Research On Macro And Micro Parameters Of Sand In DEM Based On Random Forest Algorithm

The calibration of micro parameters is the key of DEM simulation.However,the relationship between macro and macro parameters in different contact models of DEM is complex and difficult to determine.The traditional method is difficult to reflect the interaction between parameters,so it has some defects.The random forest algorithm in the field of artificial intelligence has the function of parameter analysis and prediction,which provides a better research means for the analysis of this problem.Based on the random forest method and the control variable method,the influence of macro and micro parameters of rolling resistance linear contact model and rolling resistance Hertz-Mindlin model is analyzed respectively,and the variables importance is obtained.Then,to verify the prediction ability of random forest method in DEM calibration,the shear strength of standard sand in direct shear test is predicted by this method.Finally,to further verify the correctness of the predicted parameters,the pile laboratory test and DEM simulation based on standard sand are carried out.The results show that the simulation results are close to the experimental results,which indicates that the random forest achieve excellent prediction in DEM calibration.In addition,based on the test and simulation,the mechanism and disturbance characteristics of single pile with cap and 2×2 pile group are analyzed in detail.(1)In the rolling resistance linear model,the effective modulus and stiffness ratio of particles mainly affect the deformation characteristics of samples,while the strength characteristics are mainly affected by rolling resistance coefficient and static friction coefficient.In addition,the increase of rolling resistance friction coefficient will significantly affect the dilatancy of the specimen.(2)In the rolling resistance Hertz-Mindlin model,the elastic modulus and Poisson’s ratio of particles mainly affect the deformation characteristics of the specimen,but have little effect on the shear strength of the specimen.The static friction coefficient and rolling resistance coefficient mainly affect the shear strength.Static friction coefficient and rolling resistance coefficient can significantly increase the shear strength of specimens.(3)The random forest model can better characterize the nonlinear effect of multi parameters and has good prediction performance in DEM calibration.And its variables importance function can be used in DEM parameter analysis.(4)By using the predicted parameters,the DEM simulation of single pile cap and 2×2pile group are carried out,and compared with the indoor test,it is found that the micro parameters of Fujian Pingtan standard sand calibrated by this model have higher accuracy.(5)In the static loading process of single pile cap,the range of particle disturbance gradually expands and finally tends to be stable.The inverted triangle compaction area is gradually formed in the lower part of the pile cap and diffuses outward,while the range of disturbed particles in the lower part of the pile bottom is elliptical.In the aspect of force chain,the force chain mainly appears in the lower part of the pile cap and at the bottom of the pile,and gradually dense.The distribution of the force chain at the side of the pile presents an arch shape,and the force chain at the bottom of the pile presents an elliptical distribution,and the range of the two increases with the test.(6)In the process of static loading,the range of particle disturbance gradually expands and finally tends to be stable.Due to the existence of piles,the horizontal movement of soil particles is restricted,and the soil between piles under the pile cap gradually forms a downward overall compaction area.In the aspect of force chain,the force chain mainly appears in the lower part of the pile cap between the piles and at the bottom of the pile,both of which are gradually dense with the test,and the distribution of the force chain at the side of the pile also presents an arch shape,and the force chain at the bottom of the pile presents an oval distribution,and the range of the two is increasing with the test.