Acyclic Constitutive Model And Numeric Al Implementations Considering Nonlinear Compressive Hardening And Discontinuous Shear-contraction Of Round Gravel

With the opening and operation of more and more subway tunnels,the range of train load affecting the tunnel and the surrounding building foundation soil is getting wider and wider.Although train load belongs to low-level cyclic loading,it can result in significant cumulative deformation of foundation soil in the long run.Once the cumulative deformation of round gravel is large enough,troubles against safety and applicability happen on the tunnel and the surrounding buildings.Aiming at the experimental phenomenon of cumulative deformation of the Nanning round gravel under long-term low-level cyclic loading,the elasto-plastic properties of round gravel were summarized.A cyclic constitutive model and corresponding numerical implementation methods considering the characteristics of nonlinear compressive hardening and discontinuous shear-contraction of round gravel were studied.The proposed theories and methods were verified with datum from vibration triaxial tests.Detail efforts and results are as follow:1.Establishment and calibration of the shear component in the cyclic constitutive model considering nonlinear compressive hardening of round gravel.Datum from triaxial compression tests for the Nanning round gravel were analyzed for finding the property that shear yield stress and shear plastic hardening modulus of round gravel vary with effective mean stress and relative density non-linearly.For delineating this property accurately,a new nonlinear shear yield criterion is proposed,and new shear hardening models based on the A-F kinematic hardening model and the Chaboche isotropic hardening model are developed.According to the result of the triaxial compression test for the Nanning round gravel,the proposed nonlinear shear yield criterion,the extended A-F kinematic hardening model and the extended Chaboche isotropic hardening model were calibrated in this dissertation.It was showed that the proposed nonlinear shear yield criterion in the cyclic constitutive model can describe the nonlinear influence of confining pressure and relative density on the shear yield surface of round gravel quite well.The parameters of the extended A-F kinematic hardening model and the extended Chaboche isotropic hardening model can change nonlinearly with the change of confining pressure and relative density.2.Establishment and calibration of the volumetric component in the cyclic constitutive model considering discontinuous shear-contraction of round gravel.Datum from triaxial compression tests for the Nanning round gravel were analyzed for finding the property that the tendency of shear-contraction varies with shear stress discontinuously.For describing this property accurately,the equivalent volumetric strain model is applied to derive the volumetric hardening law with piecewise gradients.According to the result of the triaxial compression test for the Nanning round gravel,the proposed volumetric hardening law with piecewise gradients was calibrated.It was showed that the volumetric hardening law with piecewise gradients can exactly describe the characteristics of discontinuous change of shear-contraction trend for round gravel.3.The stress-driven integration algorithm for the cyclic constitutive model considering nonlinear compressive hardening and discontinuous shear-contraction of round gravel and corresponding implementation by programming.Aiming at the situation that stresses of round gravel are controlled in engineering practice,based on the formulas of the proposed cyclic constitutive model as well as the backward Eular scheme,the stress-driven algorithmic form of the proposed cyclic constitutive model and related variables are derived,and a stress-driven integration algorithm of the proposed cyclic constitutive model is complied.And the model was implemented by programming based on the algorithm.4.The strain-driven integration algorithm for the cyclic constitutive model based on the characteristics of nonlinear compressive hardening and discontinuous shear-contraction of round gravel and corresponding implementation by programming.Aiming at the problem that the calculation for stresses in quadrature points is driven by strains during the finite element method computation,based on the formulas of the cyclic constitutive model as well as the backward Eular scheme,the strain-driven algorithmic form of the proposed cyclic constitutive model and related variables are derived,and a strain-driven integration algorithm of the proposed cyclic constitutive model is compiled.And the model is implemented by programming based on the algorithm.5.Verification of the cyclic constitutive model for round gravel based on the characteristics of nonlinear compressive hardening and discontinuous shear-contraction.The cumulative deformations and pore pressure of specimens of the Nanning round gravel in the triaxial vibration test with low level cyclic loading were analyzed.The vibration triaxial test was simulated according to the stress-driven integration algorithm in this dissertation.The vibration triaxial test was simulated according to the strain-driven integration algorithm in this dissertation.Finally,the test results,the simulation results by the stress-driven integration algorithm and the simulation results by the strain-driven integration algorithm were compared.It was showed that the cumulative deformation obtained by the two simulation methods was in line with the cumulative deformation obtained by the test.Therefore,the proposed cyclic constitutive model can exactly predict the long-term cumulative axial strain,shear strain and volumetric strain of the Nanning round gravel.