Modeling of anisotropy of natural clays: Constitutive laws and identification protocol

Anisotropy of a material occurs when the material properties are not the same in every direction. Natural clays often exhibit the mechanical anisotropy, especially the cross-anisotropy, because of the mode of their sedimentation and preferred horizontal orientation. Both mode of failure and deformability strongly depend on the anisotropy. As a result, several important mechanical parameters such as undrained strength, coefficient of consolidation and elastic deformability may acquire a strong directional preference. Consequently, the anisotropy of clayey soils has a significant influence on the bearing capacity of a soil mass. Thus, the effect of anisotropy of natural clays on their behavior may become critical. Neglecting the anisotropy is a very important single cause of discrepances between classical geotechnical numerical modeling and the field observations.; A mathematical model able to simulate elastic, strength and plastic hardening anisotropy of natural clays has been developed based on the kinematical rotational hardening principle. In the proposed model, the yield surface and plastic potential surface may rotate and change in size and shape due to the plastic deformation. Unlike most of existing models in which the rotation of the yield surface is governed by a tensor, the proposed model adopts two scalar constitutive parameters to define the evolution of rotation of the yield surface in off-traxial plane. This assumption makes the elasto-plasticity law more flexible. The anisotropies in triaxial plane and off plane are thus treated independently. This also leads to an advantage of a more straightforward representation of such fundamental tests as triaxial tests and plane strain tests. The anisotropy of elasticity is modeled through anisotropic elasto-plastic coupling.; The complex and arbitrary identification procedures used commonly for multi-parameter highly non-linear models raise many concerns regarding repeatability of the results and the assessment of the quality of the approximation achieved in simulation. A standard or a self-consistent identification procedure is rarely found in the literature regarding non-linear models of inelastic materials. An attempt to codify the identification procedure of the material constants for the model developed has been made and a codified procedure has been proposed. This codification is based on physically justifiable criteria, whenever such criteria could be found. When this is not found possible, as in the case of sets of the hardening evolutional parameters, a modified least square method (Levenberg-Marquardt method) is adopted to the normalized experimental data.; Two experimental sets of results found in the literature are used as examples for the identification procedure. In the first example, identification of constants for an undisturbed Haney clay is presented based on some anisotropic triaxial and plane strain tests. Subsequently, all the tests in this set are numerically simulated. In the second example, constants for a laboratory prepared kaolin are identified based on some isotropic and anisotropic triaxial tests. The tests are then simulated numerically. The numerical results in two examples are then compared with the experimental results using the method of normals. The difference between the numerical response by prediction and the experimental results can be quantified numerically using a mean value. This mean value is low, in particular, for the first example.