Micro- And Macro-mechanical Behavior Of Calcareous Sand And Discrete Element Method Analysis

Calcareous sand are widely distributed on low latitude area, such as the South China Sea. It is a marine biogenic material with calcium carbonate content more than 50%. Its particle is susceptible to breakage upon application of relatively high confining stress. Prior to developments of offshore facilities in South China Sea, it is vital to understand the characteristics of particle shape and the breakage properties of the calcareous sand. For these reasons, three aspects regarding micro-and macro-mechanical behavior of crushable calcareous sand sampled from South China Sea are addressed in this study.Firstly, the calcareous sand particles were scanned by electron microscope and analyzed by an image processing software (i.e., ImageJ). Based on the processed image, four shape parameters, namely circularity, aspect ratio, circularity and solidity, were defined and quantified. It is shown that the shape of the calcareous sand with relatively large particle size (particle size greater than 2 mm) and relatively small particle size (particle size less than 0.5 mm) tend to be circular and the particle surface is relatively smooth.Comparatively, shape of calcareous sand with median-size diameter (grain diameter ranged between 0.5 and 2 mm) calcareous sand were more irregular shape and have more surface edges. In addition, large numbers of the pictures were analyzed based on the engineering geology to identify the biological particles which form the calcareous sand, such as coral, gastropoda and bivalve.Secondly, a series of element tests (including one-dimensional compression tests, direct shear tests and drained triaxial tests) were carried out to explore the macro-mechanical characters of the crushable calcareous sands. Special attention was paid to understand the effects of particle breakage on deformation, shear strength and energy dissipation of the calcareous sand. Particle breakage was identified to occur in the triaxial tests, leading to a more uniformly graded sand packing. With the increasing initial confining pressure, the degree of particle breakage and the energy dissipation due to the breakage both increase. In the meantime, dilation of the sand is partly suppressed by the breakage. With the consideration of the shear friction dissipation only and both shear friction and volume dissipation, the dissipated energy resulting from breakage in the triaxial tests with relatively high (600 kPa) initial confining stresses could account for 25% and 18% of the total plastic energy input, respectively.Thirdly, three-dimensional discrete element analyses were performed using an open-source software Yade. In each analysis, the clump model was used to simulate the crushing particle while the rigid sphere with the same diameter as the clump was adopted to simulate the uncrushed particle. The micro-mechanical responses (such as contact force chain, incremental displacement of particle movement) of the soil samples with and without particle crushing were compared and interpreted. It is revealed by the DEM analyses that particle breakage has decreased the shear strength of the material, while suppressed the dilative behavior at relatively large strains (i.e., axial strain>10%). These effects become increasingly significant when the initial confining stress increases. Qualitatively, the computed results are generally consistent with the triaxial test results reported in the study. It is also revealed by the DEM analyses that particle breakage mainly occurs along a localized region inclined at 60° to the horizontal.