Study On Particle Breakage Mechanisms Of Calcareous Sand In The South China Sea At Multi-scale

Calcareous sand is widely distributed in low latitude sea areas,such as the South China Sea.It is the main geotechnical material of artificial reef hydraulic fill foundation in recent years.Marine biogenic calcareous sand has the structural characteristics of high calcium carbonate content,irregular particle shape and porous.Its special genesis and structure lead to the difference of its physical and mechanical properties from terrestrial sand.As a unfavorable geotechnical material with low strength and high compressibility,which tend to crush relatively easily under load.The structures of offshore engineering such as bridges,wharfs and oil drilling platforms are absolutely huge.The calcareous sand as the foundation bears a high stress level,so it is often accompanied by a large number of particle breakage.It is demonstrated that the particle crushing is a main factor affecting the property of calcareous sand.So it is very important to study the particle crushing of calcareous sand.In this thesis,the unconsolidated calcareous sand samples from an island in the South China Sea are used to explore the macro and micro mechanical properties of calcareous sand at multi-scale through a series of physical and mechanical tests.The main research contents are as follows:（1）In the micro test,the clear original image of calcareous sand particles is obtained by using the digital micro camera.The graphics of calcareous sand particles with different particle sizes are processed and analyzed with the help of image processing software ImageJ,and the shape characteristics of calcareous sand particles are described quantitatively.The shape which can characterize the calcareous sand particles are selected.For the calcareous sand in the South China Sea with different shapes,a series of single particle crushing tests are carried out to study the influence of particle shape on its single particle strength and deformation characteristics.（2）Using the high-precision biaxial loading equipment independently developed by the research group,a series of micro scale tensile,compression,pure shear and bending tests were carried out on the double particle cemented sample of calcareous sand,and the failure mode and stress displacement relationship under complex stress path were obtained.（3）In the macro test,a large number of drained and consolidation shear tests were carried out on plain sand and cemented calcareous sand.The trend of particle gradation is analyzed.The particle crushing behavior in the process of triaxial shear is quantitatively evaluated by Hardin’s crushing theory.The triaxial consolidation drainage shear test of cemented calcareous sand is carried out to analyze the effects of confining pressure and gypsum addition ratio on the stress-strain change and strength index of cemented calcareous sand.The CT test of cemented calcareous sand is carried out to analyze the shape change and particle breakage of calcareous sand before and after triaxial test.