Time Dependent Behavior Of Natural Soft Clays

Soft clays,which have been commonly encountered in real construction projects,are widely distributed in the coastal area of the southeast China.Rheological property and cyclic shearing behavior are the most important factors of natural clays that may lead to large settlements and unstability.Therefore,investigation of the effect of rheology and cyclic shearing on properties of natural clays is an important issue for understanding the behavior of clay and reliable design in practice,and this is just the object of this thesis.Based on the summary of related work,the rheological properties of K₀-consolidated natural Whenzhou clay are studied in detail from series of laboratory tests,and then an general elastic viscoplastic constitutive model is developed in this paper.The presented model is introduced to investigate the strain-rate dependency on yielding and strength behavior of clays,and the theoretical expressions of their strain rate parameters are obtained.Finally,the undrained cyclic shearing behavior of K₀-consolidated natural Whenzhou clay is observed in triaxial cyclic shear tests,and the results are fairly simulated by this rheologic constitutive model.In general,some developments for soft clays are involved in this dissertation as below:1.Experimental researches are conducted to study the rheological properties of natural Whenzhou clay.Creep and the strain rate dependency,which are the most two common behavior of clay rheology,are investigated systematically.Some features of the rheology of Whenzhou clay are obtained.The values of the model parameters can also be determined by parts of these experimental results.2.A new anisotropic elastic viscoplastic(EVP) constitutive model are presented for K₀-consolidated clays in general stress space.Just similar to the original overstress theory,two surfaces is assumed to exist for any loading history:loading surface f and reference surface(?),which are inclined ellipses in the p^'～q plane due to the effects of the stress induced anisotropy.Both of these two surfaces are constant curves of the flow functionφ, so the exact expression of flow fuctionφcan be established by the viscoplastic volumetric strain rate under one-dimensional condition.With comparisons between the simulations and the observed results of some representative clays,the validity and applicability of this model is verified.3.A rotational hardening law accounting for changes in anisotropy due to viscous strains is adopted into the presented model.By analyzing the soil's volumetric changes during isotropic loading,the expression of the new rotational parameter b is presented. Followed by comparisons with experimental data,the proposed model is capable of capturing key viscous features of soft clays,and the importance of the evolution of anisotropy is indicated.4.The strain rate dependent behavior of K₀-consolidated clays is investigated.Based on the developed anisotropic EVP model,the expressions of strain rate parameters of preconsolidation stress and undrained triaxial strength are presented,separately.The analytical solutions for undrained triaxial compression and extension strength of K₀-consolidation clays are deduced.The coincidence of the predicted and measured results shows the advantages of the new anisotropic EVP model.5.Experimental researches are conducted to study the undrained cyclic shearing behavior of natural Whenzhou clay.Considering the effects of anisotropy,all of the soil specimens are K₀-consolidated,which is very different from many existed tests.The deformation,pore pressure and cyclic strength behavior of natural clay is studied,and the effects of the loading frequency are analyzed.In addition,postcyclic degradation of strength is studied by another series of undrained triaxial tests,and some rewarding conclusions can be obtained.6.The cyclic shearing behavior is simulated by the presented rheological model.Based on their similarity,the accumulation during cyclic shearing is equivalent to a static creep process with the condition that the static incremental loadΔq=q_cyc.Then,a simple expression of the equivalent secondary compression coefficient(?)_αis adopted into the presented model which is used to simulate the accumulated cyclic shearing behavior of clays.The predicted results indicate that the creep equivalent simplification with the presented anisotropic EVP model is suitable in practice,the accumulation behavior and the postcyclic degradation of strength of natural clays can be fairly simulated.