Investigation Of The Microstructure And Fracture Mechanism Of Calcareous Sand Based On In Situ CT

Calcareous sand is a special geotechnical medium distributed in tropical oceans.The particles,which are irregular in shape and contain more than 50 percent calcium carbonate are formed by Marine organisms.Calcareous sand is mainly distributed in the South China Sea.In the past decade or so,with the exploitation of oil resources in the South China Sea,the development of tourism on islands and reefs,and the need for military preparedness,the number of modernization project at sea and on the islands and reefs has increased.Therefore,it is of great significance to carry out the theoretical and experimental research on calcareous sand for the practical engineering design,construction,safety inspection of calcareous sand in South China Sea and promote the improvement of economic level.The previous studies showed that calcareous sand particles is of high porosity,angular and thus fragile.The physical properties of calcareous sand are different from those of conventional terrestrial sand due to its formation and structural characteristics.Compared to quartz sand,calcareous sand has a much lower yield stress,i.e.the stress where a large amount of particle breakage occurs.Effects of the porosity and morphology on the particle breakage of calcareous sand is significant to understanding its engineering mechanical properties.However,the relationship between microstructure,morphology and particle breakage has not been established for calcareous sand.The main work of this paper is summarized as follows:(1)XRD,EDS and SEM are used to characterize the composition(crystal structures and elements)of calcareous sand particles.Computed tomography based on synchrotron x-rays is used to characterize the morphology of particles and their porous structures in three dimensions.The particle shape and porosity are quantified.The calcareous sand particles are mainly ellipsoid in shape.The porosity of particles vary from the fractal dimension of the porous structures,obtained via the box-counting method,increase with porosity of particles.(2)Single-particle compression tests are carried out on 20 calcareous sand particles of about 1 mm.The particles close to spheres are selected.The Weibull strength statistics are adopted to calculate the Weibull modulus m and the characteristic strength of sand particles.The Weibull modulus can be predicted with the fractal dimension of pores in particles,which is comparable to the one measured in the experiment.The results imply that it is the pore complexity that leads to the strength variation of sand particles.(3)We present for the first time in literature an in situ,3D characterization for the particle breakage process of carbonate sands under mechanic loading.Using a miniature material test system(mini MTS)implemented with synchrotron-based ?CT,the intra-granular pore morphology and evolution of grain-scale characteristics are quantified.The sequential 3D images reveal clearly the structure-property relationship for carbonate sands.3D crack networks extracted from fractured particles imply obvious cleavage along initial pores and residual cracks inside the fragments.The crack networks are fractal and the fractal dimension increases with external loading,resulting in a fractal size distribution of fragments.