Study On Hysteresis Constitutive Model And Discrete Element Numerical Simulation Of Light Weight Soil Mixed With EPS Particles Under Dynamic Load

In recent years,China has further promoted the high-quality construction of transportation infrastructure.The busy traffic network has promoted the coordinated development of different regions.However,under the long-term traffic load,engineering problems such as bump at the bridge head and ground subsidence have become increasingly serious.EPS particle light weight soil is a typical artificial structural soil,which has the characteristics of light weight,high strength,environmental protection and so on.It can be applied to infrastructure construction of high-grade highways,railways and so on,and performs well in reducing the self-weight of filled soil and controlling uneven settlement of pavement.Therefore,it is of great significance to study the dynamic characteristics of EPS particle light weight soil under dynamic load.Among the dynamic stress-strain characteristics of light weight soil,the hysteresis curve reflects the whole process of stress-strain of light weight soil under dynamic load,which is the basis of dynamic response analysis of soil.In this study,the dynamic deformation characteristics of EPS particle light weight soil with different EPS particle content,cement content and confining pressures are systematically studied through indoor consolidation undrained dynamic triaxial tests.The dynamic deformation characteristics of light weight soil are analyzed from the aspects of dynamic shear stress-shear strain hysteresis curve,backbone curve,dynamic shear modulus and damping ratio.Based on Hardin-Drnevich constitutive model and Masing rules,combined with the indoor test results,a modified hysteresis model of EPS particle light weight soil is established.Moreover,a three-dimensional numerical model of light weight soil is established by using PFC3D software.From the perspective of contact force,velocity field and displacement field,the meso-mechanical characteristics and deformation failure mechanism of EPS particle light weight soil are explored.Research shows that:（1）The hysteresis curve of light weight soil reflects the relationship between dynamic shear stress and dynamic shear strain in the process of dynamic cyclic loading and unloading.Under the dynamic load of step-by-step loading,with the gradual increase of dynamic shear strain,the area surrounded by hysteresis curve also gradually increases.Under the small dynamic shear strain,the shape of the hysteresis curve of light weight soil is roughly"spindle"or"ellipse".With the increase of dynamic shear strain,the shape of hysteresis curve gradually becomes a curved crescent shape,and its shape gradually loses its symmetry.When the strain is large,the hysteresis curves of light weight soil with different dynamic shear stress amplitudes are spaced apart,and the phenomenon of strain accumulation gradually becomes obvious.In the range of small strain,the higher the confining pressure,the less obvious the deformation accumulation phenomenon of light weight soil.（2）The backbone curve of light weight soil reflects the relationship between weekly peak dynamic shear stress and dynamic shear strain during dynamic cyclic loading and unloading.The dynamic shear stress-shear strain backbone curve of EPS particle light weight soil is hyperbolic as a whole,which has obvious strain hardening characteristics.In the process of gradually increasing dynamic load,the deformation of light weight soil can be divided into three stages,namely,vibration compaction stage,vibration shear stage and vibration failure stage.With the continuous increase of dynamic shear strain,the dynamic shear stress of light weight soil also increases gradually,rapidly in a small strain range,and then the rate of increase decreases continuously,and finally tends to a certain stable value,that is,it reaches the upper limit shear stress valueτ_ult,showing obvious nonlinear characteristics.（3）Based on Hardin-Drnevich backbone curve model and Masing rules,the modified hysteresis model of light weight soil is established,the loading and unloading criteria of hysteresis curve of light weight soil are redefined,and the restriction that the initial dynamic shear modulus of hysteresis curve is equal to the maximum dynamic shear modulus is canceled.The maximum dynamic shear modulus correction coefficient k₁and dynamic shear modulus attenuation coefficient k₂are introduced to modify the hysteresis model of light weight soil.The absolute error range between the predicted value and the test value of the modified hysteresis model is between 0.46 and 3.14 k Pa,and the relative error range is between 3.19%and 19.41%,which has a high accuracy.At the same time,it also solves the defect that Masing law can not construct a complete hysteresis curve under the condition of minimal strain.（4）The maximum dynamic shear modulus correction coefficient k₁is the ratio of the initial shear modulus of the modified hysteresis curve to the initial shear modulus of the Masing method,which describes the dynamic shear modulus of the soil at the initial moment of unloading and reloading.The dynamic shear modulus attenuation coefficient k₂is the ratio of the attenuation degree of shear modulus of the hysteresis curve of Masing method to that of the modified hysteresis curve,which describes the attenuation characteristics of dynamic shear modulus of light weight soil with the increase of dynamic shear strain under dynamic load.The hysteresis curve correction coefficients k₁and k₂jointly determine the shape and trend of the hysteresis curve of light weight soil.（5）In the numerical simulation of light weight soil,the macroscopic deformation of the sample shows,the change of the movement displacement and velocity of soil particles and EPS particles,the destruction and degradation of parallel bonding between particles,and the arrangement and reorganization of the whole particle system in the mesoscopic.When the dynamic stress is small,the contact force of the sample does not change obviously in a loading and unloading period,and the force chain is evenly distributed.The displacement field changes periodically,and the closer to the upper and lower loading plates,the greater the particle displacement,and the smallest the particle displacement at the center of the sample.The particle velocity is sensitive to the change of stress.With the change of the magnitude and direction of dynamic stress,the direction change of particle velocity near the edge of the sample is the most obvious,and the direction change of particle velocity inside the sample always lags behind that at the edge.（6）Multi-stage cyclic loading can be regarded as the superposition of multiple loading and unloading cycles under different vibration levels.During the step-by-step cyclic loading of discrete element numerical samples,the force chain experienced the change process of"dense→sparse→dense",indicating that the contact between particles was destroyed,and then the contact was regenerated under the action of external force.The displacement field has experienced a change process from"order→disorder→order".With the increase of dynamic stress,the displacement of particle system gradually increases.When the dynamic stress is small,it increases slowly,and when the dynamic stress is large,it increases quickly.When the dynamic stress is equal to 90k Pa,the displacement fluctuates briefly.The change of velocity field is similar to the change of displacement field.The closer to the sample,the more complicated the particle movement.The closer to the outside of the sample,the more orderly the particle movement.（7）In the numerical model,the rigidity of EPS particles is less than that of soil particles,and the increase of EPS particle content means the decrease of soil particle content,and the overall rigidity of the sample will decrease with the increase of EPS particle content,and its ability to resist deformation will be weaker.With the increase of EPS particle content,the critical dynamic stress of shear failure of light weight soil gradually decreases,which accelerates the instability of particle system and increases the maximum velocity and displacement of particle movement.In the process of increasing the dynamic stress step by step,the maximum velocity of the particle system first increases and then decreases,and there is a maximum velocity peak point.With the increase of EPS particle content,the peak point of maximum velocity becomes larger,and the stress corresponding to the peak point of maximum velocity gradually decreases.