Discrete Element Simulation Analysis Of Dynamic And Static Mechanical Properties Of Graded Crushed Stone Filler Considering Particle Shape Characteristics

In the new situation of rapid economic development,in order to adapt the traffic to the needs of economic development,China has vigorously promoted the construction of high-speed railway.Grading gravel is widely used as filler for railway subgrade foundation bed because of its anti-reflection crack,improving subgrade bearing capacity,reducing differential settlement,dissipating vibration energy and enhancing drainage and water isolation function.Train load,however,the long-term repeated incentives of gravel layer material space structure changes,the internal force change mechanism,intensifying function layer accumulated deformation from the macro,mechanical properties evolution,may eventually produce roadbed uneven settlement,the mud pumping in water area there may be more diseases,such as maintenance and repair costs.Therefore,the evolution of mechanical properties of gravel layer directly affects the long-term service ability of subgrade.Based on the particle discrete element method,a two-dimensional numerical model considering the morphology characteristics of graded gravel particles was established in this paper.Combined with indoor triaxial tests,the macro and micro mechanical behaviors of the lower graded gravel packing under static and static loads were explored.(1)Digital microscope was used to scan the gravel particles of different shapes,and DIP(digital image technology)was used to number and statistics the contour and geometric parameters of the scanned particles in different particle size groups,so as to establish a sample database.According to the geometric size data,the morphological characteristic parameters of shape,edges and corners and texture were introduced to analyze the morphological characteristics of gravel particles and obtain their distribution.In order to obtain numerical simulation samples of gravel particles with similar overall shape characteristics,shape index and edge index were selected to conduct multi-objective stratified sampling under large samples,and irregular blocks were created with the extracted contour files as particle units of the numerical model.(2)Based on the particle flow software PFC2 D,the biaxial model of graded broken stone is established.In order to explore the influence of porosity and confining pressure on the simulation results,three samples with porosity are set up in this paper,and loaded under three confining pressures,a total of 9 samples.In the simulation process,the flexible confining pressure boundary is adopted,and the replacement method of gravel block particles is equal displacement.Based on the stress-strain curve of laboratory test,the control variable method is used to calculate the micro parameters of single biaxial specimen.Compared with the laboratory test,the shear strength of Graded Macadam specimen increases from 143 kpa to1059kpa with the decrease of porosity and the increase of confining pressure.(3)Force chain and fabric tensor were introduced to describe the loading process of contact force with load,and the strength of force chain was divided by average contact force.With the increase of deviatoric stress,the force chains gradually approached the direction of the principal stress from the random distribution,and the strong chains in the same direction as the principal stress were the most at the peak value.With the destruction of the sample,part of the weak force chains distributed to the horizontal direction along with the deformation of the sample.According to the fabric tensor vector diagram,the contact force distribution before loading is uniform,and the distribution is like a "circle" shape.After loading,the contact force distribution in the direction of principal stress becomes larger,and the distribution is like a "peanut" shape.The particle displacement on both sides of the shear zone is the largest,and the particle rotation Angle in the shear zone is large and mostly rotates counterclockwise.Most particles outside the shear zone do not rotate,so the particles in linear motion are more than the rotating particles,which is consistent with the statistical results of particle movement mechanism.(4)The stress level of 40% is the critical value of whether the cumulative deformation converges or not,and the increase of confining pressure is helpful to improve the cumulative deformation characteristics of the material;Under the failure condition,the hysteretic curve of the material undergoes an evolution process from fullness to narrowness and then to fattening,and the cumulative plastic deformation curve presents an inverted "s" shape.(5)With the increase of load times,the force chain is consistent with the load direction gradually.Under the small dynamic stress,the force chain is random distribution,which is similar to the initial state.With the increase of dynamic stress,the stress transferred by the force chain increases,and the lines of the force chain in the vertical direction become thicker and the number increases.The stress level is the main reason for the distribution of influence chain.The larger the confining pressure is,the smaller the contact number is,the weaker the anisotropy of contact distribution is,and it tends to be quasi circular.When the dynamic stress level is large,most of the contact distribution is still consistent with the direction of vertical load,and tends to be quasi "peanut" shape.The larger the confining pressure is,the more obvious the "hoop" effect is.The particles are not easy to move relatively,and the smaller the displacement is,the less likely the material is to be damaged.