Study On Elasto-plastic Two-surface Model Of Unsaturated Soils Under Monotonic And Cyclic Loading Study On Elasto-plastic Two-surface Model Of Unsaturated Soils Under Monotonic And Cyclic Loading

Unsaturated soils are widely distributed in the shallow surface.They are porous medium materials composed of solid phase,liquid phase and gas phase and usually have nonlinearity,anisotropism,structure,cyclic plasticity and deformation accumulation.Due to the coupling of solid,liquid and gas,the constitutive properties of unsaturated soils under static and especially dynamic loading are extremely complex.At present,although many achievements have been made in the research of constitutive models of unsaturated soils under static loading,the study of constitutive characteristics and constitutive model of unsaturated soils under dynamic loading is still at an early stage in its development.Based on the background of geotechnical engineering in unsaturated land area and supported by the National Natural Science Foundation of China(No.50579002),this paper expands the plastic hardening rule proposed by Li and Meissner to describe the cyclic plasticity of the material(hereinafter referred to as Li-Meissner hardening rule)to unsaturated soil in the framework of plastic incremental flow theory.On the basis of the hydro-mechanical coupling property,the focus of this paper is the research of elastoplastic two-surface model for unsaturated cohesive soils under cyclic loading,which takes into consideration the unsaturated soil structural,anisotropy and the influence of the over-consolidated features.The main work and research results of this paper are as follows:(1)Combining the Li-Meissner hardening rule with the BBM model of unsaturated soil,on the basis of the stress deformation and yield characteristics of unsaturated soils,the geometric equations of bounding surface and loading surface are established firstly.Secondly,the evolution law of bounding surface and loading surface in stress space is derived,that is,the size and position of bounding surface and loading surface in each loading event are firstly determined,and then the plastic hardening modulus is guided by consistency condition.Finally,an elastoplastic two-surface model for unsaturated cohesive soils is established.On this basis,considering the evolution and characteristics of stress path of unsaturated soil in(p,q)stress space of triaxial tests of constant suction and constant water content,the established model is used to describe the constitutive relation of unsaturated soil in these two type tests.(2)Based on the full hydro-mechanical coupling theory proposed by Wheeler et al.,and combined with the extended Li-Meissner hardening rule developed in unsaturated soils,it is firstly assumed that the initial SI/SD yield surface in the(p?,q,s~*)stress space is the bounding surface,and any increase/decrease of suction in the bounding surface will immediately cause the plastic change of the degree of saturation of unsaturated soils.Secondly,two-surface models are established for both LC and SI/SD surfaces.Both sets of models can be simulated independently.In the model,hydro-mechanical coupling plastic hardening rule is adopted to consider the effect of water on stress and stress on water.Finally,the formula of plastic hardening modulus is derived by consistency condition and interpolation rule,and then the full hydro-mechanical coupling elastoplastic dynamic constitutive model of unsaturated soil is established.(3)On the basis of the SWCC model with pore ratio proposed by Sun et al.,and combined with the method of elastoplastic two-surface modeling,it is firstly assumed that any increase of stress in the bounding surface would cause elastoplastic deformation,and the interpolation rule was used to calculate the plastic hardening modulus of the current stress point.Secondly,SWCC model with pore ratio is used to derive the hydraulic characteristics of unsaturated soil under the influence of deformation.Finally,a hydro-mechanical coupling elastoplastic dynamic constitutive model of unsaturated soil is established.(4)On the basis of the research work(1),(a)by introducing the volumetric damage rate as a parameter describing the standard soil structure damage,the cyclic plastic characteristics and structural damage process of unsaturated structured clay under the dynamic cyclic loading process can be reflected by the evolution of bounding surface and loading surface which can represent the structure damage in stress space;(b)by introducing a hardening parameter to reflect the anisotropy.The initial anisotropy is reflected by the initial value of the anisotropy parameter and the evolution of the anisotropy produced by cyclic loading is described by rotation hardening rule.The bounding surface and loading surface evolve according to the plastic hardening and rotational hardening rules in the stress space;(c)for the overconsolidated unsaturated soils obtained from the process of isotropic unloading,the corresponding bounding and loading surface plastic hardening rules are proposed to describe the dynamic mechanical properties of overconsolidated unsaturated soil.Two-surface elastoplastic models describing structural,anisotropy and over-consolidated unsaturated cohesive soils were established by three different modeling approaches.The rationality and applicability of different types of elastoplastic dynamic constitutive model of unsaturated soil in this paper are verified by both the team dynamic triaxial test results and the published results of related researchers.These constitutive models will help analyze the problem of dynamic response of the unsaturated subgrade under traffic loading,wind loading and earthquake cyclic loading,and have important theoretical significance and practical engineering application value.