Numerical Implementation Of State-dependent Constitutive Model For Unsaturated Clays

With the increasing complexity of practical engineering,higher requirements are put forward for the discipline of geotechnical engineering to guide the implementation of practical engineering.Due to the large error of the saturated soil constitutive model in predicting practical geotechnical engineering problems,many scholars have proposed unsaturated soil theoretical model in recent years,which has high accuracy in practical application and can meet the general engineering needs.However,the theoretical model of unsaturated soil is too complex and has great limitations in practical application.With the improvement of computer computing ability,the numerical calculation of the unsaturated soil constitutive model and the efficient stress integration algorithm can greatly reduce the calculation and time cost and become an effective way to solve the engineering problems of unsaturated soil.Based on the existing constitutive model related to the state of unsaturated soil,this thesis discusses the possibility of applying the model in practice by studying the numerical implementation method of the model.The main contents of this thesis are as follows:(1)The state-dependent constitutive models of saturated soil and unsaturated soil are briefly introduced.Based on the state-dependent constitutive model of unsaturated soil,its mechanical and water retention sub-models are modified and simplified,so that the model can be better applied to numerical programming calculation.Based on the modified state-dependent constitutive model of unsaturated soil,the generalized stressstrain increment relationship,the elastic-plastic stiffness matrix of the mechanical submodel,and the expression of plastic modulus of water retention sub-model are derived.(2)Cutting-plane algorithm for state-dependent constitutive models of saturated and unsaturated soil are proposed,and the main flows of algorithms are given.In the water retention sub-model algorithm,the degree of saturation is updated.In the elastic prediction stage of the mechanical sub-model algorithm,the influence of degree of saturation is considered.In the plastic correction stage,the number of yield surfaces of the mechanical sub-model is determined by judging the positive and negative plastic operator.Combined with the algorithm,the calculation program of the state-dependent constitutive model of unsaturated soil is compiled in the FORTRAN language environment,and the isotropic consolidation and triaxial shear tests under different strain steps are simulated.The calculation results are compared to verify the validity of the algorithm.To verify the rationality of model and algorithm,the comparison between the simulation results and the test results of two groups of published triaxial compression tests of clay is carried out.(3)The secondary development function of ABAQUS and the format and usage of the user-defined material subroutine(UMAT)and user-defined field variable subroutine(USDFLD)are introduced.The UMAT subroutine of the state-dependent constitutive model of saturated soil is compiled.The subroutine is verified by simulating the triaxial test of saturated soil and the consolidation test of a one-dimensional saturated soil column.The UMAT subroutine of the state-dependent constitutive model of unsaturated soil is compiled,and the saturation and pore water pressure in the UMAT subroutine of unsaturated soil is updated by the USDFLD subroutine.The application of the statedependent constitutive model of unsaturated soil in ABAQUS is realized,and the subroutine is verified by simulating the triaxial test of unsaturated soil and the consolidation test of the one-dimensional unsaturated soil column.