Evolution Of Particle Crushing Of Carbonate Sand Under Cyclic Triaxial Stress Path

Carbonate sand is widely distributed in the South China Sea,and used as fill materials for land reclamation.Due to its special biological origin,carbonate sand basically retains inner pores from the protozoan skeleton,and has the characteristics of highly irregular shape,low particle strength,brittleness and fragility.Even at low stress level,carbonate sand also produces remarkable particle breakage,which making carbonate sand exhibits distinctive mechanical behaviors compared with terrestrial silica sand.A series of triaxial tests and cyclic triaxial tests were conducted out on a carbonate sand taken from a reef in the South China Sea.Experiment systematically investigated the stress-strain characteristics,strength characteristics and critical state of carbonate sand,compared with the mechanical behavior of carbonate sand and silica sand,expounded the influencing factors and typical characteristics of particle breakage,established the mathematical description of particle breakage evolution under triaxial and cyclic triaxial stress paths.The main research contents are as follows:(1)Triaxial drained shear tests and triaxial undrained shear tests were carried out on a carbonate sand.Under the monotonic shear condition,with the increase of confining pressure,the peak stress ratio decreases gradually,the axial strain corresponding to the peak stress ratio increases gradually,and the stress-strain relationship gradually develops from softening to hardening.The stress ratio develops faster under undrained condition than under drained condition.In addition,the peak friction angle and critical friction angle decrease with the increase of confining pressure,and the friction angle of drained shearing is greater than that of undrained shearing.By comparing the distribution of the initial and final void ratio of the samples with different relative densities,the fact that the critical state of carbonate sand is changed by particle breakage is confirmed.(2)Based on a series of triaxial shear tests under different axial strains,expounded the development law of particle size distribution,summarized the form of particle breakage and its influencing factors.Under triaxial stress path,considering the effects of confining pressure and axial strain on particle breakage,established a mathematical description of the evolution of particle breakage under drained condition,predicted the development trend of particle breakage under undrained condition.By comparing the stress-strain characteristics of carbonate sand and silica sand,it is confirmed that the difference of the stress-strain curve development trajectory between drained and undrained conditions is caused by particle breakage.(3)Under the cyclic triaxial stress path,considering the effects of confining pressure,cyclic stress ratio and the number of cycles on particle breakage,a mathematical model is proposed to describe the evolution of particle breakage under cyclic shearing with different confining pressures and axial dynamic strain amplitudes.Furthermore,based on the mathematical model of drained tests,the development trend of particle breakage in cyclic triaxial undrained shear tests is predicted.The experimental data are in good agreement with the predicted results of the model,which proves the validity of the mathematical model.