A Pproaches To The Triple-Shear Unified Finite Deformation Elasto-plasticity Constitutive Model For Normally Consolidated Saturated Clay

Many geotechnical theoretical research and engineering calculation methods are based on the theory of infinitesmal deformation.In the complex geological conditions and stress conditions,some geotechnical engineering will have a large deformation.There will be a big deviation between the calculation results obtained based on the infinitesmal deformation theory and the engineering practice.At this time,it is better to adopt finite deformation theory for analysis,so it is one of the research hotspots to apply finite deformation theory to geotechnical engineering analysis.In this paper,based on the finite deformation theory,a three-shear unified elastoplastic constitutive model of normally consolidated saturated clay is established.The main research contents and conclusions of this paper are as follows:1.In practice,the soil material has two nonlinearity,that is,geometric nonlinearity and material nonlinearity.In order to reflect the nonlinear relationship between strain and soil displacement and the influence of rigid body rotation during deformation,a constitutive model is established based on the finite deformation theory.In order to reflect the nonlinear relationship between soil stress and strain and the influence of stress state change,the triple-shear strength theory is used as the failure criterion,and the non-constant failure stress ratio is derived by using equal substitution method and coordinate translation method respectively.It is introduced into the modified Cambridge model to establish the rate-type elastoplastic constitutive model of normally consolidated saturated viscoelastic clay.In order to study the applicability of finite deformation constitutive model,the incremental elastic-plastic constitutive model was established based on the infinitesmal deformation theory and compared with it.2.Taking JiangXi red clay as the research object,a series of indoor geotechnical tests were carried out to determine the basic physical parameters of the experimental soil samples.Test samples with compactness of 80%,85%and 90%were prepared,and isobaric consolidation,isobaric rebound and isobaric reconsolidation tests were carried out by means of progressive loading and unloading to determine the compression index of test soil samples.In addition,the conventional triaxial undrained compression test and the conventional triaxial drained compression test were carried out under three confining pressures of 200kPa,400kPa and 800kPa for the test samples with three kinds of compacting degree,and the shear strength index and stress-strain relationship curve of the test soil samples were obtained.3.Two constitutive models of conventional triaxial undrained compression tests and conventional triaxial drained compression tests were numerically simulated using FORTRAN programming language.The results of numerical simulation are compared with the experimental data,and the stress-strain relationship curves obtained by the finite deformation constitutive model with equal-substitution method are most consistent with the experimental results.Through error analysis,the finite deformation constitutive model can more accurately reflect the large deformation characteristics of clay with high porosity.Under different confining pressures,the errors obtained by the finite deformation constitutive model have little change,which can better reflect the strength and deformation characteristics of soil.Without drainage,the generalized shear stress and pore water pressure are positively correlated with confining pressure,the generalized shear stress is positively correlated with the initial compaction degree of soil sample,and the pore water pressure is negatively correlated with the initial compaction degree.Under drainage conditions,generalized shear stress and volumetric strain are positively correlated with confining pressure,generalized shear stress is positively correlated with the initial compaction degree of soil sample,and the volumetric strain is negatively correlated with the compaction degree of soil sample.4.In order to reflect the strength and deformation characteristics of soil under complex stress conditions,numerical simulation of true triaxial stress state under drainage and without drainage was carried out.The effects of different initial compactness and intermediate principal stress influence coefficient b on the stress-strain relationship of normally consolidated saturated clay under true triaxial stress were studied.Without drainage,the principal stress difference and pore water pressure are positively correlated with b value,the principal stress difference is positively correlated with the initial compaction degree,and the pore water pressure is negatively correlated with the initial compaction degree.Under drainage conditions,the principal stress difference and pore water pressure are positively correlated with b value;And the main stress difference is positively correlated with the initial compaction degree,while the volume strain is negatively correlated with the initial compaction degree.