Finite Element Realization Of UH Model And Its Application To The Analysis Of Subgrade Deformation

The unified hardening model(UH model)proposed by Yao et al.is the constitutive model which can consider the influence of the complex stress path and stress history on the deformation and strength of clays reasonably.It can not only describe the characteristic of normal consolidated clays,but also can describe many characteristics of overconsolidated clays well,including stress-strain relationships,shear dilatancy and shrinkage,strain hardening and softening.The geotechnical constitutive model is the key when using the finite element method to analyze the geotechnical engineering problems.However,the elastic-plastic models such as Cam-clay model which are widely used in numerical analysis of geotechnical engineering can reflect the characteristic of normal consolidated clays and weak overconsolidated clays well,but they can not reflect the characteristic of overconsolidated clays well.So the finite element analysis of clays in practical engineering used these models has a certain limitation.If we introduce UH model into the finite element method,we can use the strong nonlinear solution platform of finite element to predict the stress-strain characteristic of clays in practical engineering reasonably and solve the complex engineering problems.So in this paper,based on UH model,the key problems needed in the finite element realization of UH model are analyzed.UH model and ABAQUS software are combined successfully by using the secondary development function of ABAQUS software.The finite element realization of UH model is finished.Three-dimensional finite element analysis of subgrade bearing capacity is carried out.And the overconsoildated ratio is defined by the undrained shear strength of clays.The relationship between OCR and depth is introduced to the material subroutine of UH model.The finite element analysis for non-homogenous undrained subgrade is performed.The overconsolided ratio of different clays layers is introduced to the material subroutine of UH model.The finite element analyses for double layer subgrade and embankment engineering soft subgrade are performed.Further,the subroutine of UH model is developed.The finite element realizations of UH model considering cohesion and UH model considering temperature are finished.The specific performance is in the following four aspects.Firstly,the key problems in the finite element realization of UH model are analyzed,and the combination of UH model and ABAQUS software is realized,and the finite element analysis of the bearing capacity of subgrade is carried out.The key problems include:(1)The loading-unloading criterion is judged by adopting the intersection of yield surface and p axis.(2)The updating expression functions of hardening parameter and internal variable are deduced.(3)The stress updating algorithm procedure of UH model is explained.(4)In the condition of no change in stress increment results calculated by elastoplatic matrix multiplied by strain increment,non-symmetical elastic-plastic matrix caused by the different stress space(transform stress space and true stress space)is changed into symmetical elastic-plastic matrix.The symmetrical elastic-plastic matrix of UH model based on transform stress three-dimentional method is deduced.Xiong' symmetrization method is developed.The convergence of finite element analysis is improved.(5)The problem that stress ratio is greater than the ultimate stress ratio is solved.(6)The process of the finite element realization of UH model is analyzed from the whole angle in detail.The user material subroutine of UH model is made,and the combination of UH model and ABAQUS finite element software is finished.The subroutine of UH model is proved to be correct through the simulation of conventional traxial test,true traxial test and bearing plate test using UH model.And then,the bearing capacity of subgrade and the stree-strain relation of clays are analyzed.Secondly,the overconsolidated ratio is defined by the undrained shear strength of clays,and the relationship between OCR and depth is introduced to the material subroutine of UH model,and the finite element analysis of non-homogenous undrained subgrade is performed.The relationship between undrained shear strength and initial overconsolidated ratio OCR can be expressed by the formulation of undrained shear strength under triaxial compression condition.In this paper,the actual OCR of clays can be calculated by the test data of undrained shear strength according to the formulation,and the relationship between OCR of clays and its depth is attained.Then through the relationship between OCR and depth is introduced to the subroutine of UH model,OCR changed with depth can be considered in the subroutine of UH model,and the actual OCR of clays can be considered reasonably.And the numerical analyses of undrained subgrade considering OCR changed with depth and normally consolidated clays are performed respectively by using UH model.The load-settlement curve,settling curve on the center of subgrade surface with time,the shear strain curve,volumetric strain curve and pore pressure curve with time in two conditions are compared.The comparisons of results show that the use of subroutine of UH model considering OCR changed with depth can consider the actual OCR of subgrade clays reasonably,and it is necessary in the finite element analysis of non-homogenous undrained subgrade.Thirdly,the over-consolidated ratio of different clays layer is introduced to the material subroutine of UH model,and the finite element analyses of double layer subgrade deformation and actual embankment layered soft subgrade are performed.In this paper,after the overconsolidated ratio of different clays layer is introduced to the finite element program of UH model.Through setting different OCR for different soil layers,the soft and hard degree of soil is controlled.The finite element analysis of strong-over-soft double layer subgrade is performed by using UH model.The additional stress calculated by using UH model,simplified double layer subgrade stress coefficient method and Boussinesq elastic theory are compared.The comparisons prove that UH model has superiority on the analysis of the stress characteristics of double layer subgrade,and it can reflect the stress diffusion phenomenon of strong-over-soft double layer subgrade.Then the additional stress of strong-over-soft double layer subgrade with different OCR is calculated by using UH model,and the effect of OCR on the stress state of double layer subgrade is studied.And the finite element analysis of the consolidation settlement in the strong-over-soft double layer subgrade is performed.The subgrade settlement,stress-strain behavior and the change of pore pressure in double layer subgrade are studied.The finite element analyses for two actual embankment engineering layered soft subgrade are performed using UH model.The settlement deformation law and pore pressure dissipating law in the consolidation progress of embankment soft subgrade are analyzed.The calculated results are compared with the engineering measured data.The consistency of the comparison results reflects the rationality of the UH model in actual engineering.Finally,the material subroutine of UH model is developed further,and the finite element realizations of UH model considering cohesion and UH model considering temperature are finished.Based on the SMP rule considering cohesion c ? 0,the transformed stress,yield function,strain increment,plastic factor and symmetric elastoplastic matrix considering c?0 are deduced,thus the UH model considering cohesion is built.And on the basis of finite element realization of the original UH model,the finite element realization of the UH model considering cohesion is finished.Then the finite element analysis for loess tunnel without lining excavating engineering is performed.The displacement and stress-strain curves of the tunnel soil calculated by the original UH model and UH model considering cohesion are compared and analyzed.And the security and stability coefficient of tunnel with different overconsolidation ratio is analyzed using strength reduction method.On the basis of the subroutine of original UH model,the influence of temperature is added,and the finite element realization of UH model considering temperature is finished,and the simulation of triaxial test under different temperature is performed.The consistency of the simulation results and unit forecast results proves the accuracy of the subroutine of UH model considering temperature.And the simulation of the bearing plate test using the UH model considering temperature is performed.The deformation of subgrade and the stress-strain relation law of soil under different temperature are compared and analyzed.