between time-dependent parameters was verified.3. Experiments on samples with various load direction with their natural deposition plane were conducted to investigate the anisotropic behavior of time-dependent parameters. These tests were conducted on Shanghai clay, Zhoushan clay and Wenzhou clay. The anisotropy of secondary consolidation coefficient and stress relaxation coefficient was specifically studied. The results showed that: the anisotropic behavior is obvious for high plasticity clay and cannot be negligible; when applied stress is smaller than preconsolidation pressure, the initial soil tensor plays an important role for the anisotropic behavior of time-dependent parameter; when the stress increases, the anisotropy will gradually vanish.4. Based on the creep tests soft clays and their physical and mechanical properties, the nonlinear behavior of time-dependent parameter was studied. With the experimental results on reconstituted clay, the evolution of Cae(secondary consolidation coefficient) with the packing density(or void ratio) of soils can be described by a simple non-linear creep formulation. For intact clay, the contribution of the inter-particle bonding and debonding for soft structured clays to Cae was analyzed based on test results on intact and reconstituted samples of the same clay. A nonlinear creep formulation accounting for both packing density and structure was finally proposed. The parameters used in the formulation were correlated with Atterberg limits, allowing us to suggest a relationship between Cae, Atterberg limits and inter-particle bonding for a given soil. The validity of the proposed formulation was examined by comparing experimental and theoretical Cae values for both reconstituted and intact natural soft clays.5. Based on the uniqueness and nonlinear behavior of time-dependent parameters, An enhanced elastic viscoplastic model(EVP-Nonliear) was developed and tested with applications. Adopting the uniqueness of the secondary consolidation coefficient, rate-dependency coefficient and the stress relaxation coefficient, one of them can be adopted as the time-dependent parameters which broadens the wide of parameter ranges. By incorporating the nonlinear properties of time-dependent parameters into ANICREEP, an enhanced elastic viscoplastic model was developed. From the aspects of effectiveness and convergence, three algorithms for elasto-viscoplastic models based on overstress theory were studied. Adopting EVP-Katona algorithm, the Umat subroutine for Abaqus finite element software was developed. The Umat subroutine was calibrated by laboratory tests and used to predict the long-term behavior of Murro test embankment.