Study On Constitution Model Of Loess Based On Disturbed State Concept

The key to the deformation prediction of the loess-mudstone interface landslide by finite element numerical simulation is the determination of constitutive model of the effected soil for the occurrence of the loess-mudstone interface landslide.This paper summarizes the research achievements of scholars both at home and abroad about constitutive models of soils and disturbed state concept.Take Q₂ and Q₁ loess of different stress conditions near the slip band of Yan Lian landslide as the research objects for undrained triaxial consolidation test and direct shear test separately in order to obtain the mechanical parameters and the stress-strain relations of the hardening responses and softening responses.On the basis of the test results above,the nonlinear elastic constitutive models and the elastic-plastic constitutive models for Q₂ and Q₁ loess are established based on disturbed state concept.The model programs are wrote by MATLAB and then tested by the observing responses.In addition,the text analyzes and evaluates the constitutive models.The main conclusions of this paper are as follows:?1?The nonlinear elastic disturbed state constitution models are established by the direct method and the indirect method.The elastic model and the nonlinear elastic model are chose to describe the undisturbed state.The pore model,the hydrostatic stress model,the ultimate gradual stress model,the elastic model and the hyperbolic nonlinear elastic model are chose to describe the fully undisturbed state.Therefore,ten nonlinear elastic constitutive models can be combined by the models above.The simulation results for the softening responses of Q₂ and Q₁ loess show that the nonlinear elastic constitutive models established by the direct method can simulate the observing responses accurately no matter what the reference state models are.However,this method depends on the test data directly.The simulation responses of the nonlinear elastic constitutive models established by the indirect method have certain errors with observing responses.This method depends on the test data indirectly.The size of the errors depends on the selection of the reference state models.The combined model of which the undisturbed state choosing the nonlinear elastic model and the fully undisturbed state choosing the ultimate gradual stress model can simulate the observing responses better than the other nine combined models for Q₂ loess.While,for Q₁ loess,the combined models of which the undisturbed state choosing the elastic model and the nonlinear elastic model and the fully undisturbed state choosing the pore model,the hydrostatic stress model,the ultimate gradual stress model and the hyperbolic nonlinear elastic model can simulate the observing responses better than the other two combined models.?2?The elastic-plastic disturbed state constitution models are established.Hierarchical single surface HISS-?₀ model and HISS-?₀ extended model are chose to describe the undisturbed state for the hardening responses of Q₂ and Q₁ loess respectively.For the softening responses of Q₂ and Q₁ loess,the undisturbed states choose the same models.The fully undisturbed states choose pore model.The disturbance function chooses plastic strain to be its variable.Furthermore,total stress path and effective stress path should be considered to the elastic-plastic constitutive models for Q₂ loess.Therefore,six elastic-plastic disturbed state constitution models can be combined,which are constitution model of Q₂ loess without superimposing disturbance under total stress path,constitution model of Q₂ loess without superimposing disturbance under effective stress path,constitution model of Q₂ loess with superimposing disturbance under total stress path,constitution model of Q₂ loess with superimposing disturbance under effective stress path,constitution model of Q₁ loess without superimposing disturbance and constitution model of Q₁ loess with superimposing disturbance.These model programs are tested by the observing responses.The results show that they can well describe the hardening responses and softening responses of the Q₂ and Q₁ loess which reflect the rationality and applicability of the models.Meanwhile,the simulation results of the models of Q₂ loess under total stress path are more reasonable than the models under effective stress path.?3?HISS-?₀ extended model is a two-dimensional interface model simplified by HISS model for Q₁ loess.HISS-?₀ model and HISS-?₀ extended model have more parameters to be determined.These parameters can be simply obtained by undrained triaxial consolidation test and direct shear test.The yield surface of HISS-?₀ model presents to be a continuous smooth closed curve in the I₁-J₂^1/2plane and a date pit in the principal stress space.The hardening parameter a,the limit constant ? and the phase constant n can control the size of the yield surface rather than the limit constant ?.The limit constant ? and ? can control the failure envelope of the yield surface.The yield surface of HISS-?₀ extended model presents to be a continuous smooth closed curve in the ?-?_n plane.The hardening parameteria,the limit constant ?_i and the phase constant n_i can control the size of the yield surface.The limit constant ?_i can control the failure envelope of the yield surface.?4?The yield surfaces obtained by model program calculation are parts of the shear yield surface of HISS-?₀ model and HISS-?₀ extended model.The yield surface is expanding and the hardening parameter is changing with plastic loading.The yield surface of HISS-?₀ model is effected by stress path,moisture content and confining pressure as well as the limit parameter g.HISS-?₀ extended model is effected by moisture content.?5?Classical plastic models?Mohr-Coulomb,Drucker-Prager,von Mises,Tresca?and modified Cambridge model are special forms of the HISS model.The failure envelopes of M-C model are closer to D-P model than to the HISS model.Therefore,both of the failure envelopes of M-C model and D-P model are below the HISS model.The failure envelopes of the HISS model are closer to M-C model under total stress path than effective stress path.