Constitutive Models For Soil Accounting For Fabric Anisotropy And Their Applications In Finite Element Analysis

Soils are common natural materials widely used in engineering constructions.Because of gravity and consolidation,soils are usually anisotropic.Moreover,the soil element in practical engineering may frequently undergo a variety of complex loading paths,such as those caused by traffic load,seismic events,and waves.Therefore,developing constitutive models for soil that can reasonably account for its fabric anisotropy and are applicable for complex loading paths is of great scientific and engineering significance.For this purpose,based on the anisotropic critical state theory,a series of constitutive models that can properly simulate the anisotropic soil behavior are developed in the present study.Moreover,to simulate the soil response under various complex loading paths,different constitutive mechanisms are introduced into the models.The model is further implemented into the infinite element software to simulate different kinds of geotechnical engineering problems.The main works of this study can be summarized as follows:（1）A series of anisotropic hypoplastic models are developed within the framework of anisotropic critical state theory to simulate the soil response under different loading paths.The fabric tensor is employed to characterize the internal structure of soil,and its evolution is described by a simple evolution rule.A state variable is proposed to qualify the relative relationship between the fabric and loading direction,and it is introduced into the model to simulate the impact of fabric anisotropy on the dilatancy and strength of soil.An internal variable is employed to reflect the influence of loading history on the soil behavior,and it is further incorporated into the model to simulate the modulus degradation of soil during undrained cyclic laoding.Moreover,the flow rule is modified to better simulate the non-coaxial response and the evolution of strain components during principal stress axis rotation.（2）Based on the characteristics of soil response under different complex loading paths,the hypoplastic model is extended to account for cyclic loading,principal stress axis rotation,and multidirectional shearing.Firstly,the fabric change effect and semifluidized state mechanism are introduced into the model to simulate the liquefaction phenomenon and the gradually increased shear strain amplitude in undrained cyclic loading test.Besides,the densification effect is properly considered to reproduce the gradually stabilized volumetric strain under principal stress axis rotation.Lastly,a new unloading criterion is incorporated into the model based on the bounding surface theory,to correctly simulate the rate of pore pressure accumulation during multidirectional shearing.（3）The J₂-deformation-type soil model originally formulated in triaxial space is extended to multiaxial space,and it is further incorporated with anisotropic critical state theory to simulate the mechanical behavior of anisotropic soil.The model is then implemented into the infinite element software to enable its application for simulating the geotechnical boundary value problems.Lastly,the model is employed to investigate the influence of fabric anisotropy on the pattern of the shear band in biaxial compression test,as well as the bearing capacity and failure mode of strip footing.