Rotation Hardening Constitutive Model Of Sand And Its Application In Marine Engineering

In marine engineering,the deformation and strength characteristics of submarine sediments under wave loads and other cyclic loads will directly affect the stability of structural foundation.Instability of the seabed or failure of the bearing capacity of foundation will lead to damage in marine structures,sometimes a certain amount of economic loss,and even marine ecological disasters.Under low stress conditions sandy soil has the characteristics of liquefaction,which makes sandy seabed foundation more likely to trigger disaster under external dynamic actions.Numerical method is the most effective technique of evaluating dynamic effects of marine engineering site;however,it needs to be combined with soil constitutive model which can describe complex loading.In this paper,based on the basic mechanical properties of sandy soil,a rotary hardening model was developed and applied in marine geotechnical engineering combined with numerical platform Abaqus.Related research work and achievements are listed as follows:1.Constructing isotropic hardening constitutive model of sandy soil:By analyzing the compression characteristics of sand and its non-linear characteristics in the0)-lgplane,a reference volumetric compression curve more suitable for describing it under isotropic compression path was proposed and described with log-power function.On such basis,unified hardening(UH)parameter was introduced to construct volumeric hardening parameters.An expression of yield surface suitable for describing dilatancy of sand was suggested,the influence of shape parameteron yield surface shape was analyzed,suitable shape parameterwas selected and a method of determining yield surface shape parameter a under equal effective pressure path was proposed.The compression curve corresponding to current stress ratio was used as the reference curve of state parameters to adjust the hardening parameters and dilatancy equation,so as to reflect the compression and dilatancy of sandy soil.This method realized the unified expression of state parameters of the sandy soil from compression to shearing.The isotropic hardening constitutive model was verified with isotropic compression tests,undrained shear tests and drained shear tests of Toyoura sand,and the influence of yield surface shape parameterand reference compression curve slopeon the predicted results of this model was analyzed.The results showed that the isotropic hardening constitutive constitutive model could describe the shear behaviors of Toyoura sand under various void ratios and pressures.2.Establishing rotational hardening constitutive model:On the basis of isotropic hardening constitutive model,the law of rotational hardening was introduced to describe the influence of stress-induced anisotropy on the mechanical properties of sandy soil.With the development of anisotropy,the proposed yield surface gradually became flat and could be retreated to the yield surface of isotropic hardening constitutive model.The anisotropic compression curve was obtained with interpolation method,and the reference compression curve varied with the rotation of yield surface.While introducing unified hardening(UH)parameter into the volumetric hardening parameter,the influence of stress induced anisotropy was also taken into consideration to make the yield surface vary with the development of stress induced anisotropy.A state parameter was introduced to adjust the hardening parameter and dilatancy equation to reflect the dilatancy of sandy soil.The rotational hardening constitutive model was verified with undrained shear tests,drained shear tests and undrained cyclic loading tests of Toyoura sand and Neveda sand.The comparison results showed that the rotational hardening constitutive model established in this paper could perfectly describe the shear characteristics of sandy soil in various void ratios and pressures of Toyoura sand.In addition,based on triaxial compression tests and triaxial cyclic shear tests,influences of rotation hardening parameters b_l,b_r and dilation correction parameters R_k on the model prediction results were compared and analyzed,and reasonable value ranges of b_l,b_r and R_k were given in allusion to Toyoura sand.3.Developing Abaqus subroutine based on rotational hardening constitutive model:According to the loading creteria of UH model,the correlation between the yield surface and the current stress point in the substepping stress method was corrected,so that the method could be more suitable for the rotational hardening model.Based on the optimized substepping stress method,the rotational hardening constitutive model was embedded into the Abaqus subroutine UMAT to realize the secondary development of the finite element software and complete the debugging of the program.By comparing the prediction results of undrained shear tests,row number shear tests and cyclic loading tests with those of Fortran program,it was proved that the finite element redevelopment program based on the substepping stress method was completely correct and the modified substepping stress method was also reasonable.Therefore,the rotational hardening constitutive model Abaqus can be applied in practical engineering.4.Analyzing interactions between waves and sandy seabed based on rotational hardening constitutive model:Mechanical characteristics of seabed under wave actions were studied with Abaqus embedded with the rotational hardening constitutive model.In this process,a geometric model of sandy seabed was constructed and material stratification was established according to the interaction mechanism between waves and seabed.In order to verify the correctness of wave loads and model boundary conditions,the isotropic elastic model was applied to calculate the interactions between waves and seabed soil within finite depth.For seabed analysis,the parameters of the constitutive model and other related parameters were determined and verified on the basis of existing literatures.Pore water pressure,shear stress variation characteristics as well as wave-induced transient seabed liquefaction under extreme sea conditions were analyzed,and the analytical results indicated that response processes of the elastic seabed and the elastic-plastic seabed to waves were totally different from each other.Under wave actions,there was a porepressure accumulation phenomenon in the seabed according to the elastic-plastic theory,which caused a sharp decline in effective stress in the seabed soil,while there was no such phenomenon in elastic seabed.