Theoretical And Experimental Research On Constitutive Modelling For Clays

Soils are one kind of geologic materials, which contain many voids. Therefore, the plastic volume change of soils may take place even under a lower pressure. According to the principle of interaction between plastic volumetric and shear strains for rock and soil, the interaction between plastic volumetric and shear strains will be induced by appearance of the plastic volumetric strain for soils, thereby leading to the complex behavior of deformation of soils. However, in metals, the plastic volumetric strain is approximately equal to zero, so there is no this interaction. The plastic deformation of metals, therefore, is a process of pure shear deformation. The traditional approach for constitutive modelling in soil mechanics originates from the theory of plastic potential for metals, in which the basic method of constitutive modelling is to seek an analytic expression of plastic potential for the soils. Obviously, since the plastic deformation of metals is a pure shear one as mentioned above, it is easy to find an analytic expression of plastic potential. However, that is very difficult for soils.Based on inverse problem theory, following an inverse course, the constitutive law for soils could be directly abstracted from the experimental results within a theoretical framework. Thus, the obtained constitutive model is able to characterize the fundamental features of mechanical response of soils accurately. In addition, by means of computer, this numerical constitutive modelling is easy realized.On the basis of a series of experiments on the mechanical properties for clays, the dissertation attempts to extend the numerical modelling method into the field of clays, the issues of constitutive modelling, constitutive relations and strength characteristics for clays are investigated within the framework of elasto-plasticity theory. The dissertation includes three parts of research work: numerical modelling of the constitutive relations of clays; study of the influence of stress path and stress history on constitutive relations and strength characteristics for clays; and parameters research and model appraisement of the classical models for clays. Main results of the dissertation are as follows.(1) The numerical modelling method has been extended into the field of clays, four triaxial compression tests under different stress paths, drained and undrained conventional triaxial compression tests and triaxial compression tests in which the mean stress remains constant and decreases respectively, have been carried out. The elasto-plastic constitutive models for normally consolidated clays under the four stress paths have been developed. Through visualization, the three-dimensional surfaces of shear and volumetric strains in the whole stress field under the four stress paths are respectively given. In particular, the elasto-plastic constitutive models under the undrained stress path and that in which the mean stress decreases provide accurate constitutive equations for computer simulation of undrained condition and excavation works. It is verified by the visualization of stress-strain relations and computer simulation that the numerical modelling method can sufficiently reflect the basic properties of deformation of clays under different stress paths. In addition, it is shown through comparison of numerical and traditional modelling methods in the aspects of theoretical foundation, basic framework and prediction results that the numerical modelling method is superior to the traditional one.(2) It is found through comparing the deformations of normally consolidated clays under the four stress paths that there are obvious differences in the stress ranges, the strain peaks, the shapes of strain surfaces and the trends of variation of volume yield loci, however the four families of shear yield loci are similar. These results demonstrate that the influence of stress path on the constitutive relations of clays is considerably large and can not be neglected, and the numerical modelling method is a effective means for describe the dependence of stress path.(3) It is found through comparing the shear and volume yield loci under the four stress paths that the stress path has notable impact on the evolution of strain hardening for clays, and this suggests that the assumption of fixed forms for the yield surfaces in the traditional elasto-plasticity constitutive models is not appropriate.(4) Based on a series of triaxial compression tests for normally consolidated and overconsolidated clays and the principle of interaction between plastic volumetric and shear strains, it is theoretically explained that the mechanism of generating two types of stress-strain relation curves for clays, and the critical state is a pure process of pure shearing deformation, in which the interaction between plastic volumetric and shear strains completely disappears, and it is independent of the previous stress history.(5) Based on the triaxial compression tests of clays under various stress paths, it is theoretically proved according to the principle of interaction between plastic volumetric and shear strains that the constitutive relations for clays are dependent on stress path and the dependency of stress path is the combination of effects of the interaction between plastic volumetric and shear strains. The mechanism of yield surface rotation was analyzed and it is explained that the rotational hardening is resulted from the dependency of stress path, which is essentially a reflection of the interaction between plastic volumetric and shear strains.(6) In order to study the influence of stress path on effective shear strength parameters for clays, on the premise of ensuring the sameness of initial condition, stress history, drainage condition, shear strain rate, tester, failure standard for soil samples, the drained triaxial compression tests under different stress paths have been carried out for normally consolidated reshaped clays. Test results demonstrate that the influence of stress path on effective shear strength parameters is considerably large and can not be neglected, in addition, the qualitative analysis about the influence is carried out.(7) In view of a hyperbolic approximation has been suggested to describe the relationship between axial strain and radial strain in Duncan E-μmodel, but the points of test results of real soils are frequently found to deviate from the ideal relationship, which results in some difficulties in parameter fitting. For best agreement of the theoretical and test results, a modified method is proposed. The validity and superiority of the modified method are attested by computer simulation.(8) In order to solve the problem of reckoning without shear induced contraction in K~G model suggested by Specification of Soil Test, a three-modulus K-G-D model is suggested which has seven parameters, the integrated incremental stress-strain relations are given. The parameter calibration need not additional tests, and the modified model is clear in concept, simple in form and convenient to use. The validity and superiority of the modified method are attested by tests and computer simulation, and the application scope of the model is discussed.(9) In order to solve the problem of the fitting result of logarithmic hardening modulus from normal consolidation line disagreement with that from critical state line in practice, based on the framework of Modified Cam clay, the relationship between logarithmic hardening modulus and mean effective stress is acquired by deduction from the concept of normal consolidation line and critical state line, which unify the two lines. The rationality and validity of the model parameters are shown in the computer simulation.