Anisotropic Constitutive Model Of Soil Induced By Intermediate Principal Stress Basing On Unified Yield Criterion

The Cam-clay model represents the beginning of modern geomechanics,which can reproduce the compressive stiffness and shear expansion of normally consolidated soils and weakly superconsolidated soils,and the resulting critical state geomechanics has become an important branch of soil ontology research.However,the Cam-clay model has the defects of not being able to characterize the soil anisotropy,such as not being able to predict the soil triaxial compression and triaxial tension test phenomena at the same time,i.e.,the yield damage surface of the Cam-clay model is shown as a circle in theπ-plane,which does not conform to the actual damage criterion of the soil and cannot consider the influence of the intermediate principal stress on its mechanical behavior.In order to overcome the defects of the Cam-clay model,this paper proposes to introduce the Lode’s anglesθand the unified yield criterion to consider the influence of the intermediate principal stress based on the original Cam-clay model,and gradually extend the model to overconsolidated soils and structured soils by adding overconsolidation and structural parameters.The main work and findings of this paper are as follows:(1)Numerical tests of equal mean effective stress triaxial compression and tension were carried out by using the secondary development function of finite element software ABAQUS.User Subroutine URDFIL is used to extract the element stress and strain information and Subroutine DLOAD is used to control the load according to the required stress path,and the data exchange between the two User Subroutines is realized by the Utility Subroutines POSFIL and DBFILE.The results show that the yield surface of any elastic-plastic constitutive model in theπ-plane can be observed using the method proposed in this paper,and the stress-strain process of the model in reaching the yield point in theπ-plane can be predicted.(2)Based on the theory of unified strength criterion of soil,the shape function g(θ),which solves the problem of convexity of the shape function,is constructed and introduced into the original Cam-clay model,and the value of M in the revised model isM_f g(θ),so that the new Cam-clay model can consider the influence of intermediate principal stress on the mechanical behavior of the soil,and the new constitutive model contains the Mohr-Coulomb,Drucker-Prager,Lade-Duncan and Matsuoka-Nakai,which can simulate the normal consolidated soil triaxial compression and triaxial tension as well as true triaxial test phenomena simultaneously.Finally,the stress integration algorithm of the new model is developed by using the UMAT subroutine with plastic partial strain increment Δε_d ~p as the iterative variable,and the new model is successfully embedded into ABAQUS.(3)Using the new Cam-clay model establishing method that can take into account the effect of intermediate principal stress,the new subloading surface model and the new super-subloading surface surface model,which can take into account the effect of intermediate principal stress,were constructed respectively,and the elastic-plastic stiffness matrices of the two models were given,and the UMAT subroutines of the two models were developed.The results show that the new models can effectively predict the triaxial compression,triaxial tension and true triaxial test phenomena of superconsolidated and structural soils,characterize the superconsolidated shear expansion and softening,structural deconstructive softening and intermediate principal stress-induced anisotropy of soils,and the stress integration algorithm of the new models is stable,with fewer iterations per incremental step,less iteration error and high computational efficiency of the algorithms.