Study On Dynamic Strength Characteristics And Discrete Element Simulation Of Light Weight Soil Mixed With EPS Beads

Light weight soil mixed with EPS particles（expanded polystyrene）is made by soil,light weight materials,cement and water with a certain proportion,and it is initially applied in some special highway subgrade projects.Subgrade soil is subjected to dynamic loads during service,and the dynamic strength characteristics of soil have an important impact on subgrade safety.The dynamic strength is related to certain strength standard,and the soil has different strength properties under different strength standard.Previous study on the dynamic strength of light weight soil focused on the influence of cement content,EPS particles content,confining pressure and other factors on the dynamic strength.There is lack of comparison of dynamic strength characteristics of light weight soil under different strength standard.Therefore,it is necessary to study the dynamic strength characteristics of light weight soil under different strength standard and to explore suitable strength standard.In this study,light weight soil of different proportions and remolded soil are prepared,and the consolidation undrained dynamic triaxial tests,conventional triaxial compression tests are conducted to obtain the hysteresis curves,dynamic stress time history curves,dynamic strain time history curves,dynamic pore pressure time history curves and effective shear strength parameters.According to the results of dynamic triaxial tests,the development laws of dynamic stress,dynamic strain,and dynamic pore pressure of light weight soil and remolded soil are studied,and the effects of pore pressure standard,limit equilibrium standard,yield standard and strain standard remolded soil are analyzed.Meanwhile,the stress-strain-pore pressure characteristics and failure modes under the undrained triaxial compression tests for light weight soil and remolded soil are discussed.With particle flow code PFC^3D,the discrete element numerical model of light weight soil mixed with EPS particles is established.According to the hysteresis curves obtained from dynamic triaxial tests,the model is calibrated with meso-parameters.The change law of contact force,displacement field and velocity field under different strain is researched.The research indicates:（1）In terms of the consolidated undrained triaxial compression characteristics,it is found that with the increase of cement content or the decrease of EPS particles volume ratio,the stress-strain curve of light weight soil changes from strain-hardening to strain-softening.Strain-softening light weight soil shares the same properties with over-consolidated clay.The pore pressure first increases and then decreases to a negative value.Strain-hardening light weight soil shares the same properties with normal consolidation clay,and the pore pressure tends to increase continuously during shearing.The remolded soil presents a shear shrinkage failure model.When the strain-softening light weight soil fails,the shear zone appears 60°,which shows brittle failure property.（2）In terms of dynamic characteristics,by the consolidated undrained dynamic triaxial tests,it is found that the cumulative change of dynamic strain of remolded soil goes through three stages.The first stage is the vibration density stage,the second stage is the vibration deformation stage,and the third stage is the vibration failure stage.The dynamic strain of light weight soil accumulates in two stages:vibration deformation and vibration failure.The dynamic strain increases rapidly when changing from vibration deformation stage to vibration failure stage.The test found that the rubber film has a compliant effect,which makes the sample drain locally when it is loaded,it causes the measured value of dynamic pore pressure to be low.（3）In terms of dynamic strength standards,for cohesive soil and light weight soil,the particles are cemented by the cementing material.During the process of dynamic load,the dynamic pore pressure is difficult to reach the confining pressure.The pore pressure standard should not be applied to cohesive soil and light weight soil.The limit equilibrium standard only considers the instantaneous limit state of the sample,and the static strength index is not equal to the dynamic strength index.Therefore,the limit equilibrium standard does not apply to cohesive soil and light weight soil.The dynamic strain time history curves of cohesive soil and light weight soil have no yield point,so the strain standard is applicable.Considering that the deformation of the soil under cyclic loading is a factor of the dynamic instability of the soil,the soil should be controlled to avoid large plastic deformation.Light weight soil uses single-width compressive strainε_d=5%as the strength standard,and take the double-amplitude dynamic strain 2ε_d=5%as the strength standard for remolded soil.（4）In terms of discrete element numerical simulation,the discrete element numerical modeling of light weight soil mixed with EPS particles is carried out using the particle flow code.light weight soil is a multi-media system.There are two main types of particles,namely soil particles and EPS particles.Cement is regarded as a kind of bonding strength in numerical modeling.The soil particles are generated according to the gradation amplification method,and the parallel bonding model is adopted.The bonding parameters are grouped according to the two particle systems of soil particles and EPS particles,and a dynamic triaxial discrete element numerical model of light weight soil is established.（5）Using the discrete element model of light weight soil,the changes of the velocity field,displacement field and contact force of the light weight soil at different total compressive strains of 2%,4%,5%,8%and 10%are studied.It is found that when the cumulative strain is2%,the particle movement direction is not obvious,and the particles in the radial direction and in the middle of the sample are present.With the increase of compressive strain,the particle velocity direction is aligned,and the particle velocity at both ends of the sample is distributed along the radial direction.The particle displacement gradually decreases from the outside to the inside,and the displacement value at different locations with the smallest displacement in the middle vary greatly,showing fluctuations across orders of magnitude.In the process of particle movement under vibration,the middle particles are always in a compressed state,the contact between the middle particles is dense,and the EPS particles at both ends of the sample are more distributed.As the compressive strain increases,the distance between the EPS particles becomes larger and the contact chain of force decreases accordingly.（6）When the volume ratio of EPS particles is constant,the displacement value of the particles increases with the increase of strain,but the increase is small and the change is not large.In the later stage of vibration,the bonding of soil particles is gradually destroyed,and the displacement of soil particles with a smaller displacement value increases with the vibration.The particle velocity directions at both ends of the sample are distributed in the radial direction,and there are particles pointing to the center of the circle and away from the center of the circle in the radial direction.With the increase of the volume ratio of EPS particles,the direction of particle velocity in the middle part changes from vertical to the center of the sample to along the diameter.Distributing in the direction away from the center of the circle,the particle velocity at both ends is greater than the particle velocity in the middle.The increase of the amplitude increases the displacement of the particles and the acceleration of the accumulation rate of soil deformation.