Abstract: The obvious particle size effect of the calcareous sand gravel with irregular particle shape and uneven particle grading is shown after loading. A series of shear tests on calcareous sand gravel under various particle sizes as well as dry and wet conditions was carried out. The relationship between shear stress and displacement of calcareous sand gravel under different shear modes was analyzed, and the particle size effect of shear strength and deformation of calcareous sand was studied. The results indicate that the peak strength of calcareous sand gravel first increases, then decreases and then increases, when the particle size increases from 0.075 mm to 60 mm. The inflection point of the peak strength change appears when the particle size is from 0.25 mm to 0.5 mm, and the calcareous sand particles generate more fragmentation under this particle size. Due to the influence of inflection point particle size, the internal friction angle shows the rule of increasing first and then decreasing with the increase of particle size. While the quasi cohesion shows the rule of increasing first, then decreasing and then increasing with the increase of particle size. The existence of water film effect makes the shear strength of calcareous sand under a certain water content lower than that under air drying. The quasi cohesion parameter obtained by direct shear test is larger than that by simple shear test, since the effective stress path and dilation angle under different shear methods have difference, and the deviation will increase with the growth of particle size.

Key words: calcareous sand, simple shear test, particle size, strength index