Numerical Analysis Of Unsaturated Soil Strength With PFC^3D

Soil is some kind of particles assembly,and most of them which we met in the nature are unsaturated soils.The soil is a mixture of solid phase,liquid phase and gas phase,and this multi-phase particle system determines its complexity on mechanical and failure properties.Laboratory test and numerical simulation are the most common ways used to obtain mechanical behavior of soils.Compared with laboratory test,the numerical simulation method can not only maintain the identity of the initial sample,but also can arbitrarily change the boundary,to achieve the different conditions of the loading numerical test.By introducing PFC3D theory and developing simulation tools,this paper will focus on the simulation of tri-axial shear test and uniaxial tensile test based on the existing experimental results.There are three main steps during the numerical test:to establish an effective numerical model which could reflect the particle gradation of the real soil samples;to determine the reasonable micro-parameters to make the model consistent with macroscopic properties of real soils;to reveal the internal mechanism of this properties from the microscopic view.The main research works and results are as follows:1.Based on the theory of particle flow and the existing experimental results,a method to establish the relationship between macroscopic and microscopic parameters is proposed.The relationship between the bond strength and the water content is established by using the yield stress as a bridge.In order to realize function of the existing PFC3D discrete element program in the simulation of unsaturated soil behavior,the improvements we take are as follows:the use of dynamic expansion in the particle generation to reduce the initial contact force;the use of stress boundary to simulate the flexible boundary of the rubber membrane;the real-time monitoring of the position of the particles in order to prevent its escape in the shear process;using the relationship between the macro and microscopic parameters to reflect the impact of moisture content on the property of the soils.Based on the proposed methods,a discrete element model of unsaturated soils with different initial conditions such as porosity and water content can be established under different stress paths.2.Based on the proposed sample generation method of unsaturated soil and the existing experimental results,the discrete element calculation model of tri-axial shear test of unsaturated soil is established by generating unsaturated soil numerical samples with different porosity ratio and water content.Then we perform the discrete element numerical simulation of the consolidation-drained shear test,and it is revealed the macroscopic mechanical properties such as the stress-strain relationship in tri-axial shear test and the strength properties of the unsaturated soil samples,at the same time to the macroscopic mechanical properties such as the stress-strain relationship in tri-axial shear test and the strength properties of the unsaturated soil samples,at the same time to obtain the microscopic information such as the velocity field,the force chain distribution,the number of bond damage and the energy change.The comparison between the numerical simulation results and the existing test results verifies the applicability of the proposed model and calculation program to simulate characteristics of unsaturated soils in the tri-axial shear test..3.Based on the proposed sample generation method of unsaturated soil and the existing experimental results,the discrete element calculation model of uniaxial tensile test of unsaturated soil is established by generating unsaturated soil numerical samples with different porosity ratio and water content.Then we perform the discrete element numerical simulation of the uniaxial tensile test for,clay and sand respectively.It reconstruct the stress-strain relationship of clay and sand samples with different initial porosity and water content in uniaxial tensile test and revealed the changes of uniaxial tensile strength characteristics of clay and sand with pore ratio and water content,and the sensitivity of microscopic parameters was analyzed.The comparison between the numerical simulation results and the existing test results verifies the applicability of the proposed model and calculation program to simulate characteristics of unsaturated soils in the uniaxial tensile test.