The Analysis Of Packing Density And Packing Structure Of Round Gravels Based On Discrete Element Method

The packing density and the packing structure reflect the structural characteristics of the arrangement and distribution of the soil particles,so it is a basic work to explore the physical and mechanical properties of many granular materials such as coarse granular soil,and it is also a starting point for the analysis of granular material characteristics at the macro and micro scale.Focusing on the characteristics of granular medium and the diversity of morphology distribution of round gravels,this paper studied the packing density and packing structure of round gravels with the help of PFC and in combination with corresponding physical tests.The specific contents are as follows:(1)Using digital image processing technology,the 2D pictures of 7 grain groups(2-60mm)were collected and their binary values were converted.Based on Image pro plus software,the geometric parameters and morphological parameters of round gravels were analyzed,and it was considered that the particle shape characteristics of round gravels were similar to ellipsoid but distributed in a discrete way,so it was necessary to establish particle models with different shape parameters to reflect the actual distribution law of round gravels.(2)The shape characteristics of boulder particles were characterized by the axial factor,and the ellipsoid model of boulder particles was established by using the clump function of PFC3D.Three sampling methods of numerical samples,namely simple random sampling,normal distribution sampling and stratified sampling were studied.(3)The physical packing tests and numerical simulation packing tests were carried out on7 groups of 2-60mm in total.The results show that the void ratio of each grain group is basically the same,and the larger void ratio of the larger grain group may be caused by the size effect.In the numerical simulation,when f is less than 0.4,the particle packing void ratio increases with the increase of the sliding friction coefficient between particles.When f is greater than 0.4,the void ratio increases slowly.According to the results of physical packing tests,the sliding friction coefficient f of the ellipsoid model of round gravels was calibrated to be 0.3.(4)Based on the method of"Separating particle size and volume filling",the method of determining the packing structure of gravel based on discrete element simulation was constructed in PFC3D software by using the microscopic parameters of porosity.The discontinuous uneven grading of C_c=4.5,C_u=8,C_c=1.06,C_u=1.73 were selected for numerical simulation,and the corresponding packing structure was determined.The data show that the packing structure is closely related to the gradation of specific packing particles.In addition,the simulation test verifies the feasibility of judging the structure based on discrete element packing,and also reveals the shortcomings of using fixed particle size analysis.