Research On Deformation Behavior And Constitutive Model Of Sands Under 3D Principal Stress Rotation

The constitutive behavior of granular materials under principal stress rotation?PSR?is an important and challenging topic in geotechnical engineering.Due to limitations in mechanical control in existing geotechnical testing equipments,such as hollow cylinder torsional shear apparatus,the traces of the principal stress directions during PSR can only be two-dimensional?2D?curves.However,the stress variations in actual geotechnical structures under engineering loads often follow complex three-dimensional?3D?paths.By developing a novel technique for the application of any arbitrary stress state on numerical samples,basic deformation patterns and laws of granular materials under 3D PSR are first revealed in this thesis.A new cyclic constitutive model for sands under general loading considering 3D PSR is then developed.The main novel achievements are as follows:1.A novel technique has been proposed for applying specified stresses at boundaries of spherical numerical samples,allowing for the application of any arbitrary stress state and ensuring the stress homogeneity inside the sample.Existing physical tests can be qualitatively and numerically reproduced,confirming the essential effectiveness of proposed methods.2.The macroscopic deformation pattern and microscopic fabric evolution of granular materials have been studied under PSR with rotation planes oriented at different angles?with respect to the bedding plane.Cumulative volume contraction is always found under continuous PSR.Relative magnitudes of the volume contraction under various?-values are known to be closely related with the magnitudes of the intermediate principal stress??₂?,which can be interpreted by the combined action of the?₂-value and the fabric anisotropy intensity in the direction of?₂.When?is neither 0^o nor 90^o,significant non-coaxial deformation accumulation occurs in the plane perpendicular to the PSR plane due to forming the oblique angle between the PSR plane and the bedding plane,which is attributed to the major principal fabric direction's being neither parallel nor perpendicular to the PSR plane.In addition,other macroscopic or microscopic quantities,such as the deviatoric strain increment,the non-coaxial strain in the PSR plane,and the coordination number,are also influenced by the?-value.3.Deformation patterns of granular materials under PSR about the axis of the major or the minor principal stress have been studied and compared with the existing findings of PSR about the intermediate principal stress axis.Samples under PSR about different principal stresses exhibit similar deformation patterns.However,more significant deformation occurs when PSR conducted about the intermediate principal stress axis.The magnitude of the deformation is closely related with the stress ratio inside the PSR plane.4.Compared with cyclic changes of principal stresses in their magnitues or directions,complex 3D PSR observed in geotechnical engineering induces much more significant macroscopic deformation and microscopic fabric evolution for granular materials.5.An anisotropic plasticity model for large post-liquefaction shear deformation for anisotropic sands has been extended for the PSR case,by incorporating the deformation hardening and fabric influence on the flow of plastic strain observed in physical or numerical PSR tests.The new constitutive model is able to achieve unified description of behaviors of sands under fixed principal stress axes shearing and PSR,and main features of soils under 3D PSR can also be captured.Good agreement of the tested and calculated results show the essential effectiveness and comprehensive serviceability of the proposed model.