Study On Application Of Thermodynamic Methods To The Constitutive Modeling Of Soils

Traditional constitutive models of soils may be divided into two groups.One is experiential models obtained by fitting experimental data,which aim at improving fitting precision.The other is theoretic models,which can be obtained based on Drucker's Stability Postulate,Ilyushin's Postulate and plastic potential theory.However,in fact,Drucker's Stability Postulate or Ilyushin's Postulate only can be used to distinct between hardening and softening material,is not equivalent to the second law of thermodynamics.Classic elasto-plastic theory adopt several elementary factors including yield function,plastic potential,hardening rule and elastic law to describe mechanics behavior of soils.Generally speaking,these factors are determined independently and contradict each other sometimes, and the plastic potential surface does not always exist for geomaterials.To avoid these differences,starting from modern ideas of thermomechanics,this research discussed the construction of the free energy function and dissipation incremental function,and energy dissipative characteristics for soils.Energy conservative unloading and reloading part of Duncan-Chang E-B model,dynamic deformation mechanism and threshold shear strain for soils are studied.Besides,develop a constitutive model taking into account the fabric tensors and their evolution modes based on thermodynamics approach.For the widely used Duncan-Chang E-B model in engieering,the unloading-reloading modulus and the bulk modulus are usually defined through the pressure-dependent expression. But such a model leads to a non-conservative elastic response during the unloading and reloading process,which means that(for instance) multiple cycles applied to such a material could lead to continuous production of energy.This research formulate elastic component of Gibbs free energy function for soils from the elastic component of Duncan-Chang E-B model based on contribution of elastic bulk modulus and elastic shear modulus to Gibbs free energy function,and modify the compliance matrix starting from elastic component of the Gibbs free energy function.A very important result of modified model is that leads to coupling between elastic volumetric stress(or strain) and elastic deviatoric strain(or stress) behavior.The magnitude of this coupling reflects a degree of material anisotropy,which is determined by the value of the stress ratio.Besides,the material behavior is modeled as elastic with additional dilatancy term in the bulk modulus due to shear modulus dependency on pressure. The appearance of these additional terms demonstrates that the modified model can accurately model elastic component of stress and strain relationship curves of undrained and drained triaxial tests for dam material of high rockfill dam under the different consolidation pressures, at the same time,it is energy conservative in closed stress cycles.Starting from the skeleton curves of Hardin-Drnevich model and Ramberg-Osgood model and formulating the hysteresis loop by use of Masing's rule,the research construct dynamic dissipation function for soils using the assumptions of the beeline and the skeleton curve shift laws by use of thermodynamic approaches.Then discuss corresponding yield surface and energy dissipation mechanism of materials of two high core rockfill dams at different dynamic strain amplitudes.Two types of cyclic threshold shear strain,called the first threshold shear strain and the second threshold shear strain,are proposed for dynamic characteristics of rockfill non-cohesive materials.Two threshold shear strains represent boundaries between fundamentally different dynamic characteristics of cyclic soil behavior. For cyclic strains below the first threshold shear strain,soil behaves as a constant friction coefficient material.Between the first and the second threshold shear strain,the yield of soil is controlled by the variable friction coefficient.Above the second threshold shear strain,soil becomes increasingly nonlinear,with significant dilatancy-induced microstructural changes taking place under cyclic loading.Both the first and the second threshold shear strain do depend significantly on the maximum dynamic shear modulus coefficient and exponent.In addition,friction angle of cohesionless soil also influences them to some extent.The second threshold strain is equivalent to that defined by traditional pore pressure increasing and volume varying.From the engineering application aspect,if dynamic strain for soil is smaller than the first threshold strain,then maximum dynamic shear modulus and constant damping ratio can be used to analyze dynamic characteristics for soils.Develop a constitutive equation taking into account the fabric tensors and their evolution modes.Link modern ideas of thermomechanics opinion to the theory of void fabric tensors. The dissipation incremental function of anisotropic model considering the effect of fabric characteristic can be obtained automatically from the modified Cam-clay constitutive model. After discussing the relationship between essential independent variables in constitutive equations and the state of granular materials from a viewpoint of the evolution mode of the void fabric tensors,the results show that with the development and change of void fabric,the pore of granular materials can rearrange and show less symmetry(more anisotropic).In the true stress space,fabric not only affects the deflection of the yield surface,but also affects the hardening rule.