Macro- And Meso-mechanical Properties Of Frozen Soil And Meso-deformation Mechanism Of Subgrade

A great many infrastructures such as railways,highways,pipelines are widely distributed in frozen regions of China.The frozen soil with typical discrete characteristics is the foundation of the infrastructures,its macro-and mesomechanical properties and meso-deformation mechanism of subgrade are the keys to safety construction and normal operation of the infrastructures.This paper fully considers the discrete characteristics of frozen soil and investigates the meso-contact model of frozen granular material,the effect of ice on granular material under frozen condition,the macro-and meso-mechanical properties and failure mechanism of frozen soil and the meso-deformation mechanism of subgrade by means of theoretical analysis,laboratory tests,field monitoring and numerical simulation.The main contents and results are presented in detail as follows.(1)Based on the Particle Flow Code(PFC)theory,the meso-contact model of frozen granular material is proposed according to the characteristic of frozen granular material.For the conventional meso-contact model which is widely used to investigate the meso-mechanical properties of granular materials,two adjacent particles are in contact at a discrete point.For the meso-contact model in this paper,two adjacent particles are in contact over a width in order to reduce the effect of particle shape on particle sliding and the roller is adopted in order to reduce the effect of particle shape on particle rolling.Based on the consideration that the physical and mechanical properties of ice(bonding material)are not constant during loading process,this paper assumes that normal bonding force changes with the loading process.In addition,the particles are not allowed to slide or roll before bond is broken.(2)The granular material is instantiated into glass beads.The static and dynamic mechanical properties of frozen glass beads are studied through low temperature static and dynamic triaxial tests and the stress-strain curves are fitted by generalized hyperbolic model and Hardin hyperbolic model.The effects of temperature,loading rate,frequency and confining pressure on static and dynamic mechanical properties of frozen glass beads are also studied.Based on the meso-contact model of frozen granular material,the PFC model of frozen glass beads is established according to the static and dynamic triaxial tests.It is assumed that normal bonding force changes with the loading process and tangential bonding force is invariable.The nonlinear effect of ice on granular material under static and dynamic condition is studied.Moreover,the effects of normal and tangential bonding forces on macro-mechanical properties,energy evolution and crack growth of frozen glass beads are also investigated.(3)Frozen soil is taken as the research object.The static and dynamic mechanical properties of frozen soil are studied through low temperature static and dynamic triaxial tests.Based on the meso-contact model of frozen granular material,the PFC model of frozen soil is established with full consideration of the discrete characteristics of frozen soil.The nonlinear effect of ice on frozen soil is gained by properly adjusting the effect of ice on frozen glass beads.The effects of contact property between soil particles such as contact stiffness ratio and friction coefficient on macro-mechanical properties,energy evolution and crack growth of frozen soil are also investigated.In addition,the failure mechanism of frozen soil during loading process is studied through the research of force chain evolution and crack growth.(4)The coupled model of frozen subgrade is established based on the coupling principle of FDM-DEM.It not only solves the problem of time-consuming computation effectively,but also studies the meso-mechanical properties and meso-deformation mechanism of soil in key areas thoroughly.For the calibration of the coupled model,the comparison of simulated results and test data is shown.The meso-deformation mechanism of subgrade is revealed by studying the evolution of force chain between soil particles and average coordination number.