Dem Investigation On The Effect Of Particle Breakage Of Carbonate Sand On The Mechanical Behaviors

Calcareous(or carbonate)sand,which is widely distributed in the south China sea and other tropical marine areas,is one of the indispensable materials for the development of the south China sea.Different from ordinary terrestrial sand,calcareous sand is rich in inner pores due to its special biological formation,combined with irregular shape and low particle strength,which leads to obvious particle breakage even under low stress conditions.Many scholars at home and abroad have finished a lot of research on calcareous sand by means of physical or numerical experiment.The results show that particle breakage is the main reason that calcareous sand exhibits mechanical properties different from ordinary geotechnical materials.DEM is more helpful than physical experiment in simulating particle breakage.In this paper,the open source discrete element program YADE was used to set up a particle breakage model,which was calibrated and verified by the existing calcareous sand laboratory test data to reproduce the results of calcareous sand physical experiment.This model is used to systematically study the stress-strain characteristics of calcareous sand and the evolution laws of particle breakage under different stress paths and confining pressures,the effects of particle gradation,particle breakage and its gradation evolution on the critical state.The main research contents and results are as follows:(1)Based on YADE's linear and cohesive contact models by adopting BPM method,a discrete element model of triaxial test considering particle breakage was established.Many triaxial shear tests were carried out under undrained condition,and the influence of different mesoscopic parameters(such as contact Young's modulus,stiffness ratio,rolling stiffness,friction angle,bonding strength)on the macroscopic response of the model was studied by using control variable method.According to the test results,each mesoscopic parameter was adjusted repeatedly,a numerical model of particle breakage was established which was consistent with the trend of the laboratory test results of calcareous sand,and the calibration steps of the mesoscopic parameters of the triaxial test model were summarized.(2)The established calcareous sand test model was optimized for computational efficiency,and particle breakage in the loading process was quantitatively described by Hardin crushing theory,and a series of triaxial shear tests were carried out under different stress paths and different confining pressures.The deformation characteristic,strength characteristic,stress paths and particle breakage evolution laws of calcareous sand under different stress paths and confining pressure were analyzed systematically.In addition,a comparative test was carried out on the crushable and uncrushable particles,and the effect of particle breakage on the test results was analyzed.(3)The calcareous sand model and the uncrushable particle model with the same initial gradation were shear to critical state under different stress paths and different confining pressures.The effects of stress paths,particle gradation and gradation evolution caused by particle breakage on critical state were studied experimentally.In addition,the calcareous sand model was consolidated and crushed under different preloadings,and the particles were set not to be broken during the shear process.Triaxial tests were carried out on the samples with different initial gradations,and the corresponding relationship between critical state lines and fixed gradations was determined.As a comparison,the same experiment was carried out on the samples with the possibility of particle breakage during shear loading,revealing the change mechanism of critical state lines of calcareous sand model in triaxial tests.