| Soft soils, which have a high water content, large void ratio and great compressibility, are distributed widely in the coastal areas and riverside areas of the China. When the infrastructure constructions are built on the soft clay subgrades, the issues of settlement and especially long term settlement are particularly prominent. The process of deformation of soft soils is time dependent, including both consolidation and creep, which influence each other and determine the long term settlement of soft soils together. In the process of consolidation, the excess pore water pressure is dissipated and the soil volume is reduced by compression, however, the process of creep is caused by the viscosity of soft soils. Therefore, based on the time dependent characteristics of the deformation of soft soils, how to establish a constitutive model which can reflect the creep behavior of soft soils and realize the coupling effect of creep consolidation further are the hot problems of engineering and academic circles. In this study, the methods of laboratory experiments, theoretical analysis and numerical simulation were performed to investigate the consolidation and creep characteristics of soft soils, and the governing equations were established further. The main contents and achievements are as follows:(1) A series of oedometer tests, undrained triaxial creep tests and undrained triaxial shear tests were carried out to analyze the time dependent characteristics and deformation mechanism of soft soils, and provide the experimental basis for the establishment constitutive model. The test results show that :1) The secondary consolidation coefficient decreases with the growth of consolidation time or preloading. The secondary consolidation coefficient increases with the consolidation pressure, which has a turning point appearing in the vicinity of preconsolidation pressure, and the increasing amplitude of secondary consolidation coefficient depend on the consolidation state. There is a good linear relationship between the secondary consolidation coefficient and compression index, and the ratio is 0.028~0.049. 2) In the undrained triaxial creep tests, when the upper load is small, the creep deformation of soft soils weakens with time gradually and finally tends to keep stable, however, once the deviatoric stress reaches the critical value, the soil sample will be damaged due to the accelerated creep. 3) In the undrained triaxial shear tests, the undrained triaxial shear strength of soft soils increases with the increasing of strain rate, and the rule of rate dependence can be well characterized by rate parameters.(2) Based on the oedometer tests and the Bjerrum’s theory of e lgp equitime curve, the suppose which considers the creep deformation is time dependent plastic deformation was verified as reasonable. An one dimensional elastic viscoplastic constitutive model under the K0 confined condition was built from the perspective of both creep effect and strain effect, and the physical meaning of the model parameters and the method for obtaining the parameters were given. By inserting the nonlinear elastic viscoplastic model into Terzaghi’s consolidation theory, one dimensional governing equations which can couple the effect of creep and consolidation were established, and the Crank Nicolson finite difference method was used to solve the equations. The calculation results of this model were contrasted with the results of oedometer tests and Imai&Leroueil’ s tests, and verify the validity of this model. The calculation results show that, when the primary consolidation is finished, the difference of calculated deformation between the model in this paper and the hypothesis A model will become bigger with the increasing of sample height or the viscosity property of soil.(3) Based on the improved K0 oedometer, the experiments defined as flexible geometric confined tests, in which the vertical stress and lateral stress ratio is kept as a constant, were carried out to analyze the compression characteristics of soft soils when a certain lateral deformation was allowed, and the tests results were compared with the results of traditional oedometer tests. The results show that, the development of law of compression curves, compression index and secondary consolidation coefficient is similar under different confined conditions. However, due to the lateral deformation, the compression index and secondary consolidation coefficient of flexible geometric confined tests is about 1.08~1.16 times and 1.1~1.3 times respectively as oedometer tests. Based on the experiment results, Darcy’s law, the principle of effective stress and continuity principle, a consolidation equation under the flexible geometric confined condition was built, and coupled with the elastic viscoplastic constitutive model to obtain the governing equations which can couple the effect of creep and consolidation. The physical meaning of this model parameters and the method for obtaining the parameters was given. This creep consolidation model was used to simulate the tests results under the flexible geometric confined condition to verify its rationality and usability. The calculated results show that, the influence of sample height or secondary consolidation coefficient on the strain, which is generated in the primary consolidation process, is bigger in flexible geometric confined condition than K0 confined condition. For the foundation model in this paper, when the primary consolidation is finished, the strain was calculated under flexible geometric confined condition is about 1.17 times as the strain under K0 confined condition.(5) Based on the results of creep tests, the fractional Merchant model was built, and the empirical model was used to fitting the tests results of oedometer tests and undrained triaxial creep tests further, and the ranges of parameters were given. This empirical model can avoid considering the complicated mechanism of creep and consolidation, which can predict the long term deformation of engineering from macroscopic aspect. By comparison with the fitting results of traditional Merchant model, the fractional Merchant model can simulate the creep deformation better. The fractional Merchant model has obvious advantages in fitting the deformation at the inflection point of the trial curve, and can reflect the degree of nonlinearity that soil deformation increases with time. The analysis of parameters shows that, the order "α" has a minimum variation range under different pressure, and the coefficient of viscosity "η1" has a maximum variation range. In general, fractional Merchant model can satisfy the requirement of engineering.(6) Based on the Perzyna’s overstress theory, the one dimensional nonlinear elastic viscoplastic model was extended to 3D space by introducing the concept of reference surface, and the three dimensional nonlinear elastic viscoplastic model under general stress condition was built, which can consider the effect of initial anisotropy. By inserting the three dimensional nonlinear elastic viscoplastic model into the Biot’s consolidation theory, the creep consolidation control equations under general stress condition were obtained, and the expression of the stress increment and the incremental form of Jacobian matrix were derived. The UMAT subroutine was written and inserted into Abaqus, and the surcharge loading on the soft foundation is simulated. The calculated results suggest that, when the drainage condition is poor, the pore water pressure will increase with time due to the characteristic of creep under a constant load, which will affect the deformation and long term deformation of soft foundation. |
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