Constitutive Relationships Of Anisotropic Sands Under Fixed Principal Stress Axes Shearing

Anisotropic sands are ubiquitous in natural environment and engineering practice.They have great influence on the strength and deformation issues of geotechnical engineering.Therefore the constitutive relationships of anisotropic sands have been difficult and hot issues over the past few decades for many domestic and foreign scholars.Nevertheless there are still strong needs for systematic research for the constitutive relationships of anisotropic sand.The previous experimental and theoretical research results are summarize.Based on those,the objective of this dissertation this study is to investigate the constitutive relationships,strength criteria and mathematical model of anisotropic sand through discrete element method and theoretical analysis.The following three novel achievements can be drawn:1.The basic strength and deformation laws of anisotropic sands and other granular materials are systematically summarized and relevant physical mechanisms are clarified.The peak strength of an anisotropic sand first increases then decreases with the increase of bedding plane angle.It gets its minimum value when the maximum shear-normal stress ratio acts on the bedding plane.The plastic shearing modulus increases with the increase of the peak strength and the irreversible dilatancy ratio increases with the increase of bedding plane angle.Moreover,there exists two modes of non-coaxial deformation under fixed principal stress axes shearing are clarified.2.A novel anisotropic variable is proposed to measure the distance between the bedding plane and the maximum shear stress ratio acting plane.The variable is adopted to define a new anisotropic strength criterion with two experimental parameters of clear physical meaning for anisotropic sands and granular materials.The criterion is validated against experimental results of several types of anisotropic granular materials,including sands,ideal 2D rod-like material and DEM model,showing its simplicity and applicability.Also,a general methodology is developed to build such a new anisotropic strength criterion only through replacing the constant parameter in an isotropic criterion by a linear function of the anisotropic variable.The above-mentioned two parameters can be easily determined by least square method with more than two sets of experimental results.3.Based on the constitutive relationships revealed,the anisotropic plastic shearing modulus and the irreversible dilatancy ratio are quantitively expressed by an anisotropic variable and an initial anisotropic experimental parameter.A unified plasticity model for large post-liquefaction shear deformation is extended to be able to deal with anisotropic sands.Simulations of several kinds of tests,including the drained and undrained fixed principal stress axes shearing tests and the undrained cyclic loading tests were conducted.Good agreements o the tested and calculated results show the essential effectiveness and comprehensive serviceability of the anisotropic plasticity model.