Hydro-mechanical Constitutive Relationships Of Unsaturated Soils And Numerical Algorithm

Unsaturated soils widely exist in nature.Massive infrastructure,such as railways,highways and airport runways are built on unsaturated soils.The performances of infrastructure are closely related to the mechanical behaviour of unsaturated soils.Unsaturated soils consist of soil particles,pore water and pore air,and show complex mechanical behaviour.Research on unsaturated soils in still in the exploratory stage.The theory of unsaturated soils is not perfect.The study of constitutive relationship for unsaturated soils has great significance.This thesis study the mechanical behavior of unsaturated soils,unsaturated expansive soils and overconsolidated unsaturated soils.The constitutive relationships of the unsaturated soils are built.The constitutive relationship are incorporated with the finite element method to analyse the engineering problem.The main conclusions are as follows:(1)The analysis about the mechanism of the swelling behavior of the unsaturated expansive soils points that it is possible to build the constitutive relationship of unsaturated expansive soils based on the macro phenomena.The concept of macro-structural neutral loading line is introduced by adopting a simple assumption concerning the plastic swelling potential of unsaturated expansive soil.A macro-structural volume change equation of unsaturated expansive soil is derived using macro-structural neutral loading line.The constitutive relationship for unsaturated expansive soils can be built easily by incorporating the model for non-expansive unsaturated soils with the concept of macro-structural neutral loading line.The model do not need the coupling function and micro structural strain to calculate the plastic swelling strain.Only a new macro-parameter is introduced into the constitutive model compared to the original model.The parameter in the model is easy to determine.Finally,this paper presents the comparison between predicted results and experimental data.The comparison indicates that the proposed model represents remarkable ability to reproduce the mechanical and hydraulic response of the unsaturated expansive soils.(2)The mechanical behavior of unsaturated soils is the result of the coupling between the hydraulic and mechanical properties.The concept of hardening effect of unsaturated soils was proposed based on the experimental data of compression tests.The experimental data indicated that these is a linear relationship between hardening effect and degree of saturation in the space of hardening effect and logarithmic degree of saturation.A hydro-mechanical constitutive model for unsaturated soils using skeleton stress and degree of saturation as basic variables was proposed.The new model indicates that the volume change for unsaturated soils can be divided into two parts:one part is for the stress change,and another part is for the variation of degree of saturation.The model also indicates that the normal compression lines of unsaturated soils parallel to the normal compression lines of saturated soils.The governing equations for unsaturated soils under local equilibrium were derived based on porous medium theory.The comparisons between the experimental results and model predictions indicated that the proposed model represents great ability to reproduce the mechanical and hydraulic response of unsaturated soils.(3)A hydro-mechanical constitutive model for unsaturated soils with different ratios was derived by incorporating the concept of subloading surface.The implicit integration algorithm and consistent tangent modulus of the model was given.Only a new parameter was introduced compared to the original model.The model is capable of predicting the behaviour of overconsolidated unsaturated soil,such as the change of degree of saturated under constant suction,the strain hardening and softening,shear contraction and dilation.The validity of the proposed model was confirmed by experimental results.(4)The mechanical behavior of the unsaturated soils under constant degree of saturation is explored.Controlling the degree of saturation of specimen was achieved by change the void ratio and suction.The consolidation tests under constant degrees of saturation were conducted.The experimental data indicates that the normal compression lines of unsaturated soils under constant degrees of saturation parallel to the normal compression line of saturated soil.The experimental data confirm interpretation about the volume change behavior of unsaturated soil.(5)The three-phase coupled governing equations for unsaturated soils were derived based on the porous medium theory.The finite element formulation for unsaturated soils was given by incorporating the constitutive model proposed in this thesis.The finite element formulation was validated by the experimental data of leaking flow in the soil pillar.The numerical simulations indicate that the rise of underground water level has a negative effect on the strip foundation and soil slope.