Abstract: Calcareous sand is a special type of rock soil developing in the ocean. This marine sediment with irregular shape has different mechanical properties from the land originated sand. On one hand, microscopic 3D imaging was conducted in this paper for comparing the micro properties of calcareous and quartz sands. It can be found that calcareous sand particles are multi-angular and flatter in comparison with quartz sand. On the other hand, the effect of different particle fraction contents on compressive deformation properties of calcareous sand was investigated in compression tests. Then the 3D morphology was used to discuss the effect mechanism from a microscopic viewpoint. Quartz sand was used as a reference for comparison. It is shown that the compressibility of calcareous sand is 1.6-2.6 times that of quartz sand under the same gradation and compactness. The effect of the coarse fraction (5-1 mm) content in the gradation plays the most significant role in this feature. When the content of coarse particles is less than 25% and the mass ratio of middle and fine particles (M) is constant, there is the worst coarse fraction content causing the greatest compressibility of calcareous sand. With the increase of the mass ratio of middle and fine particles, the worst coarse fraction content decreases, and an empirical formula is proposed.

Key words: calcareous sand, compressibility, particle morphology, gradation